Ground loops

(From Thomas Maier)

When you have two circuits that are tied together electrically, but one of them is high current then you should direct the ground and power paths to "feed" them separately. You want the current of the driver to stay on the driver side and the current of the logic to stay on it's own side. The thin trace inbetween is still needed because this is not galvantic isolation.

 e->
<-e |------------|
|---------- | |
| | <<<< physical CURRENT POWER FOR
--REGULATOR LOGIC separation >>>> DRIVER DRIVERS
| | | |
| e-> | ground | <-e |
-----|--------------------------------------------------------------
^ thick ^ thin ^
traces traces very thick
from reg to traces from
logic load drivers to supply

The common mistake is to "daisy chain" the ground by having the ground of the high current item seek it's current path through the ground of the logic. This causes ground spikes on The common mistake is to "daisy chain" the ground by having the ground of the high current item seek it's current path through the ground of the logic. This causes ground spikes on the logic and thus logic errors due to bad voltage levels at the logic chips.

Physical separation is to prevent electromagnetic coupling, of course. Even getting the grounds proper won't help if you couple the magnetic field back into the logic traces.

Always image traces to be resistors. Thick ones are small resistance and thin ones are large. The objective in laying out the board is to encourage the large currents to take the path back to their own source without getting onto the other grounds.

Separating them in this way can make a micro run right along side of a vicious current driver and not have logic problems in most cases. The cases in which it usually doesn't work is when the signal being sent to the driver is analog instead of digital. You're going to get some amount of ground differiental with the separate ground paths and so the analog signal will reflect this differance in the signal voltage relative to ground.

Current loop coupling of the signal to the driver could solve a really bad problem of ground differientals, but I have never used that technique. If your going to go to that extreme then you may as well isolate them altogether.

If your signal is digital you can clean it up abit by having a schmidt trigger on the driver side of the loop with it's ground relative to the high current load. This can provide a volt or more of tolerance in the ground differance.

If you get the currents going right you will see less problems with the logic side, but you might see more problems with the driver because it's signal from the logic is corrupted by lifting of the ground potential because of the high currents. When you have reduced this effect by minimizing the high current ground ohmage to the point where you cann't minimize it any more AND you have included schimdt buffering, then it's time to admit defeat and galvanically isolate the two circuits.

By using these guidelines I have been able to run six amp chopper drive circuits on the same board with micros and no galvanic isolation. The layout of the current loops is critical.

Peltier coolers/heaters

(From Chris Webster)

A typical peltier device consists of a number of series-connected N- and P-type semiconductors sandwiched between two ceramic plates, such that the flow of majority carriers (electrons or holes) in each semiconductor occurs in a single direction, as shown below:

 (hot side)
============================== <-- ceramic
/|\ __________ __________
| majority | | | | | | | |
| carrier | N | | P | | N | | P |
| flow (-) _____|___| |___|__|___| |___|_____ (+)
=============================== <-- ceramic
(cold side)

The device works by depleting the cold side of thermally-generated carriers and moving them to the hot side; in essence, the device is a heat pump. When a fixed potential is placed across the device's terminals, a fixed temperature difference will be maintained between the hot and cold sides. If the hot side has a sufficiently "beefy" heat sink, this temperature difference can be several tens of degrees C.

The process is reversible -- placing a temperature difference across the device will cause a voltage to develop across its terminals.

Generating -5VDC from +5VDC

(From Richard Friesen)

If you happen to have the March 1984 issue of Radio-Electronics, turn to page 78. This issue has the very first instalment of Robert Grossblatt's "Designer's Notebook" column. In it, he shows a simple circuit which will supply a negative voltage, given a positive voltage. It's basically a 555-based oscillator, and a voltage-doubling rectifier. He claims the negative-voltage output should be good for about 60ma. No-load voltage should be pretty close to the input voltage (but negative), although the voltage will drop a bit, depending on the load. If you put +5V into the circuit, it'll give you around -5V out. load. If you put +5V into the circuit, it'll give you around -5V out. If the load makes the voltage drop too low (-3V or -4V), you could always just feed the circuit with a higher voltage (like maybe 9V or 12V) and then just regulate the output down to -5V using a 7905 regulator. I've used this circuit a couple of times for powering op-amp's, and it works great!

I'm not that great at ASCII-art, but I'll give it a shot. If the following schematic doesn't make sense, let me know, and I'll try it again...

 +V
^
|
+-------+---+
| | | -V Output
R1 |8 |4 +----+---> | 7 ------- | | Parts List:
+-----| | D2 | IC1 = 555
| +--| | + | | R1 = 1.5K
R2 | 6| IC1 |---C1--+ | R2 = 10K
| | | |3 | | C1 = 10uF,16V
+--+--| | D1 C2 C2 = 22uF,16V
| 2| | | |+ C3 = 1500pF
| ------- | | D1,D2 = 1N4001 diodes
C3 |1 | |
| | | |
+---------+-----------+----+
|
===
(GND)

Note: In the above "diagram", both diodes point down (the anodes are at the top). Also, watch the polarity of C1 & C3.

The circuit is set up to oscillate at about 45kHz, with a duty cycle pretty close to 50%. None of the values of any of the parts are terribly critical, so if the capacitors or resistors are "in the ballpark", it should still work okay.

HV supply: 12VDC in, 12KV out

(From Sam Goldwasser)

Simple High Voltage Generator - 12 V in, 12,000 V out inverter. Modify appropriately for 24 V in, 30,000+ out, lower power.

 *VCC Q1 +-----------------C ||
o | C ||
| B |/ C C ||
| +------| 2N3055 C ||
| | |\ E 5T C || C-------|>|----------o +HV
| | | C || C HV Diode, usually
| | -_- C || C built in
| | C || C
+--|--------------------------C || C
| | Q2 _-_ C || C
| | | C || C Secondary (HV) winding,
| | B |/ E 5T C || C intact.
| | ----| 2N3055 C || C
| | | |\ C C || C
| | | | C || C
| | | +-----------------C || C
| | | || C
| | ------------------------C || C-------------------o -HV
| | 2T C ||
| | +----------C ||
| | | 2T C || T1 - Flyback transformer from BW
| +--------------------------C || or color TV or computer monitor.
| |
| R1 | R2
+----------/\/\/\--+--/\/\/\---+
110 27 _|_
2W 5W -

Read in Entirety!

1. Obtain flyback transformer with known good HV secondary winding. primary may be left intact if it is known to be in good condition - non shorted. A flyback removed due to failure may be used if it was the primary that failed and the primary turns can be removed without damaging the HV secondary or losing the secondary return connection! Flybacks fail in both ways (primary and secondary).

