I decided to build a simple water/methanol injection system for my car and now I'm thinking about a simple way to activate the water pump only from certain boost level. For the boost reading I'm planning to use map sensor. The map sensor has +12v supply, ground and output pin (5v max). The output voltage varies depending on the boost pressure.

What would be the easiest way to activate a relay which turns on the pump when certain voltage (or more) is received at the output from the map sensor? Ideally with a potentiometer for the boost level adjustment but that would be just a nice to have option.

  • \$\begingroup\$ A Schmitt-trigger comparator (with a significant hysteresis and pre-filtering) is the first thing that comes in mind \$\endgroup\$
    – Eugene Sh.
    Jul 22, 2019 at 15:05
  • \$\begingroup\$ An adjustable pressure switch maybe? \$\endgroup\$
    – Solar Mike
    Aug 19 at 20:23

1 Answer 1


Use a comparator. Connect the middle pin of your potentiometer to the negative input and the sensor output to the positive input. Connect the potentiometer between 5V and ground. The comparator should have a schmitt trigger input, to avoid toggeling of the relay.


simulate this circuit – Schematic created using CircuitLab

You will propably have to put a FET at the output of the comparator to drive the current of the relay. And don't forget to put a flyback diode over to relay to avoid destroying your FET.

  • 2
    \$\begingroup\$ Remember that many comparator devices have open-collector outputs, so they will require a pull-up resistor on the output. \$\endgroup\$ Jul 22, 2019 at 16:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.