# How to make circuit to reverse dc motor direction with a 12v+ trigger wire

I have a motorized retracting front license plate on my car that works great via a double pole momentary switch currently. However I really would like to make it automatically work when the car starts.

It is very hard to modify the device itself due to the way it is installed so I am looking to build a circuit that would operate it based off a 12v + trigger that will be on when the car starts.

The device that retracts the plate is very simple with 2 wires (red and white) to push the plate out you connect positive to red and negative to white. To retract it you put positive to white and negative to red. There is no stop switch on the device so it takes about 10 seconds to fully extend or retract.

So the circuit would need to provide power in one config for about 10 seconds when it senses 12v + and the provide power in the opposite configuration when the 12v + ends on the trigger.

I have some basic electronic knowledge and can solder components to a board but creating this circuit is beyond me. I would greatly appreciate some guidance to get this built.

Thank you.

• "There is no stop switch on the device so it takes about 10 seconds to fully extend or retract." - What does this mean exactly? Commented Jun 8, 2020 at 4:52
• Are these legal for cars used on the road? Commented Jun 8, 2020 at 6:14
• Bruce - So there is no switch on the device that cuts the power to the motor when the plate is fully retracted or extended. So in an ideal world I would need to have the power cut to the motor after about 10 seconds when the car starts or stops. Commented Jun 8, 2020 at 14:56
• Mike - They are legal for front plates in most states. Many people like me that frequently bring their cars to shows have a systems like this to hide their front plates as a precaution. In the case of shows most of us remove our back plates also. Commented Jun 8, 2020 at 14:58

If i understood you correctly, you need something like this:

simulate this circuit – Schematic created using CircuitLab

When you have power on "Direction" relay, motor will rotate in one direction. When you don't - it will rotate in opposite direction. "Power" relay works as stop switch.

• Except the power relay would always be on. Commented Jun 8, 2020 at 7:32
• Thank you Morris so the key is a DPDT relay right? That makes sense. I forgot those exist because in the car world most are single pole. Any ideas on how to make the voltage cut after a specific amount of time? Commented Jun 8, 2020 at 15:00
• Look at this question: electronics.stackexchange.com/questions/502381/… Commented Jun 9, 2020 at 5:21

Peter Walker's and AnalogKid's solution to the question, Need a circuit to detect if something has gone from off-on or on-off, is also valid for this question.

The timing diagram is nearly identical.

AnalogKid's timer circuit 'Motor Trigger' is to be used with the value of C3 increased to 100μF for a motor 'on' time of 10s.

The trigger voltage (+12V - engine on, 0V - engine off) is to be tapped from the D+ terminal of the vehicle alternator.

The timer relay 'Motor' is to run the motor for 10s with its direction determined by the trigger relay K1.