2. Wind 10 turn center tapped drive winding and 4 turn centertapped feedback winding using #16-20 guage insulated wire. Make sure both halves of each coil are wound in same direction.

3. VCC should typically be in the range 12-24 volts at a couple of amps. Circuit should start oscillating at around 5 volts VCC or so. If you do not get any HV out, interchange the connections to the transistor bases. Heat sinks are advised for the transistors. Be aware of the capability of your flyback (BW monitors up to 15KV, color up to 30 KV). You risk destroying the secondary windings and/or HV rectifier if you get carried away. Running this on 24 volts will probably cause an internal arc-over in a small flyback, at which point you start over with more caution and a new flyback.

4. Actual output will depend on turns ratio of the flyback you have. For typical monochrome computer monitors or video display terminals, you should be able to get around 12,000 volts with 12 volts input. I made one from a dead MacPlus flyback from which I removed the (dead) primary windings.

5. Frequency of operation will be in the KHz to 10s of KHz range depending on VCC, load, and specific flyback characteristics.

6. You can experiment with # turns, resistor values, etc. to optimize operation and power output for you needs.

7. CAUTION: contact with output will be painful, though probably not particularly dangerous due to low (a few mA) current availability. HOWEVER, if you add a high voltage capacitor to store the charge, don't even think about going near the HV!

Parts list:

Q1, Q2

2N3055 or similar NPN power transistors (reverse polarity of VCC if using PNP transistors.) Maximum stress on transistors are about 2-3 times VCC. Heat sinks will be needed for continuous operation.

R1

110 ohms, 2W resistor. This provides base current to get circuit started.

R2

27 ohms, 5W resistor. This provides return path for base feedback during operation.

T1

Flyback transformer from/for BW TV, color TV, or computer monitor modified according to text above. Most modern flybacks include built-in HV rectifier diode(s) so output without additional components will be high voltage positive pulses. Note: this kind of flyback transformer drives the CRT directly and uses its glass envelope as the high voltage filter capacitor. (A foot square piece of 1/8" Plexiglas with Aluminum foil plates makes an adequate filter capacitor.)

Wire

A couple of feet of #16-#20 hookup wire, magnet wire, or any other insulated wire for home made primaries. Use electrical tape to fix windings to core. Wind feedback winding on top of drive winding.

RR LED flasher (alternating)

(From Darren Leigh)

You can get a 50% duty cycle if you use a CMOS version (a 7555 for example). CMOS has a rail-to-rail output swing which lets you get away with this:

 +--------+
| |
| 7555 |
| |
+----|2 |
| | 3|--+---> output
+----|6 | |
| +--------+ |
| |
+-----/\/\/------+
| R
-----
----- C
|
|
gnd

To make little railroad lights alternately flash, hook the output to a pair of LEDs -- one via a 3904 (small NPN) and the other via a 3906 (small PNP).

 ^ Vcc
|
\
/ R
\
/
|
|
---
\ / LED (glows when 555 ouput is high)
V
---
|
|
/
|/
in -\/\/\--| 3904
R |\
V
|
|
gnd

and:

 ^ Vcc
|
/
|<
in -\/\/\--| 3906
R |\
|
|
|
\
/ R
\
/
|
|
---
\ / LED (glows when 555 output is low)
---
|
|
gnd

---

(From rschramp@...)

I think the easiest way is to build a relaxation oscilator with an opamp (e.g. 741), some resistors and an capacitor. Use two LEDs as lights.

0V--cap-+--resist--+
| | +5V
| |\ | _|_
+5 +-|-\ | \ / LED
| | \ | +----resist--+
resist | >----+-----+
| | / | +----resist--+
+-------|+/ | _|_
| |/ | \ / LED
| | |
+----resist------+ 0V
|
|
resist
|
0V

This is a smitt-trigger (a circuit with hysterysis) which alternating charges and discharges a capacitor through a resistor. If the output voltage is low the upper led is lighted the current throug the led is limmited by the series resistor. If the output is high the lower led is lighted.

Ultrasonic transducer oscillator circuit

(From Chris Abbott)

Allows the transducer to oscillate at its self-resonating point, with no tedious setup.

As far as I can tell, the circuit will run on 5V-9V.

 +V --o----------o--------------------o-------------o-------o
| | | | |
| | R3 / | |
| | +----+ /-C | |
| o---+ 15K+---o---B | TR1 | |
| | +----+ | \-E / |
| | | \ /-C | 47nF
+++ +++ | ----o-----B | TR2 =====
| | 2M2 | | 10K | C1 | \-E | C2
| | R1 | | R2 | D1 ===== \ |
+++ +++ | | /| | 10nF | |
| | /---o---|< |----o---------o-------o
| | /-C | \| |
| /--o-----B | TR4 1N4148 | Earthy
| /-C \-E ----- Side
o----B | TR3 \ XXX
| \-E | -----
| \ | | Transducer
| | | |
o--------|-----------|--------------------------------o
| | |
| | /| | |
o--|< |--o-----------o------- 0V All transistors 2N3707
| \| NPN, Emitter at bottom.
D2 1N4148

Phase shifter circuit

(From Richard Karlquist)

Here is a well known op-amp phase shifter. I am surprised no one has posted it yet, so I guess I will have to. The circuit will have 90 degrees phase shift at:

 FREQ = 1/(2*PI*R3*C)

At low frequencies it has 0 degrees of phase shift and at high frequencies it has 180 degrees of phase shift. By making R3 a pot, you can vary the phase at a given frequency from nearly 0 to nearly 180 degrees. Since you want to work at 1 MHz., you will need to use a high frequency op amp, like the Burr-Brown OPA620.

 R1=R2 |---\/\/\/\/\\/\--|
| R2 |
| |
R1 | |\ |
| INV | \ |
INPUT --------\/\/\/\/\/\/----------| \ |
| | \ |
| | \-------- OUTPUT
| R3 | /
| NON-INV | /
|---\/\/\/\/\/\/----------| /
| | /
| |/
|
|
------- C
-------
|
|
|
|
GROUND

---

(From David Medin)

How about the simple single-transistor phase splitter?

 V+
|
R
V+ |
| +---------+
10K / |
0.1 uF | |/ P 0.1 uF
IN>----||------+-------| 2N2222 O<--||---> High impedance output
| |\E T (should be buffered)
10K \ |
| +---------+
GND |
R
|
GND

The circuit uses the inverting properties of an emitter follower with a collector load. You have to experiment for a value of R and the pot that will keep the transistor's power dissipation within limits given the V+. The output stage should be buffered by another emitter follower stage, or an op amp, etc. Note that this circuit induces some loss in the p-p value of the signal, too. It is not completely distortionless, but reasonable if you do it right.

