Stop, wait, go circuit

In all likelyhood my title has probably already informed you that I know little about electronics. What I need is a circuit that when 12v power is applied it will drive a motor until a sensor is tripped, wait a preset amount of time, then drive the motor again. The cycle would then repeat until power is disconnected. The time it waits should be adjustable and will need to be in milliseconds. I would probably use an IR LED and photocell as the sensor.

What I want to do is turn a wheel and have it stop at 20 positions along the wheel as it rotates. The IR LED would be on one side and the photocell on the other with holes drilled through at the point I want it to stop. When the photocell receives light the motor should pause for the preset time then resume. The other option was a microswitch and notches along the outside of the wheel to trigger it. This wheel will probably be about 6" in diameter and turn at up to 300RPM (including the pauses).

I have a feeling this is probably a fairly simple circuit I just don't know how to go about it. Any help would be greatly appreciated.

Tim

What type of motor is it? I don't think a normal DC motor will stop fast enough and stop at the desired position. I think you will need a stepper motor. Did you make any tests?

How much of a load is on the motor becides The disc? Do you have the motor? Just a thought : Motor runs continusouly driving disc & load thru a slip clutch; detected stop drives magnrtic brake for given delay. Not much time to play with. Look for 18 degree step motor,easier to control.
















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There will be some friction on the wheel plus I'm planning to incorporate a ball detent or similar idea to stop and hold the wheel at the proper position. If you can design me a circuit that will do what I need I should be able to figure out the rest. There may be a fair bit of load on the motor, so it should be able to handle that.

Thanks

Here is a monostable circuit with 555 which can do the work. Once you press the trigger switch it outputs high for a preset time and then it goes low. Put a transistor on its output (indicated by the device box on the diagram) and drive a relay. Connect the motor to the supply and to the normally closed contact of the relay. The motor will run until you trigger the 555 (with the sensor) and the relay switches to the normally open contact. Thus the normally closed contact will become open circuited and the motor will stop.

http://www.eleinmec.com/article.asp?4

Okay I read over the description of that circuit and think I get the hang of it, but in my situation the trigger pulse will continue to be present until the timer stops and restarts the motor. Will this cause an issue? I may be able to set up the sensor to trigger slightly before the stop point and allow the wheel's momentum to carry it past the sensor; should I do that?

If you can do it because if the sensor triggers the timer all the time, the timer will never start the motor again.

Okay thank you. I will put this circuit together so and make try it out. Thanks for your help.