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Re: [Electronics_101] Re: Two LED Flasher Circuit

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  • Jong Kung
    Tony, As Steve noted, you have to be careful that your LOAD (the actual circuit outside of 555 config) is not too big. With this configuration, your 555 output
    Message 1 of 17 , Jun 18, 2013
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      Tony,


      As Steve noted, you have to be careful that your LOAD (the actual circuit outside of 555 config) is not too big. With this configuration, your 555 output (pin 3) is doing double duty (no pun intended). The pin 3 (output) is being used to charge and discharge the timing capacitor AND to drive the LED (the load). There's only limited current in / out of the pin 3. If the load is too big, there's nothing left to drive the timing capacitor.

      If you ask me this is the fun part of analog electronics - finding new ways to wire up the same old chips and maybe even do useful things with it.

      =====

      Are you doing a project or just playing and learning?


      Jong


      On Jun 18, 2013, at 6:23 PM, "Tony" <tbmoab@...> wrote:

      > Hi Steve: Many thanks. So something like a 470 ohm resistor from pin 3 back to pins 2 and 6 ?
      >
      > --- In Electronics_101@yahoogroups.com, "AlienRelics" <alienrelics@...> wrote:
      >>
      >> I think you mean that they are not on for equal lengths of time, IE, the duty cycle is not 50%. It is impossible for them to blink at different rates.
      >>
      >> The problem is that electrolytic capacitors are very wide tolerance, and vary with time, temperature, and voltage. And in a two transistor multivibrator aka astable, each period is controlled by a different capacitor. With a 555 or similar circuit, one capacitor is used for both time periods.
      >>
      >> There are a variety of ways to get variable duty cycle using a 555 timer. If you only mean to get 50% (equal on times for each LED), then simply putting a resistor from pin 3 back to pins 2 and 6 and then the timing capacitor to the ground will get you a 50% duty cycle that does not need adjustment and won't vary appreciably with changes in supply voltage.
      >>
      >> However, you must be careful then not to load the output too much. A couple of LEDs with 10mA flowing through each should be fine with most varieties of 555 timer.
      >>
      >> If you wish to have an adjustable duty cycle, you need what are called steering diodes. The simplest of those, however, alters the frequency when changing the duty cycle. Designed correctly, you can get the frequency to stay fairly constant.
      >>
      >> To get a more accurately fixed frequency with the 555, it requires two 55 timers or a dual 556 timer. One functions as a clock, the other as a variable monostable. But if you are going that far, might as well program an 8 pin microcontroller.
      >>
      >> Steve Greenfield AE7HD
      >>
      >> Steve Greenfield AE7HD
      >>
      >>
      >> --- In Electronics_101@yahoogroups.com, "Tony" <tbverse@> wrote:
      >>>
      >>> I've tried the Basic LED flasher circuit using NE555 timer IC at http://wild-bohemian.com/electronics/flasher.html but the two LEDs do not flash at quite the same rate and I cannot quite see what is wrong with the circuit.
      >>>
      >>> Any other 555-based circuits out there and also where I can adjust the flash rate of EACH LED in a two-LED circuit?
      >>>
      >>> Thanks.
      >>>
      >>
      >
      >
      >
      >
      > ------------------------------------
      >
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    • AlienRelics
      Yes, but much higher resistance than that. How fast do you want it to flash? Better to use higher resistance values so you can use lower value capacitors, as
      Message 2 of 17 , Jun 19, 2013
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        Yes, but much higher resistance than that. How fast do you want it to flash? Better to use higher resistance values so you can use lower value capacitors, as the larger the capacitor, the worse the leakage and tolerance (in general).

        Timing for a 555 wired up in that way (assuming not too much loading from the LEDs) would be about:

        Ton = Toff = 0.7RC

        Ttotal = 1.4RC

        freq = 1/(1.4RC)

        Pin 7 is not connected. The resistor from pin 3 back to 2 and 6 is the only timing resistor, then the timing capacitor is connected from pins 2 and 6 to ground.

        So a 680k resistor with a 1uF capacitor will give you about 1 second total time, or about half a second length flashes from each LED. That is about the maximum resistance for a TTL 555 timer, but with a CMOS version such as the TLC555, you could use 10M. For a 1uF capacitor, I'd use a tantalum rather than aluminum electrolytic as tantalums have a lot less leakage and better tolerance. Don't try using one of those high value ceramic capacitors, their capacitance changes drastically with voltage.

        Steve Greenfield AE7HD

        --- In Electronics_101@yahoogroups.com, "Tony" <tbmoab@...> wrote:
        >
        > Hi Steve: Many thanks. So something like a 470 ohm resistor from pin 3 back to pins 2 and 6 ?
        >
        > --- In Electronics_101@yahoogroups.com, "AlienRelics" <alienrelics@> wrote:
        > >
        > > I think you mean that they are not on for equal lengths of time, IE, the duty cycle is not 50%. It is impossible for them to blink at different rates.
        > >
        > > The problem is that electrolytic capacitors are very wide tolerance, and vary with time, temperature, and voltage. And in a two transistor multivibrator aka astable, each period is controlled by a different capacitor. With a 555 or similar circuit, one capacitor is used for both time periods.
        > >
        > > There are a variety of ways to get variable duty cycle using a 555 timer. If you only mean to get 50% (equal on times for each LED), then simply putting a resistor from pin 3 back to pins 2 and 6 and then the timing capacitor to the ground will get you a 50% duty cycle that does not need adjustment and won't vary appreciably with changes in supply voltage.
        > >
        > > However, you must be careful then not to load the output too much. A couple of LEDs with 10mA flowing through each should be fine with most varieties of 555 timer.
        > >
        > > If you wish to have an adjustable duty cycle, you need what are called steering diodes. The simplest of those, however, alters the frequency when changing the duty cycle. Designed correctly, you can get the frequency to stay fairly constant.
        > >
        > > To get a more accurately fixed frequency with the 555, it requires two 55 timers or a dual 556 timer. One functions as a clock, the other as a variable monostable. But if you are going that far, might as well program an 8 pin microcontroller.
        > >
        > > Steve Greenfield AE7HD
        > >
        > > Steve Greenfield AE7HD
        > >
        > >
        > > --- In Electronics_101@yahoogroups.com, "Tony" <tbverse@> wrote:
        > > >
        > > > I've tried the Basic LED flasher circuit using NE555 timer IC at http://wild-bohemian.com/electronics/flasher.html but the two LEDs do not flash at quite the same rate and I cannot quite see what is wrong with the circuit.
        > > >
        > > > Any other 555-based circuits out there and also where I can adjust the flash rate of EACH LED in a two-LED circuit?
        > > >
        > > > Thanks.
        > > >
        > >
        >
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