The same topology can be employed with op amps: One inverting and one non-inverting with the pot between their outputs, buffered by a third op amp.

Bringing NiCd's from the dead

(From no-idea)

The failures the article talks about occur in mutli-cell Ni-Cd battery packs, and are due to the voltage differences between cells. Say you have four 1.25 V cells in a pack connected to a 200 ohm load. The load "sees" 5 volts and draws 25 mA. Since each cell must pass the entire 25 mA and each cell's potential is 1.25 volts, Ohm's Law tells us that each cell sees the equivalent load of 50 ohms.

But in practice, no four cells in a battery ever exhibit exactly the same output voltage. Assume that one cell is delivering only 1.2 V, and the others are at 1.25 volts. Now, the 200 ohm load sees 4.95 volts and draws 24.75 mA. Since all four cells must pass the entire 24.75 mA, each of the strong cells at 1.25 volts sees an equivalent load of 50.5 ohms; the weak cell sees only 48.5 ohms. The weak cell works into the heaviest load and as a result will discharge more rapidly than the other cells. If the pack is charged for only a short period of time, the weak cell, which has been working the hardest, is also the one that receives the least charging power.

This usually doesn't matter if you trickle charge after each day of flying. The inequality is small for any given charge or discharge cycle, due to the relatively flat output voltage NiCd cells exhibit over most of their range. But a combination of incomplete charges and deep discharges will exaggerate the energy difference between a weak cell and the other cells. Operated continually in this manner, the weak cell invariably reaches its "knee," the point at which its voltage decreases sharply, long before the other cells reach the same point.

Now comes the problem! Suddenly, the weakest cell sees an increasingly heavy load, which causes its voltage to drop even faster. This avalanche continues until the cell is completely discharged, even as the other cells continue to force current to flow. The inevitable result is that the weak cell begins to charge in reverse, which eventually causes an internal short. Once an internal short develops, recharging the cell at the normal rate is futile. The short simply bypasses current around the cells active materials. (Even though the cell is apparently dead, most of its plate material is still intact.) If the small amount of material that forms the short could be removed, the cell would be restored to virtually its original capacity once again.

 300 ohm Charge
5W / Switch
20-40 + O---/\/\/\----o------o o------------o----------------o
VDC | | |
| Zap | |
| Switch | +|
| ___|___ | -----------
o------o o---------o -----
| | + Shorted |
6000 micro- | + ------- Cell |
Farad, 40V _________ | | |
Capacitor --------- |_____| Volt |
| | meter |
| | |
- O-------------o----------------------o----------------o

Using the circuit shown, the internal short can be burned away in a few seconds. In operation, energy stored in the capacitor is rapidly discharged through the dead cell to produce the high current necessary to clear the short. Current is then limited by the resistor to a safe charge rate for a small A cell.

Several applications of discharge current are usually necessary to clear a cell. During the "zapping" process, it is a good idea to connect a voltmeter across the cell to monitor results. Momentarily close the normally open pushbutton switch several times to successively zap the cell, allowing sufficient time for the capacitor to charge up between zaps, until the voltage begins to rise. Then, with the toggle switch closed, watch as the potential across the cell climbs to 1.25 volts. If the potential stops before full voltage is reached, some residual short remains and another series of zaps is in order. If you observe no effect whatsoever after several zaps and shorting out the cell and taking an ohmmeter measurement indicates a dead short, the cell is beyond redemption and should be replaced.

Once full cell potential is achieved, remove the charging current and monitor battery voltage. If the cell retains its charge, it can be returned to charge and eventually returned to service. But if the cell slowly discharges with no appreciable load, the residual slight short should be cleared. To do this, short circuit the cell for a few minutes to discharge it, zap again, and recharge it to full capacity.

Condenser microphone hookup

From: Andrew Mitz

 capacitor (0.5 uf or so)
| |
+---------------------+--------| |--------------> to amplifier
| | | |
| (positive lead) | _ _
| +----------/ \_/ \_/----------+
MIC (resistor 1-2K or so) |
| | +
| (ground lead) (-) -------
| (battery 9v or so) ---
| |
| |
+---------------------------------------------------+----> ground
to amp

'Nixie' display tubes

From: mkuhn@... (Martin W Kuhn)

Nixies are lots of fun. I just recently built a digital clock out of Nixies, so I think I can help you here. (BTW, if anyone is interested in details of the clock, let me know)

There is no heater connection; these are not like vaccuum tubes. There may be some unused pins, however, or perhaps they are for a decimal point or other symbol the old equipment didn't use.

Nixies have a cathode for each symbol, and one anode which is used for all the symbols (usually a grid-shaped element in the middle of or in front of the symbol elements.)

Here's a super-simple power supply to try experimenting with them:

 D R
<------>|-------/\/\/\/---
^
|---------------> To anode terminal
To |
120 VAC | C
----|(----
|
<--------------------------|----> To selected cathode
D : any rectifier diode of >= 250 PIV
R : 100 K or so variable resistor
C : 20 uf (or more) at at least 250 WVDC (observe polarity!)

CAUTION: Take care when using the above circuit! If you have an isolation transformer around, use it! Output of this supply can be over 150 VDC! Also, make sure you discharge C when you are done using it--- It is a good idea to wire a resistor across C when you use this circuit as a shunt. (something like 100K to 1M or so is fine) Make sure you connect it to the tube BEFORE plugging it into an outlet, in order to avoid a nasty shock by accidently touching the output leads! A .5 amp or so fuse might be a good idea too, to protect against short circuits.

NOTE: This might not be the best possile circuit, and others might want to suggest changes to it, but it works, and is simple enough.

After you wire it up, connect it to the anode and one of the cathodes. Make sure R is at its MAXIMUM resistance. After you plug it in, slowly adjust R until the selceted digit just manages to glow completely. If the wire lead connecting the number to the pin on the base of the tube also glows, then R is too low; turn it back! The exact voltage necessary to light the tube will depend on the particular tube you are using. Also, you may want to wire an ammeter in series with the tube when you first try this out. The tube should only draw a few mA. If it draws much more than 5mA or so, something is probably wrong!

BTW, the Nixies should glow a "pure" even amber-orange color. If they are sort of a lighter color with blueish fringes, then they are somewhat gassy, but still usable. If you see blue "clouds" in the tube and/or the symbols are fuzzy and indistinct, then the neon has gotten too contaminated, and the tube should not be used.

If you are planning to use Nixies in something like a logic circuit, you can easily drive them with any NPN transistor with a CEO of at least 200V and an Ic of at least 10mA or so. (actually, low-power high-voltage transistors of this type are not so easy to get these days)

If you have any specific problems/questions let me know; I am not sure what sort of info you are looking for. Hope this helps, anyway.

