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Re: generating low current 35% duty cycle 140Hz square wave pulse

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  • Steve
    Low current is relative. A TTL 555 timer only draws about 1mA. A problem you may run into using a 7555 is that the CMOS version does not provide as much drive
    Message 1 of 5 , Jul 6, 2003
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      Low current is relative. A TTL 555 timer only draws about 1mA.

      A problem you may run into using a 7555 is that the CMOS version does
      not provide as much drive current, source and sink, as the TTL
      version. And power transistors require quite a bit of drive.

      Yes, I was talking about using a transistor as an inverter, that
      serves the same function. Pull the base of an NPN high (emitter
      connected to ground) and the collector goes low.

      Why would you use PNP power transistors? If the solenoids are pulled
      to ground to trigger them, it is easier to use NPN in common emitter
      configuration.

      Yes, a 555 (or 7555) will go low enough to turn of the transistor.

      Study how a 555 timer works in astable mode (ie, oscillating). Ton =
      .7x(R1 + R2)xC and Toff = .7xR2xC so Ton can never be less than Toff,
      therefore the dutycycle Ton/(Ton+Toff) will always be greater than
      50%. Steering diodes: Put a diode across R2, with cathode connected to
      the low side and anode connected to the high side. Now for Ton,
      current flows through R1 and D1, bypassing R2. (simplistically, as
      that .7V does affect things) So now (roughly) Ton = .7xR1xC1 and Toff
      = .7xR2xC1. Therefore since Ton and Toff can be any arbitrary time
      related only by C1, you can adjust to virtually any duty cycle.

      Steve Greenfield


      --- In Electronics_101@yahoogroups.com, Eliah Kagan
      <bewarethefro@y...> wrote:
      > I have virtually no electronics knowledge, so please
      > forgive me if I make any mistakes here that border on
      > the insufferable. (-;
      >
      > > Do you mind if I ask what this is for?
      >
      > It's for operating the hydraulic system on my friend's
      > '92 Dodge transmission. We're replacing the existing
      > system with a manual-like system (it's operated that
      > way--it doesn't work the same way). The four clutches
      > (UD, OD, 2/4, & L/R) are operated by a quad solenoid
      > pack that opens the four hudraulic valves when 12V ~1A
      > goes through them. The system operates at 140Hz (from
      > the info I have so far, it looks like this shouldn't
      > vary too much) and a 35% duty cycle (this can vary a
      > bit). The pulses for the four solenoids are produced
      > by opening and closing their ground connections.
      >
      > > bewarethefro, you do realize that a 7555 is just the
      > > CMOS version of
      > > the 555? So except for special cases (like extremes
      > > of voltage or
      > > component values) you can consider them the same.
      >
      > I had thought that the 7555 was the only low-current
      > one and that the all the 555's (non CMOS) were high.
      > Since we're using this for switching, we figured we
      > would wire each of the solenoids' ground connections
      > into the collector and out the emitter of an NPN
      > transistor, and source the base with a low-current
      > square-wave from pin 3 on the 7555 (actually from pin
      > 5 or 9 on the 7556, since we'd use 2 of those instead
      > of 4 7555's).
      >
      > I didn't know a 555 IC will only go as low as 50% duty
      > cycle when figuring this out. Could we just generate a
      > 65% duty cycle and sink the base of a PNP instead of
      > sourcing the base of an NPN, so that it wouldn't be
      > necessary to add an inverter? (Or is that what you
      > were saying when you said that a common emitter
      > switching transistor circuit would do that?)
      >
      > Are there other low-current 555's besides the 7555?
      > It's preferable for it to run at a lower current
      > rather than a higher one, as while it doesn't have to
      > operate when the car is off (there's a built-in
      > secondary mechanical system that makes it work right
      > when you put it in neutral and push it), there's going
      > to be a lot of other stuff drawing current from the
      > alternator.
      >
      > It seems to me (not knowing anything, of course), that
      > ideally it would be better *not* to use a CMOS timer,
      > since I read that if there's hardly any discrepancy
      > between the magnitdes of the (+) and (-) connections
      > with a CMOS circuit then the whole thing can stop
      > working. Is this the case?
      >
      > Also, is the low produced by the 555/7555 lower than
      > the threshold of the transistor we would use (low
      > current switching 12V 1A), or would we need to find a
      > diode with a lower threshold and put it in between the
      > PNP's base and pin 3 on the timer?
      >
      > > [...] but if you use steering diodes you
      > > can get a duty
      > > cycle less than 50%
      >
      > What is a steering diode?
      >
      > --- Steve <alienrelics@y...> wrote:
      > > Or one 555. The duty cycle of a simple RRC astable
      > > 555 is always
      > > greater than 50%, but if you use steering diodes you
      > > can get a duty
      > > cycle less than 50%.
      > >
      > > Or design for a duty cycle of 65% and use an
      > > inverter after the 555,
      > > and you'll have 35%. A common emitter switching
      > > transistor circuit
      > > will do just that.
      > >
      > > bewarethefro, you do realize that a 7555 is just the
      > > CMOS version of
      > > the 555? So except for special cases (like extremes
      > > of voltage or
      > > component values) you can consider them the same.
      > >
      > > Do you mind if I ask what this is for?
      > >
      > > Steve
      > >
      > > --- In Electronics_101@yahoogroups.com, "rtstofer"
      > > <rstofer@p...> wrote:
      > > >
      > > > You could use two timers: one as an oscillator at
      > > 140 Hz and one as a
      > > > monostable multivibrator (one-shot) set at 2.5 mS
      > > On-Time.
      > > >
      > > > See
      > > http://www.uoguelph.ca/~antoon/gadgets/555/555.html
      > > > for a nice tutorial.
      > > >
      > > >
      > > > --- In Electronics_101@yahoogroups.com,
      > > "bewarethefro"
      > > > <bewarethefro@y...> wrote:
      > > > > I need to generate a low current 35% duty cycle
      > > 140Hz square wave
      > > > > pulse. (It will be amplified with a switching
      > > transistor and I can
      > > > > take it down to a lower level with a resistor,
      > > so I don't have exact
      > > > > requirements for voltage and current.)
      > > > >
      > > > > 1.) Will a 7555 timer/oscillator IC do this, or
      > > is the frequency
      > > > too low?
      > > > >
      > > > > 2.) Generally, how can I do this?
      > > > >
      > > > > Thanks!
      > > > >
      > > > > -Eliah
      > >
      > >
      >
      >
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