LED Warning Circuit for EIS
By: Doc Green

This circuit is an alternative to "the big red lamp" supplied by Grand Rapids Technologies, Inc. for use as a warning light in conjunction with their electronic flight information systems (EIS).

This circuit simply replaces "the big red lamp" with a series string of three ordinary red LEDs (light emitting diodes) and an essential current limiting resistor. LEDs can be obtained that are encased in a mounting bezel, for the sake of appearance.

The EIS output for the warning light goes low (acts like a ground) when the alarm is activated. Therefore, one end of the series string (LEDs plus resistor) connects to the positive aircraft power where the power connection is made for the EIS unit. The other end connects to the EIS warning light output.

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With a resistor value of 560 ohms, the current through the LEDs will be about 15 mA. This is probably adequate for acceptable brightness of the LEDs, but the current can be increased by lowering the resistance to 470 ohms, or perhaps 390 ohms. The exact resistance required will depend to a certain extent on the characteristics of the LEDs actually used. Lowering the resistance will make the LEDs brighter.

Two comments in regard to the LEDs: (1) Do NOT connect an LED directly to a battery without a series resistor. Doing so will burn it out immediately. (2) LEDs are polarized. This means you have to have the positive and negative voltage applied in the correct polarity or the LED's will not light up.

LEDs that are mounted in a bezel will likely come with color-coded leads or other information to indicate the polarity. For example, one lead may be shorter than the other. In this case, the short lead is the negative.

The suggestion is to wire up the series string of LEDs and the resistor and then connect it to a 12 volt battery to test the circuit. The LEDs should light up. If they appear dim, use a smaller resistor as suggested above. If they do not light up at all, the polarity is probably reversed on at least one of the LEDs. All three have to be right, or it will not work.

You would think that the tiny LEDs would not be conspicuous or bright enough to be noticeable in the cockpit while paying attention to flying the plane. However, they are. Using three of them scattered over the instrument panel seems to make them more noticeable. If you wish, you can use only two instead of three without having to change the resistor values.

Good luck and happy flying!

Doc Green