Video amplifier

From: iisakkil@... (Mika Iisakkila)

" Got some questions about video amps. I've seen an NE592 used as a video buffer amp at the end of a 75 ohm line. Used so that the 75 ohm line could drive all kinds of neat processing stuff without affecting the signal (that's what a buffer is after all, right?) Now National Semiconductor makes an LM592 that's also a video amp. Do these two chips cross reference to eachother? "

They are the same chip. Sources for NE/SE/LM/uA592 include TI, Harris, Philips (Signetics) and Motorola. Be aware that there are 8 and 14 pin versions of it, the difference being that the larger package has two additional gain control pins. It's not really an op-amp, so you can't use feedback to control the gain. Additionally, they're _fast_ circuits, so use a ground plane and ceramic bypassing caps as close as possible to the supply pins.

" Also, is there a relatively simple video buffer amp I could make with discrete components? I really don't want capacitive coupling, since video has DC components. "

The DC components in video are normally a non-issue. Most video equipment are AC coupled (at least the input), which is the reason why you can't get away without black level clamping if you plan to process the video signal. Nothing is said about the actual voltage levels of the video signal, they are just referenced to the black level which may float anywhere (well if I remember right, you're guaranteed to have less than 1W power dissipation in the terminating resistor with standard video...). A typical video input has a 75 ohm terminating resistor to ground and then the signal is fed to the input buffer via a ~50uF electrolytic cap.

Anyway, here's a simple discrete video output stage. Can't get much simpler than this. Note that there's a serial matching resistor on the output, so you'll have to feed 2Vp-p video into the buffer to get the usual 1Vp-p into the equipment you're driving. This is the way it's usually done. Sorry for the crude transistors, but I hate doing ASCII graphics.

 o +5v
|
+---+
| |
1k R |
| |c BC108B
+-b
2Vpp | |e 75ohm
video |e +--------R-------> video out
in >---b | 1Vpp @ 75 ohm
|c R 1k +-->
BC178B| | |
+---+ ---
|
o -5v (yes, two-sided power supply, not ground)

And while I'm at it, here's the input stage to go with it. It provides the 2x voltage gain you need to feed the output buffer above.

 o +5v
|
+----+-----------+
| | |
3k8 R R 680R R 56R
| | |
| | |e BC178B
Video in | +---------b
1Vpp/75R + | |c |c 100n plastic
>---+----||---+--b BC108B +-----||--------> to black level clamping
| 47u | |e | 2Vp-p
| | | R 220R
R 75R R +-----------+
| |1k8 R 150R
--- --- |
--- (single power supply this time)

The simplest black level clamp consists of a signal diode (1N4148) reverse-biased to ground from the output line of the input buffer above and a 4k7 resistor in parallel with it. That forces the sync tips to be at (gnd - threshold voltage of the diode), which shifts the black level of a 2x amplified video reasonably close to ground. Add that and you can connect the two circuits above together and see how they work. They should be very good as far as the signal quality goes (maybe not broadcast quality, but no visible signal degradation). Don't forget good power supply bypassing, use at least 220u of electrolytics and 100n ceramic caps near the transistors on both circuits (the output stage needs them on _both_ supplies).

FM Oscillator

From: dthomas@... (Dave Thomas)

Here's a dandy circuit for a VCO and buffer that operates across the entire FM broadcast band (88-108 MHz). I stole the main idea from the local oscillator in a radio shack scanner (pro2004). I like this design because it doesn't require a tapped coil, it tunes very broadly, it's stable, and it has a nice, hot output.

FM BROADCAST VCO AND BUFFER

 VR1
<G>-+-/\/\/\--+--+-----------+--------+--R6--+----+-----+-----<+12VDC>
| ^ ^ | | | | | |
C2 | CR2 C8 R4 | c R8 | c C11
+---+ | | | |/ | |/ |
| <G>+--+ +--C5--+---+--| +--| <G>
audio R1 | | | |\ | |\
in | | | C6 | e | | e
O--C1---+--R2--+--C4--+ R3 | | | |
| ^ | | +----+--C9--+ +--C10--O RF out
C3 CR1 L1 | C7 R5 R7 R9
| | | | | | | |
<G>-----+------+------+------+---+----+------+----+-----------<G>
Q1 Q2
+ connection<G> ground connection
^ cathode of a diode
C1, C2, C8, C11 .1 uF
C3, C4 .001 uF
C5, C6, C7 39 pF NPO or silver mica
C9 10 pF NPO or silver mica
C10 22 pF
CR1 ECG616 varactor (tuning) diode
CR2 9v Zener
L1 5T #20 wire, 1/8 inch I.D., adjust spacing for tuning range
Q1 2N3663
Q2 2N3904
R1, R2 47 K
R3 22 K
R4 15 K
R5 1 K
R6 390 ohm
R7, R8 4.7 K
R9 100 ohm
VR1 100 K, linear taper

As with all VHF circuits, pay particular attention to construction technique. I recommend cutting little square islands on one side of a two-sided copper-clad board. Use the remainder of that side as the ground plane, and leave the bottom side to serve as a shield. If you keep all lead lengths short, the circuit is quite stable.

With the parts listed here, effective frequency range extends well beyond the FM broadcast band in both directions. If a 6V zener is substituted for CR2, the circuit will run from a 9V battery, with a slightly smaller tuning range. The output is hot enough that the signal can easily travel a city block using just a clip lead for an antenna.

----Original Message Follows---- From: "radu"

Reply-To: home_electronics@yahoogroups.com

To: home_electronics@yahoogroups.com

Subject: [Electronica] wireless phone

Date: Mon, 12 Nov 2001 22:12:15 -0000

hi,

does anybody knows about any kind of wireless phone with

a larger range of action than usual(at least 5 km)?

Thanks,

Radu Soptea

Crystal Oscillator

(From Goran Olsson)

The world is full of xtal oscillators twiddled by digital designers lacking in the analog design knowledge necessary. Just look at all the PC real time clocks that lags or leads by several minutes per day. And they eat backup batteries too! IC's with pins that say "Xtal here" can't be trusted either!

The design below, for 1 MHz, is a good starting point for a discussion:

 CMOS or HCMOS inverter
|\
+--| >0---+----> OUT
| |/ |
| |
+--\/\/\--+
| 1 Mohm |
| |
| \
| / 2.7 kohms
| \
| /
| |
| 1MHz | parallel resonant
+---|[]|--+
_|_ _|_
55pf ___ ___ 60pf
_|_ _|_
\ / \ /

First of all, all crystals have two modes of resonance, the series and parallel resonances. These are closely spaced, and the circuit design must ensure that the resonance mode specified for the crystal is in operation, or you will end up with a frequency different from what is stamped on the crystal.

