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simple pulse circuit?

Discussion in 'Electronic Design' started by [email protected], Oct 23, 2007.

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  1. Guest

    Hello,

    I would like to provide a 3V pulse, for about 1/2 second to 1 second,
    then rest for, oh, 30 seconds to two minutes, and then repeat the
    process.

    Any ideas for the simplest circuit that could do this? (I'm certain
    this is covered in basic EE courses, but alas, I am not an EE.)

    Why I want to do this:

    I have a car windshield pump motor from Kragen, designed to squirt
    water at 12V. I've heard it cannot do this for very long at sustained
    power, and will burn out. I would like to modify it for use in a home
    science project (continuous ethanol process), basically providing
    around 1 mL per minute (or less).

    I'm thinking of powering it at 3V to 6V, to reduce the heat (yes the
    pump gets hot!). Short bursts are ok for my science project.
    Accuracy would be great, but +/- 25% should be enough.

    I could do the microcontroller thing, but I'd rather not. I could do
    the parallel port control thing, but then I would need to get a
    computer with a parallel port running, and that is a lot of effort
    (and I no longer have a laptop with a parallel port). I do have a 555
    PWM circuit in pieces somewhere, but if something even simpler than
    that can do, that would be great. (I've seen my dad run a tiny
    speaker from just a coil, a battery, and a transistor, so it can't be
    THAT tough.)

    Thanks,

    Michael
     
  2. This is exactly the kind of timing jog the LM555 was
    invented to do. The low power CMOS version (the LMC555)
    will run on 3 to 6 volts.
    http://www.fairchildsemi.com/ds/LM/LM555.pdf
    http://cache.national.com/ds/LM/LMC555.pdf
    http://www.uoguelph.ca/~antoon/gadgets/555/555.html

    You can use a logic level (low gate voltage) power MOSFET to
    accept the timer output pulse and switch the motor supply.
    Don't forget to put a diode across the motor to prevent a
    high voltage pulse each time the MOSFET turns off.
     
  3. colin

    colin Guest

    you can use the serial port instead,
    you can drive the rts/cts line even from msdos,
    and use a delay loop.

    im not sure the pump would atualy work at such reduced voltage,
    its probably best to test it with an adjustable power supply
    if you have one and see what voltage it works at without getting hot.

    or even use a variable power resistor, or collection of fixed power
    resistors.

    another idea is to have a flow rate switch/pressure switch/level switch
    to turn the motor on or off and a resorvoir tank.

    you could even have a thermal/temperature switch on the motor, ....

    Colin =^.^=
     
  4. Guest


    My new laptop doesn't even have a serial port, if you can believe it.
    And there's something a bit ridiculous about running an old AT
    computer, at about 200W, continuously for several days, to power a
    little pump at 1 mL/min. (haha)

    I've got a low power 555; I'll use that along with a TIP31A. I'll
    probably use NiMH or NiCd batteries first, measure the current, then
    use a low voltage wall wart.

    Thanks guys,

    Michael
     
  5. Guest


    Tried it, and control wasn't as precise as I'd hoped. I'd set the pot
    to a good drip, then suddenly the pump would turn on full blast, ...

    I'm running at 5.5V, using a low power 555 (will get precise #s
    tomorrow), with an IRF530. Maybe the gate isn't fully saturated?
    Using a TIP31A, the transistor got really hot really quickly. The IRF
    didn't get so hot.

    I'm starting to wonder if I should just run a recycle loop with a fish
    tank motor to handle 99.9% of the flow, then put a needle valve in
    there to get 0.1% of the flow out... crude, but no electronics
    required.

    Michael
     
  6. Rich Grise

    Rich Grise Guest

    The pump needs to be completely submerged in the water; that's its
    coolant. Without it, the pump will burn, as you've noticed. Then, since
    it's a DC motor, you might try PWM (like with the 555) go control
    the speed - when you turn down the speed with PWM, you turn down the
    power dissipation accordingly. Also, with PWM, you should run it at
    the full 12V - PMDC motors lose torque terribly fast if you just
    regulate the voltage.

    A 555 and power MOSFET circuit should be almost trivial. (a w.washer
    motor probably takes a couple of amps!)
    There are other circuits that will do the job, but I don't think any
    of them is as simple as a 555 that you already have on hand. ;-)

    Cheers!
    Rich
     
  7. Guest


    Ah... and here I thought you s.e.d magicians would be able to do it
    with just a cap and a transistor... ;-)

    Thanks for the reminder re: torque vs. voltage.

    Michael
     
  8. Rich Grise

    Rich Grise Guest

    Well, there's magic, and there's magic. :) Can you use a knob for speed
    control? I once slapped together an astable with a pot for both base
    resistors - 100K pot with 1K at either end, those to the bases, and
    the pot wiper to Vcc. I got an amazing variation in duty cycle. I
    couldn't use a 555 (I'd already tried once, and it died), because
    the circuit is a PWM motor controller for a spool gun. This is a
    thing used in MIG or FCAW welding, where the handpiece has a small
    spool of weld wire. And the weld current goes right alongside the
    motor controller board.

    My new "design" worked, and survived, and the guy who was promoting the
    whole thing turned out to be a scam artist. I got paid, but not enough. >:-[
    Always a pleasure when something I share turns out to be useful. :)

    Thanks,
    Rich
     
  9. Guest

    A small PIC
     
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