Electronic Fuel Level Warning System

If you fly a regular training program with your glider, you’ll soon develop a feel for landing on time even without a watch. If the engine happens to cut out, it’s usually no big deal: with a little luck, you’ll even be able to bring your fast delta glider back safely to the runway.

For now, the situation is quite different with helicopters. An engine failure is usually associated with a serious crash. I say “for now” because, as of yet, there is no proven system that allows for safe autorotation landings.

Let’s say, over the course of your career as a helicopter pilot, you wear out three sets of rotor blades because you’re flying overtime. Then you’ve gotten off easy—just imagine a total loss! Still—you’ve spent so much more time and money than would have been necessary for a reliable fuel warning system. Based on these considerations, I came to the conclusion that a fuel warning system is an indispensable piece of equipment, especially for beginners in helicopter flying.

So the decision has been made: We’re going to build an electronic system that uses a flashing light to indicate when the fuel level has dropped below a certain minimum. This circuit is so simple that it can be built without a measuring instrument. If you wire the specified components correctly according to the schematic, the flashing system is guaranteed to work.

Inputs 1 and 2 are connected to the fuel tank sensor, which changes its resistance depending on the fuel level.

When both electrodes are submerged in the fuel mixture, the resistance is approximately 20 ks. If the fuel level drops below level A, the system goes into idle mode. When the probe is submerged, a tiny current flows through the electrodes; this current is amplified by transistor T1 and triggers the astable multivibrator via T2. The multivibrator (composed of T 3, T 4, C 1, C 2, and R 6–R 9) controls the indicator lights via the two medium-power transistors T 5 and T 6.

This electronics system has proven reliable so far, with no failures during many hours of flight. I use a 4.5 V dry cell battery to power it. Since the circuit draws only a few mA of current when the tank is full, such a battery lasts for many hours.

In addition to the listed components, you’ll only need a soldering iron and a piece of Veroboard (a hardboard with printed circuit traces). You can purchase the components at an electronics store (e.g., Pusterla in Zurich) for about Fr. 12.—.

It’s best to start the assembly with the probe inputs and then wire the rest step by step to the right, following the schematic.

The only critical component in the entire setup is the indicator lights. It is essential that they be mounted on a soft foam rubber base; otherwise, the filaments will break due to vibrations after only a short time. Install at least two lights—this provides double the reliability and also looks good. If you want to use more than two bulbs, you can connect them in parallel with the others without any concerns. (See the diagram -----).

And now, enjoy your helicopter flying!
J. Steinmann

The author will be happy to provide further information.

Jürg Steinmann, Tel. 01/26 81 17
Bucheggstr. 24, 8037 Zurich