In the series mode, the crystal shows a low impedance at the resonant frequency. This impedance is on the order of 100 ohms to a few kohms. In the parallel mode, the crystal together with a specified capacitance in parallel, normally 30 pF, shows a high impedance at the resonant frequency. The 30 pF value is used regardless of the frequency.

All crystals have resonances at the odd harmonics, 3, 5, .. times the fundamental (overtones). At frequencies above 25 MHz, crystals are often made to operate at one of the harmonics. In all cases the external circuit must be made to suppress operation at the wrong harmonics or fundamental.

Normally the crystals are specified for the parallel resonance mode. The circuit above is designed for such a crystal. The crystal and the 30 pF parallel capacitor are here transformed into a pi filter network by dividing the 30 pF cap into two 60 pF caps and grounding the middle node. When one end is driven from a low impedance, this network has a 90 deg phase shift at the frequency of maximum gain. With a suitable driving impedance, the phase shift is brought close to 180 degrees. Thus the 2.7 kohm resistor. Other good reasons for it are that harmonics are damped by the resulting RC filter, and that the inverter output is removed from the strange load of the crystal network. A rule of thumb for determining the value of the output to crystal resistor is that it should have the same impedance as the capacitor at the operating frequency:

 R:= 1/(2*pi*f*C)

For a 32 kHz oscillator this resistor becomes 160 kohm.

The gain and 180 degrees phase shift of an inverter is now all that is needed to make this circuit oscillate at the right frequency with no twiddling necessary. The resistor between input and output is essential to put the gate in the range of linear operation so the necessary gain to start oscillation will be there. Since a CMOS inverter has very high input impedance, the value can be large. It is not critical, but a low value will increase power dissipation. Use 1000 times the series resistor if you have no other preferences. Note that the inverter *must*not* be a Schmitt trigger. Also note that one of the capacitors is adjusted to correct for the input capacitance of the inverter. In an actual circuit, corrections should also be made for other stray capacitances. The frequency is fine tuned by trimming the capacitors.

At higher frequencies account must be taken to the phase shift of the inverter. The phase shift for a gate when operated as a linear amplifier is certainly not to be found in any data sheet. Just note that 8 ns delay corresponds to 45 degrees phase lag at 16 MHz. Use this type of info as a starting point for adjustment by reducing the R & C:s.

An 4000 series CMOS inverter is usable up to around 5 MHz. Use HC to 25 MHz, AC to 40 MHz. Above that you are into F, ALS or AS families. The same principles apply, but the DC feedback must be arranged by a voltage divider, and the impedance is much lower, on the order of kohms.

To use a 3:rd or 5:th harmonic crystal, you need to insert a bandpass filter into the feedback to avoid oscillating at the fundamental or other harmonic. A series resonant LC filter is something that easily could be inserted between the output and the resistor in the above circuit. Zero degrees phase shift at the center frequency means that the other design criteria still hold. The Q of the filter should be low, around 1-3. Example for 30 MHz: (Just the filter.)

 .47 uH 56 pF 180 ohms
from inverter output-----(((((------| |---------\/\/\/-+- to xtal
|
_|_
___ 56 pF
|
_|_
\ /

A C-L-C pi filter and series resonant crystal is another solution:

 180 ohms .47uH
from inverter output----\/\/\/--+---(((((----- xtal --+-- to input
| series |
_|_ _|_
___ 120pF ___ 100pF
| |
_|_ _|_
\ / \ /

Component values should not be taken literally. (No indication of inverter type given!)

Some additional hints:

Don't distribute the inverter output node over a large PC board. Instead use free inverters of the same chip for buffering.

If you use the other inverters of the same chip for other signals, be aware that there is crosstalk that causes phase jitter in the oscillator output that might be disturbing in critical applications. For a clean noise-free output, a local voltage regulator to supply the inverter is also a good idea.

Also apply care in the PC board layout of the oscillator. Ground plane, good power decoupling, no signal wires routed under the crystal circuit.

http://www.aaroncake.net/circuits/index.asp

Guitar Fuzz Effect

---

Fuzz is one of the classic guitar effects, and this simple circuit generates it quite well. The circuit is so compact that it can be built into guitars or amps that do not have built in fuzz to add that capability to the instrument. The circuit does not use much battery power, so a standard alkaline battery will last many years even with daily use.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
R1, R2	2	100K 1/4W Resistor
R3	1	1K 1/4W Resistor
R4	1	1M 1/4W Resistor
C1	1	100uF 16V Electrolytic Capacitor
C2, C3	2	0.47uF Ceramic Disc Capcitor
D1, D2	2	1N4148 Diode
U1	1	741 Op Amp
MISC	1	Socket for U1, Jacks for Input/Output, Wire, Board, 9V Battery Snap, Case

Notes

1. You can use any input/output jacks you want, but the standard for musical instruments such as guitars is a mono 1/4" headphone plug and jack.

Crystal 32.768KHz CMOS Oscillator

From: cs911225@... (KEN E WILLMOTT)

Try a Pierce oscillator, with the following specific component values:

 1/6 4049 or equivalent

|\ |\
*--| >0---*----| >0--->
| |/ | |/
| |
|--\/\/\--|
| 15M |
| \
| / 330K
| \
| /
|32.768KHz|
----|[]|---
_|_ _|_
10pf ___ ___ 39pf (variable)
_|_ _|_
\ / \ /
V V

Info on CO2 lasers wanted (ho hum)

From: wouter@... (Wouter Slegers)

I'd like to get some info CO2-lasers... I read the following on a BBS.. If anyone has a sequel on this, please email it to me..

Wouter Slegers
(wouter@...)

 Supreme 7 (S7) Productions proudly present....
Palm Beach BB Uk - (0303) 265979
LASER WEAPONRY / PART 1 / LASER SIGHTS
by The Deceptor and Flip

WELCOME!.....To part 1 of LASER WEAPONRY - to be included in each issue of ELEKTRIX. This first tutorial deals with building your own laser sights as seen in films such as 'THE TERMINATOR' and actually used by US military and UK M.O.D. for such weapons as ANTI-TANK GUNS, APRLs and other high- power military weapons. The type of sights mentioned here can be strapped onto the barrel/tube and provide perfect targetting - also phreaks people out if you walk down the street pointing the beam at people.

NOTES:

• The laser used is a helium-neon one which emits a bright red beam.
• Pointing it in someone's eyes will probably blind them so be careful with where you point it - unless ofcourse you intend to do damage (?!)
• It can't burn skin or paper or anything - it's only really useful in this case as laser sights - although in future issues I others will explain how you can use it for Data Snooping.
• Total project cost is about 40 quid.

