How much fuel in the tank! This question always concerned the driver of the car, and the motorcyclist and the pilot of the trike. And if the vehicle with varying degree of accuracy to this question can be answered, then the rider or the pilot of a light aircraft to make it much more difficult. Offer device DPJ alarm on the minimum remaining fuel in the tank based on reed switch contacts, operating on the circuit. As you know, under the influence of permanent magnet on the reed switch contacts are closed they are to be used to enable the audible signal and warning light.
The alarm sensor is a glass or plastic tube within which can move freely, the float attached to it a permanent magnet. Reed switch is installed on the outside of the tube. Tube is mounted directly in the fuel tank is either mounted outside, far from it. When the tank is full, the float is at the top of the tube and, accordingly, the contacts of the reed switch are open. As the development of fuel the float drops, and in a position corresponding to the minimum fuel level, influence of magnetic field triggers the reed switch contacts and the circuit is closed.