Low Fuel Light

Written and conceived by Jeff Anderson

Please note the following warning:
"In case of gross construction error it is possible to blow out the fuel tank's sending unit real quick if the tank is full, and less so if the tank is at the 1/8 tank indicated level that's needed for setting the turn-on threshold. Other gross errors could be more likely protected by fuses in the car."

Five years ago I designed and added a low-fuel warning light to my 1991 Miata.

I find that the warning light is useful, in addition to the analog gauge.  I never have to worry about my fuel level until
the light comes on.  But then perhaps I just like warning lights that are helpful, e.g., the brake lights on the car in front of me when they put on their brakes.  I could stay attentive to the closing distance just as I could stay attentive to a reduced fuel gauge level.  Both of these warning lights clearly notifies me of when to be particularly alert to the situation at hand.

The circuit I designed, and use, is shown below:
(Editors note - the RED line doesn't connect the 3.3Kohm resistor or the small PNP transistor to the 470k or 10k resistors, it goes OVER that connection!)

It's a GIF, not ASCII!

D1, D2 and D3 are diodes.
Most any generic Silicon type will do just fine, i.e. 1N4001, or similar.


After dropping the steering wheel, I took the whole gauge shroud off and apart.  Then I scraped off the black paint covering one of the unused warning light openings and inserted a transparency of a gas pump image with a red background color.  Thus, when my fuel is low, I see a picture of a gas pump and a red light in a formerly unused warning light circle.  The printed circuit board behind the unused warning light openings are designed to accept a light socket. I bought the warning dash lamp (the brighter type) and socket from a Miata dealer.  If I recall correctly, the +12V connection for the lamp was available, designed into the circuit board and the +12V connection was made simply by inserting the lamp socket into the circuit board.

In the first few circuits I tried the car's unstable voltage regulation caused the circuits to perform somewhat in unison with the speed of the engine and the state of the car's battery.  Regulating the car's variable voltage to the gauge and lamp circuit resulted in a full tank of gas not showing full on the gauge.  The circuit shown above solves that problem because it is a four leg bridge circuit.  Leg 1A is the gauge and leg 1B is D1 + D2 + 1.5K resistor.  Leg 2A is the fuel level sending unit and leg 2B is the adjustable 2K resistor.  D1 and D2 compensate for the two transistor's, and D3's, curves near their cut-off threshold and results in an extremely stable bridge balance that is independent of supply voltage variance.  D3 assures high temperature cut-off.  The 470K resistor provides a one or two gallon hysteresis, i.e., once the lamp turns on, it tends to stay on, and once it's off, it tends to stay off.  This stabilizes the circuit so that splashing fuel in the tank won't cause the light to flash with each splash.  The functional operation of this circuit is quite satisfactory.

However, it could be improved by adding a timed delay circuit so that the warning lamp will not light until it receives a signal for about one solid minute.  Now, upon a hard turn, or fast acceleration, the warning lamp tends to flash if the fuel is a little below a half tank.