WHAT YOU NEED:

• A Helium-Neon laser tube.
• A portable power supply (easily transformed into a backpack).

A fairly decent tube can be obtained from BULL ELECTRICAL for about 25 quid

J&N BULL ELECTRICAL
250 PORTLAND ROAD
HOVE, BRIGHTON
SUSSEX BN3 5QT

The specs. of the JN BULL tube are as follows:

Maker: Philips (holland) - could buy cheaper direct I guess.
makers ref.no: LHN-VLP/04
type: Helium-Neon
Size: approx 260mm x 40mm
Striking voltage: 6-8KV
Running voltage: 1.2-1.5KV
Output: 1.6mW (not bad at ALL! - usual MAX is around 2mW!!!)
Running current: 5mA
Polarity: white lead = negative, black lead = positive
Estimated life: 5000 hours
Warm-up time: 1 second
Wave length .63+0.01um (62.8nm roughly - red part of spectrum)

Although the light emitted is actually red you'll only see the actual beam if the air is misty or dusty...though the spot at the end of the beam is perfect - the invisible beam is an advantage though if you don't want to be detected.

I've included that info in case you pickup the beam as surplus stock some- where without the info. The tube isn't too fragile either - I dropped mine from two foot once - nothing.

As you're going to need a power supply that's portable then you'll be hard pressed to find a company that does one....so here's the docs.

Part 1:
~~~~~~~ C4-C10 - DISK CERAMICS
1M-1Watt
R3 100p 100p 100p 100p
----====----||---------||--------||--------||-------
D3-D11: | C4 /\ C6 /\ C8 /\ C10 / |
high- | Diodes follow / \ / \ / \ / |
voltage | direction to D5 D6 D7 D8 D9 D10 D11 |
3kV diodes| D11 from D5 / \ / \ / \ / | |R4
A | / C5 \/ C7 \/ C9 \ / | |47K
<--------------|------D3>|--------||--------||--------||----- |
_ | | 100p 100p 100p | |R5
Diode /|\ D4 | | |33K
going | | C2===0.1/1.6KV |
up.....> | | | ____|
B | | +VE -_|_
/_____________/|\____________| | |
\ | | | | LASER
| | | | TUBE
<--To part 2 | C3===0.1/1.6KV |___|
| | GND - |
| | |
| | |
|_____________|_________________________________|

________________________________
| /\||
Part2: | \/||
~~~~~~ | R1 /\||
| ______----______________\/|| ----- <- From Part 1
| | ---- ____|____ ||/\ (A)
| | 110 2W / | /\||\/
| | ______/ | | \/||/\ <--Ferrite core
| | / R2|_|1K/\||\/ transformer
| |_____|/ / __\/||/\
DU 1n4002 100v | DU |\X C1 |------|__ ||\/
Going: UPWARDS | __|_______|___)|_| \ /\|| ----- <- From Part 1
| | DD | )| | \/|| (B)
DN 1n4002 100v |__/___|_____|/X 10uf | /\||
Going: DOWNWARDS \ |\______________\__\/||
| /
| |
| |
| \ - Switch
X - Collector | |
| |
Transistors are: | =====
D4005 NPNs | --- 12V of AA NICADS in series
| ===== (BATTERY)
| --- (11x1.2V is sometimes a
| | little more reliable)
| |
-------------------------

I hope people understand those schematics - TXT files make life so hard!...

If you can't understand something then contacts us at Palm Beach and we'll send u a photocopy of our own docs if you like (fax optional).

Use:

Strap the tube (maybe put it in a protective casing too) to the barrel/tube and then FIRE AWAY!!!!

If you point it at someones eyes or your own then you/they can say goodbye to them for good.

Phun with your tube:

As well as being GR8 for sights you can use it for other things:

• Shine it at a neighbours window while they're sleeping and the room will fill with a eerie red light!! haha - gr8....and this can be done from a few hundred feet with excellant results.....

• Another good use is to point it just in front of someones feet while they are walking....The red dot on the ground will make them REAL paranoid!

• Picking up conversations in buildings by bouncing the beam off a window and via modulation of the beam and conversion at the opposite end you can hear very high quality conversations without being seen or heard - this is a bit more tricky though....details in the next ELEKTRIX issue.

• PLAIN TERROR! YERRRRR! PHUCK this is the bit I love.....walk down the road real casual or through a town centre....carrying your nice laser....aiming the beam at people as they go by....This really phreaks them out...best if you and a m8 piss about and make out he's gone blind. People will run fer their lives - you need to add a nice buzz noise to it - that really makes it seem quite an awesome device!

That's it for this LASER WEAPONRY issue.....a great addition for any launcher ,tube, rifle, or just about any direct beam weapon......It could also be a fantastic sight for a carbon dioxide laser (needed since CO2 lasers are not at all visible - phun).....YUP! CO2 laser is in Part 2 of LASER WEAPONRY... fully portable and with over 200W output we are talking *** SERIOUS POWER ***

LED Display controller

From: byron@... (Byron A Jeff)

" I would like to make a big LED scrolling display (like the ones you find in a bank), about 10 "characters" long. Do I have to make a huge array of led's myself or is there a supplier of premade displays? I suppose I can wire up the logic to drive the display, but again, is there a chip or board that will display and scroll characters? "

No need to wire up individual LEDs. Look in magazines such as Circuit Cellar Ink or Radio Electronics for 5x7 multiplexed LED display modules. They are designed where common cathodes are wired along the columns and common anodes are wired along the rows. One advertiser (in CCI I believe) sold these at a price of 8 for $15.

Driving these displays takes a little work. Since they are multiplexed only one row/column can be active at a time. I found that lighting up a column at a time is best (less flicker due to only 5 columns per pass as opposed to 7 rows per pass). To get even brightness each column must be left on the same amount of time and each common anode needs a current limited connection to +5. I accomplished this by connecting the output of a 7407 open collector TTL buffer to the anode along with the current limited +5. When the buffer was on it would sink the current and the LED would not light. If the buffer was off then the current flowed through the LED and it would light up. See diagram below:

 +5VDC
|
\
/ 270 ohms
\
/
Control|\ 7407 |
-------| >-----------+
|/ |
Control = 1 LED on |
Control = 0 LED off |
|
LED
anode

Note: 7407 can sink up to 30 mA of current. Current limiting resistors down to 166 ohms can safely be used. Also remember not to exceed the breakdown current of the LED. LED's can be pulsed at greater than average current for more brightness.

