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Circuit to control a relay at a given voltage

Blog entry posted in 'Need Help', Jan 4, 2017.

0.00/5,
Need a basic circuit that will turn on and off a DC transistor relay but only when the secondary voltage reaches a set value and will turn off the relay when it drops below the voltage.


The circuit will be 12v (for a car)
I have 12 volts and neg coming directly from the battery.


12V --------------- Relay (Normally Open) Relay Out ------------- LED Light Pos.
Neg ------------------------------------------------------------------------ Led Light Neg

Relay Control has + and – input and will trigger the relay at .03 volts. The circuit can handle up to 42 volts DC.


I have a second 12v DC pulse generated source as the input to the circuit.
It averages about 6V on an analog meter. It is a 0 to 12 square wave.
I want to turn on the relay when the voltage average exceeds 7 volts and turn off when below set value Would be great if it could be adjusted.

If your circuit works, I will be happy to contribute some funds.
  1. Max Geiszler
    I'm trying to do the same thing. Thought it would be easy... Wanted to use a NO relay to turn off a fan when a diffuser with its own circuit switches off its own power due to low water. Im not very circuit savy.
    Max Geiszler, Jan 19, 2017 at 3:59 AM