compact direction-switching for pager motors.
October 27, 2010
I have a quesiton for the wiser people with more time than I do.
I am driving a small pager motor with a PWM signal from a RC+inverter system (similar to a "neurone" in a beam bot with the output forming its own input, or even a "solar theremin" curcuit).The signal is modulated via a resistor+LDR to change the frequency of the signal pulses and piped off to drive a tiny motor. If necessary, with a bit of amplification.
The situation is this - As the neuron controls the power to the motor according to ambient light levels. The motor pushes, or pulls a nut via a thread that is mounted directly on the motor spindle. When the nut gets to one end, I want the motor to change direction. the initial direction on startup doesn't matter.
What I don't want to have is a massive motor driver for the motor. I want to make an array of these - an intermediate objective is something like a 5x5 array, but probably just a 1x2 array will be fine to start with. Ideally each of the units can be up to 10mm square, but the smaller the better.
My question boils down to; given a pwm supply, what is the most compact way to reverse the direction of a motor, assuming that the signal to reverse is either a 0->1 or 1->0 pulse (either, or both, doesn't matter).
It would be most perfect to use an inverter or three (with perhaps other descrete components, but no other ICs) to accomplish this, because I'm already planning to use an inverter+cap+resistor+LDR to form the pwm supply.
Note that simply inverting the pwm output won't work, because the time between pulses is necessarily different to the pulse-width, so inverting this signal will result in a different motor speed. At least, that's what I think.
Thanks for your comments, it seems that my fantastic ideas were a bit too fantastic, so I haven't yet persued in reality the concept I had in mind, but some simulations with yenka showed that it requried a few too many additional components to make sense.
While the other remaining option seems to be that offered by Ignoblegnome - a few relays (and this might be the best way yet), however there actually IS an IC that does what I want, and can cope with 200mA of pulsed (50% duty cycle) input. This is the maximum rate and current that I expect from these pager motors: MAX4749 SPD dataswitch with three such SPDT switches on it, each can be controlled separately (or the MAX4748 takes a single control input for all three switches). As my input will be digital (pwm from an inverter-occilator), and the control too, this chip is ideal. I'll be needing 9 of them for my application.
Tragically, maxim are no longer taking sample requests from those with google email addresses... bugger.