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.

 

Welcome to AAC!

Use a comparator with a zener diode and your pot to set the turn on voltage. You'll need a transistor to switch the relay, a snubber diode, and few passive components.

 

Welcome to AAC!

Use a comparator with a zener diode and your pot to set the turn on voltage. You'll need a transistor to switch the relay, a snubber diode, and few passive components.

However, you may need to add hysteresis to your circuit. Once the target voltage is reached, it’s value may oscillate around the target. Resulting in your relay “chattering”.

A better solution would use two comparators and a SR flipflop before the relay. One comparator would turn on the relay; the second would be set for a slightly lower voltage and used to turn off the relay.

 

Do you know the lowest MAP sensor output voltage you will want to detect to turn on the relay?

 

However, you may need to add hysteresis to your circuit. Once the target voltage is reached, it’s value may oscillate around the target. Resulting in your relay “chattering”.

A better solution would use two comparators and a SR flipflop before the relay. One comparator would turn on the relay; the second would be set for a slightly lower voltage and used to turn off the relay.

Hysteresis can be added to the comparator with a couple resistors.