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From: tgm@psuecl.bitnet
Newsgroups: rec.models.rc,sci.electronics
Subject: Re: Electric motor speed control
Message-ID: <1991Jan18.183648.526@psuecl.bitnet>
Date: 18 Jan 91 22:36:48 GMT
References: <1991Jan4.051028.24807@ux1.cso.uiuc.edu>
Distribution: na
Organization: Engineering Computer Lab, Pennsylvania State University
Lines: 57

In article <1991Jan4.051028.24807@ux1.cso.uiuc.edu>,
forbes@aries.scs.uiuc.edu (Jeff Forbes) writes:
> Does anyone have the schematic for an electric motor speed control that
> will operate with a standard RC receiver? I don't care to spend $40++
> for $10 worth of parts, and I would like to build one.
>
> Jeff Forbes

I don't know much about standard RC receivers, but if you can get a voltage
from it, you could build a Pulse Width Modulation motor controller, dirt
cheap. What you need is a 556C low-power timer IC, a rugged NPN transistor
for switching the motor on/off, a couple misc components like 2N2222
transistors, caps, resistors...

The theory behind the circuit is to switch the motor full-on to full-off
repeatedly and very quickly. The amount of time the motor is on in
relation to the period of the on/off cycle determines how fast the motor
runs.

I don't have a schematic, but the idea is simple, and 555-type oscillator
and PWM shematics are in just about any manufacturer's application books.

Here it goes:                          ^    ^ Vhi
                                       |    |
                                       |    *----|
                                       |    |    |
                                       |   Motor _  Diode prevents spikes
          RC Receiver--------|         |    |    ^
                             V         |    *----|
          +----------+    +-----+     /     |
          |OSCILLATOR|--->| PWM |----|     /
          +----------+    +-----+     \---|  TIP-31
                                2N2222     \
                                            |
                                            V GND
1. Set up 1/2 of the 556C as an oscillator with a frequency of 100-1000 Hz.
   The higher the frequency, the less "jerky" the switching action will
   be. The output should be high almost all of the time, briefly dropping
   low once per cycle. This is necessary because the PWM's output is held
   high, and thus the motor on, when the oscillator is low--no matter what
   the PWM control voltage (RC receiver) is. So, even when your receiver
   says "off", the motor will be partially on, but only a teensy bit.

2. Set up the other 1/2 of the 556C as a Pulse Width Modulator, triggered
   off the osciallator's output. The control voltage for the PWM will come
   from your RC receiver. Here's the tricky part: Pick your timing resistor
   and capacitor such that when your receiver says "full tilt", the PWM's
   output's high time is equal to the oscillator's period.

3. Use a 2N2222 and TIP-31 (for up to 3 Amp capability) in the above setup
   to switch the motor on and off in accordance with the PWM's output. Don't
   forget a clamping diode across the motor's leads or else the spikes built
   up across the motor could fry your TIP-31.

That about does it. BTW, all the above parts can be found at Radio Shack.

--Tom Moertel
thor@chopin.psu.edu