looking for motor or servo tips

[Dbell5555]

i'm looking for a motor or servo that will turn a set number of revolutions and only that number of revolutions when i hit a button (with no programming), like something that is used to turn a car stereo flip display. it has to be able to go in reverse to flip back as well. can anybody tell me what kind of motor i should look into?

thanks much,
dave

[sanddrag]

You can use either a regular permanent magnet brushed motor with a shaft encoder or you can use a stepper motor. Stepper motors require a little bit more complicated control circuitry though.

[Kevin Sevcik]

I think your best bet would be to use a standard motor and some micro switches. It'd take a little careful wiring. The basic idea is that you have a switch to run the motor in one direction or another. Once you have this wired, you put a micro switch in the middle of one of the wires for one direction on the common and normally closed terminals. Then when the switch is hit, the motor stops turning in that direction. It will still turn in the opposite direction when the switch is flipped.

"But Kevin," you say, "I want it to make multiple revolutions before stopping. How can I do that with microswitches?" Well now. It's possible and done every day. The trick is to base your motion off a screw getting turned by the motor. Then put a nut on it or something that will get moved back and forth and have that trigger the microswitches. Adjust the stop positions by moving the micro switches forwards and backwards.

Now. This method does take a bit of mechanical finesse and possibly some machining. On the other hand, encoders and stepper motors need a control system watching them constantly.

[Gui Cavalcanti]

If it's a low enough voltage, you could also just put a rubber stop in and stall it. I have a lot of small <$20 RC cars that steer by stalling out against the steering stops.

[Mr. Lim]

I would suggest a servo, although I'm a LITTLE bit sketchy on how to do this without programming.

Does a servo react to a full 0 to 100% duty cycle range on the PWM signal input? If so, you could get the full range of motion on a servo just by hooking up an SPDT switch to the input line between +7.2V and GND.

Hopefully someone with a little more servo experience can shed some light.

-SlimBoJones...

[sanddrag]

I think a full or zero PWM signal is still a signal, not a constant voltage, nor would you ever average it out to a volatage. Pulses to the speed control need to be kept just that, pulses, in order for it to be a readable signal. But pulses to the motor can be equivalated (new word ) to a constant (average) voltage because that motor is going to turn no matter if it is getting a lot of higher voltage little bits every now and then or a lower voltatge constant flow. As long as the juice is being applied, the motor will turn. But the same cannot be said for the control module. It reads pulses, not constants, because it is digital, not mechanical.

If it really is just to flip out a car stereo screen or something, you may want to just use a little microcontroller like a Basic Stamp and a servo. If a servo gets the range of motion you need, I'd go for that. Nice and easy. You can have a switch directly input to the BS and then the BS directly gives a pulsing output to the servo. Easy and pretty cheap. But I do get the feeling that a servo is not quite what you are looking for. You want it to have a nice slow smooth action with quite a bit of degrees of rotation right? Is this going to be like the stereos with pop out screens that they put in all the Pimp My Ride cars?

[Dbell5555]

Yeah, the pimp my ride car stereo look is what I'm trying to achieve except instead of flipping a stereo it will flip out a nitrous control panel.

This has to run on a 12V system, will those little RC car motors run on that?

[billbo911]

Originally Posted by Dbell5555

This has to run on a 12V system, will those little RC car motors run on that?

Typical RC car motors run on 7.2 V. You can find them running on 8.4V and 9.6V, (notice the 1.2V increment, guess why). But they can be a bit more costly.
You can always build a fairly simple voltage regulator with a power resistor and a zener diode. This will limit the maximum voltage to the motor, but you will need to carefully calculate the power rating for the resistor. Simple circuit designs are available with just a Google search.

Good luck!!!