Feb 152021

Fuel pump cut-off circuit for theft prevention

A lot of times thieves steal a car even though it has a burglar alarm installed. This fuel pump cut-off circuit for the car prevents the activation of the pump by removing the supply voltage (12 volts) from it. The car will be able to run for a while longer, but then it will stop due to lack of fuel.

The purpose of this alarm project circuit is to increase the security level of a car, which already has an alarm installed.

The fuel pump is disabled when the car’s engine is off. To enable the pump, a normally open (NO) button is pressed at the same time as the car ignition key is activated. The button locks the relay that activates the fuel pump and keeps it that way (operating normally) as long as the car’s engine is not turned off.

This button should be conveniently located in a place inside the car known only to the driver.

Once the car’s engine has been turned off, the circuit and the fuel pump are disabled, waiting for a new start.

How the fuel pump cut-off circuit works?

When the car’s ignition switch (SW2) is activated, the first SPST relay (RL1) is supplied with 12 volts. Relay RL1 in turn activates relay RL2 as long as the momentary contact button SW1 is pressed.

Fuel Pump Cut-off Circuit for theft prevention

If these two conditions are met, a voltage of 12 V is applied directly to the fuel pump and a self-interlock is simultaneously performed.

This self-interlock keeps the relay that supplies the 12 V supply to the fuel pump active and remains so, even if the SW1 button has been released.

When the car engine is turned off, the power supply is removed from the circuit and the gasoline pump, leaving everything as at the beginning for a new start of the engine.

Relay (Electromagnetic Switch)

SPST Relay – Single Pole Single Throw Relay

If the fuel pump is activated by a car’s own relay, the output of this circuit must be connected to the car’s relay activation input. If not, the relay in this circuit can directly supply the fuel pump.

Circuit component list

  • 1 12 volt relay, single pole single throw (SPST), automotive type (RL1)
  • 1 12 volt relay, single pole two throws (DPST), automotive type (RL2)
  • 1 momentary contact switch, normally open (NO) (SW1)

As you can see, this circuit has no electronic components.

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