Now for the cathodes. Remember due to multiplexing only one cathode can be on at a time. Otherwise all columns with multiple cathodes on will show the same LEDS on (due to common anodes). So I chose to use a demultiplexer to select a single cathode line at a time. I chose a 7445 BCD demultiplexer because it can sink 80 mA of current. However this was just an experiment and I wasn't looking to send messages to my neighbors down the street. Since the 7445 has 10 outputs it can drive two of these displays. For brighter displays the cathodes need more sinking current. Sprague makes a BiMOS driver IC that is serially driven and can sink more current. I've also seen LED driver chips in various mags. Or you could simply wire a transistor for each cathode that is driven by a TTL output.

I don't know of any board to control this. I controlled mine from my PC parallel printer port (Just an experiment). But if I was doing this for real I'd probably use one of the nifty microcontrollers on the market. An Intel 8031 can be wired up in 4 or 5 chips plus the LED driver circuitry. Since it has a built in serial port is could have messages downloaded to it through from a PC through an RS-232 link.

http://www.crosswinds.net/~staticwire/start.html

Flyback inverter

PARTS
C1: 1000uF
C2: 0.1uF
C3: 0.1uF
R1: 1K
R2: 10K
R3: 1k
IC1: 555 Timer IC
T1: MJE 340
D1: 1N4004 or similar
L1: 10mH

EXPLANATION

This circuit works by collapsing a magnetic field in a small coil to induce high voltage spikes. The spikes are then fed through a diode to the output. The output cannot sustain anymore than 300V since this is the highest voltage the transistor will handle across the collector and emitter.
I have not yet worked out the most efficient on-time for the coil so you are welcome to experiment with R1, R2 and C1.

APPLICATIONS

This circuit is ideal for strobe light applications.

TESTS

I have not yet performed any tests since i am still experimenting with different value parts.
I can say that i got it to run at about 500mA and it charged a 10uF capacitor fast enough to strike a flash tube several times a second.

Adjustable flashing LED

From: dwg@... (David Grieve)

Use a 555 timer IC as the (resistor controlled) frequency source, choose component values to run at 2 x desired flash rate - get the data sheet for this part, it's pretty comprehensive. Use the output to clock a flip flop (e.g. 74HC74). Feed flip-flop Q and /Q outputs to simple transistor stages, viz...

 ___+5V
|
_|_
_\_/_ LED
____ |
TTL ____|6k8 |__________|/
|____| | |\e
_|_ _|_
|4k7| |270|
|_ _| |_ _|
| |
_|_______|_GND

Drive transistor 1 from Q, drive transistor 2 from Q or /Q via a switch. Run the whole thing from +5V. If you want to run it from +9V, no problem, us a 4000 series CMOS flip-flop and change the emitter resistor to 560. If you don't want the hassle of transistors then the ULN2003 darlington driver array could replace the transistor stages.

---

From: NURDEN1@... (Dale Nurden)

Yes, you do get variable capacitors, but they are usually very low values so they probably won't be much good to you. The easiest way to do this IMO is just a pair of transistors plus 4 resistors and 2 capacitors. I threw one together the other day and it works perfectly. To change the flash rates, you just change two resistor values (one for each LED). To make both LEDs flash together, you would your switch to switch them in parallel.

The circuit is called a monostable multivibrator (should find one in a good elementary electronics book), and goes something like this (drawn from memory, so don't count on it being 100% error free):

 -----+--------+--------+--------+--> +9v (whatever)
| | | |
R1 R2 R3 R4
| | | |
+------+ | | +------+
| +---C1---+ +---C2---+ |
D1 c| | | | D2
| \ | | |c |
| Q1 |------|--------+ b / |
| / b +---------------| Q2 |
| e| \ |
| | |e |
-+------+--------------------------+------+--->GND
Q1, Q2 ... anything NPN : BC108, 2N2222, etc
R1, R4 ... 1k0
R2, R3 ... 10k0
C1, C2 ... 10uF
D1, D2 ... Your LEDs

I think it is R2 and R3 that you vary to change the frequency, but you can just fiddle a bit to figure it out. Also, the capacitor values will also affect the frequency. If this doesn't work this way round, then try swapping the values of R1 and R2, and R3 and R4. I can never remember which way round they go - I always do it by trial and error.

(from ASCII circuits)

CMOS Oscillator

From: mjohnson@... (Mark Johnson)

> What I am looking for is a low power oscillator (<.5 mA @ 5V)
> running at a frequency of roughly 1 MHz. However, the frequency-
> determining component should be an inductor with a value of
> approx. 75 uH

The circuit below uses a single CMOS lowspeed 74C14 inverting Schmitt trigger chip, your 75uH inductor, and two 10K resistors. It draws about 400uA and oscillates at about 4MHz.

The oscillator period will be approximately linearly related to the inductor value, Period ~=~ K1 + (K2 * L) [Note also that K1 will not be zero]

 +-----------> Output
|
|
|\ |\ |\ | |\
| \ | \ | \ | | \
| \ | \ | \ | | \
+--| >O------| >O------| >O----+-----| >O-----|
| | / | / | / | / |
| | / | / | / | / |
| |/ |/ |/ |/ |
| |
| +5.0V |
| | |
| \ |
| / R1 |
| /| \ 10K |
| / | / L1 |
| / | | 75 uH |
+----------------O< |--------+---+----------)()()()(------+
\ | |
\ | /
\| \ R2
/ 10K
\
|
GND

Applying the Brakes: When you first press the brakes, this circuit will turn on your 3rd brake light via the main brake lights. After about a second a series of short strobe pulses occur. The number of pulses range from approximately 1 to 10, depending on the setting of P1/P2 and when the brake pedal was applied last. After the pulses have been applied the third brake light assumes normal operation. The prototype was set for five flashes which seemed more than enough. Two days later I re-adjusted the trimmer potentiometers for 4 flashes--1/2 second pause--4 flashes. Looks pretty cool!

Circuit Description: The schematic consists of two 555 timer/oscillators in a dual timer configuration both setup in astable mode. When power is applied via the brake pedal, the brake light driver T1 is switched on via the low-output pin 3 of IC2, and timer IC1 begins its timing cycle. With the output on pin 3 going high, inhibiting IC2's pin 2 (trigger) via D2, charge current begins to move through R3, R4 and C2.
When IC1's output goes low, the inhibiting bias on pin 2 of IC2 is removed and IC2 begins to oscillate, pulsing the third brake light via the emitter of T1, at the rate determined by P2, R6, and C4. That oscillation continues until the gate-threshold voltage of SCR1 is reached, causing it to fire and pull IC1's trigger (pin 2) low. With its trigger low, IC1's ouput is forced high, disabling IC2's trigger. With triggering disabled, IC2's output switches to a low state, which makes T1 conduct turning on the 3rd Brake Light until the brakes are released. Obviously, removing the power from the circuit at any time will reset the Silicon Controlled Rectifier SCR1, but the RC network consisting of R4 and C2 will not discharge immediately and will trigger SCR1 earlier. So, frequent brake use means fewer flashes or no flashes at all. But I think that's okay. You already have the attention from the driver behind you when you used your brakes seconds before that.
The collector/emitter voltage drop accross T1 together with the loss over the series fed diodes D4/D5, will reduce the maximum available light output, but if your car's electrical system is functioning normally in the 13 - 14volt range, these losses are not noticeable.

