On the annunciator panel, there are eight indicators:

Fuel Pump On
Pitot Heat On
Low Oil Pressure
Low Voltage
Starter Engaged
Pilot Door Unsafe
CoPilot Door Unsafe
Parking Brake Set

Most of these are activated by a wire going to ground which completes the circuit and turns the indicator on.  The Low Oil Pressure indicator wire goes to the Grand Rapids Engine Information System (EIS) alarm.

When I powered up the EIS and annunciator panel I noticed a minor problem.

The indicators don’t photograph too well, but you can see the Low Oil Pressure is lit. Just not very bright. The problem is that I haven’t programmed the EIS to alarm on anything. Which means that the indicator is not on.  This occurs because EIS was designed to illuminate an incandescent light bulb. This results in some power leaking through the circuit that wouldn’t illuminate a incandescent bulb but does cause an LED to come on partially.

The standard fix for this is to put a 1k resistor across the LED to drain off the leaking voltage. But when I did this on the oil pressure LED, in addition to stopping the partially lit LED when it was supposed to be off, it also made it dimmer than the rest when it was supposed to be on.

Now this wouldn’t be a huge problem if it weren’t for being able to adjust the brightness of the annunciator panel. When I turn the brightness down, the Low Oil Pressure LED actually goes out while the other LEDs are still on. This could lead to not seeing a low oil pressure warning.

I tried numerous fixes and none worked. I eventually got a tip to use a MOSFET to control the circuit. This required quite a bit of hacking on the original PCB but it worked.

It ain’t pretty, but it gets the job done. If/when I ever make a version 2 of the PCB, I’ll integrate the MOSFET circuit into the low oil pressure line.

Here’s the current annunciator circuit for anyone interested.