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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.

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  • \$\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

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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.

schematic

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.

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

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