I need a simple electronic circuit that will help me accomplish what is depicted in the attached pic. This is for a more personal project but I know there is talent in this forum and I am no "wizard" with electronics.

Basically when the switch is off I want the motor to turn in one direction indefinitely until and if it hits a stop at which point its Voltage would drop and its current would go sky high and then it shuts off (I'm guessing a current limiting or voltage drop cut off can be used)

When the switch is placed in the on position(12V) i want the motor to run in the opposite direction (reverse polarity) until and if it hits a stop at which point it shuts off again.

Any help is greatly appreciated.

Best regards,

Chris

 

How much current will the motor draw when spinning indefinitely, and how much current will it draw when it hits a stop? Is the stop going to be sturdy enough that the motor can bang against it without damaging it? This is just a plain old DC motor that can be connected to 12V, right?


I can't promise that I'll be able to help (already behind on a few of my own projects including my car), but those are a couple of questions you'll probably need to answer regardless of who helps you out. Do you need to make a lot of these, or is it pretty much a one-off thing?

 

The stop is sturdy enough, the motor is small by comparison. I don't know how much current it will draw while turning but I can assure you that it is a very load free operation....probably close to free spin. since not only is the mechanism very loose and simple(it opens or closes a valve but there is no force on the valve pushing against its motion), there is also a huge ratio transmission that slows the operation. When it stops it will draw a considerably larger amount.....is there a way to set this up with a potentiometer so i can adjust how much it takes for it to cut off? If I could get my hands on a circuit diagram and a formula I would probably be able to calculate what components to use for this particular motor's current draw etc.

Yes the motor is a small DC 12V motor...I am going to go out on a limb and say it is a 180-280 sized motor.

I am looking for just one for my car so yes a one off....at least for now.

Thanks a million for the reply.

Chris

 

There are plenty of ways to skin a cat, but here's how I would do it because this is what I'm accustomed to:

Use four transistors in an H-bridge configuration, that's a pretty standard circuit for DC motor control. Two transistors (Q1 and Q3) can provide power to the motor (one on each side of the motor), and two others (Q2 and Q4) can provide ground to it (again one on each side). If Q1 and Q2 are on (Q3,Q4 off) the motor will spin clockwise.

Personally I'd use a microcontroller to measure current, measure the switch input, and turn the four transistors on and off to control the motor direction. Measuring current is relatively easy, use a low-value shunt resistor (something like 0.01-1.0 ohms depending on the current the motor draws) between the Q2 and Q4 transistors and ground so that all. The voltage drop across the shunt resistor will be pretty small, you will most likely need an amplifier to increase the voltage (current measurement) that the microcontroller sees. You could use a potentiometer to tell the microcontroller what current level stops the motor. Only problem is you've got to know how to write code and you've got to have the tools to program the controller.

It's probably possible to get things done without a microcontroller using logic gates but that's a bit more old-school (and more difficult IMHO)... it would probably take me longer to come up with a circuit that works.

 

I'm glad you ask this...I want to know some things about this too...