simple pulse circuit?

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.

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.

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 =^.^=

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

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

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

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

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

A small PIC