Abstract: A liquid level transducer actuates a relay to sound an alarm when there is a change in a liquid level in a tank. A bridge circuit is balanced at an existing liquid level to maintain the relay normally deenergized. A change in the liquid level unbalances the bridge circuit causing the relay to be energized. The system may be applied to stationary storage tanks or to vehicle fuel tanks to warn of theft of fuel. In a vehicle application an unauthorized movement of the vehicle would also cause sufficient shifting of the fuel in the tank to sound the horn and give warning.