The motor is a volume control for a stereo, when you hit volume up on the remote control, the volume knob physically turns, increasing the volume. I want to control that from other parts of the house.
The 12 volts I have is actually from a regulated power supply, I probably should have said that but I was hoping it would be simpler, so wasting power is not a problem.
I thought this would be a simple problem. Who knew I would get this many responses!!
Well on this note Im gonna have to say use a PWM full bridge inverter.
It is a DC machine and thus the direction of rotation is defined by the polarity of the voltage applied. A straight resistor (if it worked), zener, reg would only provide positive volts thus only one direction of rotation.
Use two P-N type inverter legs to make a full bridge thus making easy control. You can get 15V logic (I think it is the 4000 series) so no need to get a 12v-5V reg to power logic. all you need then is some logic to say what gate on teh inverter to fire to PWM the right polarity onto the DC machine. This way bi-directonal and you improve efficiency
I have drawn the cct that would be needed but I havn't got anywhere to host it. It is very simple and you dont even need a 555 timer. I am using an LM311 comparator as a PWM generator and by running the limited logic (one inverter and two AMD gates) also off 12V
2x N-type MOSFET - these will have anti-parallel diode within
2x P-type MOSFET - these will have anti-parallel diode within
1x LM311
1x Inverter package
1x AND package
resistors and caps as required.
possible PNP,NPN as push-pull on the output of the AND gates that feed teh FETS if they cannot source enough.
Should be enough source/sink since only low amp FETS. You should be able to set the switching freq pretty high (high 10s of kHz) so the ripple is low and have a duty of 33% for an average of 4V. These are all set with the resistors/cap around teh LM311
Also since you say a 4V motor, 4V will probably have quite a high RPM for its size and also for your application. By changing the comparator feedback threshholds you can drop the duty => drop the machine volts thus drop the speed.
If anyone can host I have a bmp of the cct