Building Tips: You can easily build this circuit on perfboard or on one of Radio Shack/Tandy's experimentors boards (#276-150), or use the associated printed circuit board listed here.
Keep in mind that T1 will draw most likely 2 or 3 amps and mounting this device on a heat sink is highly recommended. Verify that the scr is the 'sensitive gate' type. In incandecent bulbs, there is a time lag between the introduction of current and peak brightness. The lag is quite noticeable in an automotive bulb, so the duration of a squarewave driving such a bulb should be set long enough to permit full illumination. For that reason, and because lamps and car electrical systems vary, adjustment via P1 and P2 is necessary to provide the most effective pulse timing for your particular vehicle.
The reason that the third light is connected to both brake lights is to eliminate the possibility of a very confusing display when you use your turn signal with the brakes applied.
The cathode of D4 and D5 are tied together and go to point 'B' of the third brake light in the component layout diagram. Point 'A' goes to the other leg of the third brake light. Most if not all third brake lights in Canada & USA have two wires, the metal ones also have a ground wire which obviously goes to ground. I don't know the wiring schema for Australian and European third brake lights.
Don't forget the three jumpers on the pcb; two jumpers underneath IC1/IC2 between pin 4/8 and the one near T1/R6.
If you use a metal case, don't forget to insulate the D4/D5 diodes.

Some 90's cars, like my 1992 Mercury Sable, have two bulbs inside the third brake light, each bulb is hooked up seperately to the left and right brake light for reasons only Ford knows. Click here for a possible 2-bulb hookup. It shows how I modified mine to get it working; and that was easier than I expected. Current draw with the two bulbs was measured at 1.85Amps (1850mA). Even with double the current none of the circuit components were getting hot. I had to re-adjust the two pots to make it flash since the bench testing was done with one bulb.

Bench Testing:I tested different semiconductors like the 1N5401/1N5404, NTE153 for T1, and 4A type powerdiodes for D4/D5. All worked very well. As expected, T1 is getting very hot. Current draw was measured between 680 - 735mA with a regular automotive 'headlight' bulb, extra heavy duty to make sure the circuit was safe. I tested several other power transistors including some darlingtons like the TIP125 and the TIP147. I eventually settled for the TIP125 myself because I had it available but any thing with 5A or more will do fine.
The actual third brake bulb is a lot smaller. Adjusting the trimpots (P1/P2) may take a bit of patience but really fine-tunes the circuit well. The only drawback of this circuit is the discharge lag coming from the electrolytic capacitor C2 and the R4 resistor. Especially if the brakes are used often or at short intervals the third brake light will not flash or maybe flash once or twice. Again, this is because the R-C combo does not have enough time to discharge in between braking. It takes about 12 seconds to discharge C2.

 The pcb measures 2 x 2.5 inch (5 x 6.4cm or 170 x 200 pixels)
at 2 colors and is shown smaller when you print these pages. If you need a direct, full size copy of the pcb I suggest to load the gif file into a program like Paint Shop Pro or one of the many
gif viewers available.

 The layout is enlarged a bit for a better component view.
Note that T1 is drawn soldered
on the pcb but if you have a metal case you can put it anywhere on the metal case
(as a coolrib) and use havy duty wiring between T1 and the PCB.


CORRECTION: SCR1's anode/kathode were shown reversed (fixed: 2-26-2000).

Frequency and Capacitance meter

From: kenny@...

An [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty cycle will be proportional to capacitance.

Circuit will look something like:

+5 ---+---------------+---+
| | |
R +----------+
|(see below) | 8 4 |
+---+------+-----|7 3|------/\/\/\---+------- Vout
: | | LMC555 | |
C to test +-----|6 | -----
: | | -----
ground +-----|2 5|----+ |
| | 1 | | ground
| +----------+ 0.1 uF
| | |
Clock ----+ ground ground
^
(see | next section - CMOS Oscillator)

The ON time for the monostable is about 1.1RC, so component values that should work would be a 50 Hz clock, say a 1 Hz low-pass filter on the output, and R = 9.09K, 1%. That combination will give an output of one volt per microfarad. Switch R in decades for smaller capacitors. Trim R for calibration.

NOTE: Error Fixes! D1 and SCR1's anode/cathode were shown reversed in the circuit diagram. Corrected all capacitor designators to right values. Updated the layout to reflect the changes. Will do the same for the pcb shortly!
PCB: Forgot to connect the cathodes of D4/D5 to point 'B' of the brake light. IC2 was also marked IC1; Should be IC2. Circuit diagram above is correct. Corrected writing mistakes in the Documents.
Layout Diagram: SCR1's anode/cathode were yet again drawn in reverse. All ok per 2-26-2000. Apologies for the errors!

Parts List

Semiconductors:
IC1,IC2 = 555 Timer, RS# 276-1723
SCR1 = NTE/ECG5402, RS# 276-1067, or equivalent
T1 = NTE/ECG197, SK3083, TIP125, or equivalent
D1,D2,D3 = 1N4148, 1N914, NTE/ECG519, RS# 276-1122
D4,D5 = 1N5400, NTE/ECG5850, RS# 276-1141, or equivalent
Resistors:
R1 = 18K
R2 = 330 ohm (RS# 271-1315)
R3 = 270K
R4 = 82K
R5,R6 = 1K2
R8 = 100 ohm (RS# 271-1311)
P1 = 50K, 10-turn
P2 = 10K, 10-turn
Capacitors:
C1 = 100µF/16V (RS# 272-1016)
C2 = 22µF/16V (RS# 272-1014)
C3 = 220µF/16V (RS# 272-1017)
C4 = 10µF/16V (RS# 272-1013)

T1 is a PNP Silicon Audio Power Out/Medium Power Switch Transistor, 7A, with a TO-220 case. As long as you have a transistor which is close it will work fine. The SCR is a 100vrm, 0.8A, sensitive gate with a TO-92 case. Diodes D1, D2 and D3 are standard small signal diodes. Power diodes D4 and D5 are the 6A, 50prv types, cathode case. The 60vrm type will work as well. I used for IC1 & IC2 the LM555 type. P1 controls the 'on' and pulse-duration, P2 controls the pulse-timing.