Fuse Status Panel Project

Last updated - 1/12/06.

Since I am planning on using mostly fuses instead of circuit breakers, I thought it would be a good project to design an enunciator panel that would show the status of each fuse.

Initially, I was going to just wire a fuse and resistor to each of the outputs from the fuse box and have them poke through the panel. This would tell me which fuses had power, and which did not. Then, I figured I'd better put the LEDs all in one place, and maybe it would be good to put them on a perf board. Next thing, I was thinking "Enunciator Panel" and started thinking about all of the features that I wanted. I had some ideas about blinking LEDs when a fuse went out, being able to stop the blinking, having switches to disable an LED, etc.

This started getting complex, and I finally decided to just do a simple board that showed fuses with power as green and fuses without power out as red. Since I had some available, I started experimenting with a CMOS NAND gate and some LEDs. The NAND gate becomes an inverter if you tie both inputs together. I put an LED on the input and one on the output. With the right resistors, this seemed to do just what I wanted.

What I came up with was a very simple circuit that uses three CMOS hex inverters, one dual-color led per fuse input, and a resistor for each led. Three hex inverters gives you 18 outputs, which seemed about right. I can always install two boards if I need more. The CMOS is good because it operates from 2 to 15 volts, which is just right for our 12v electrical system.

I did a board layout in a few days, placed my standard 3-board order with ExpressPCB for $51, and had boards back within a week. At the same time I bought 20 Red/Green LEDs and 10 CMOS 4039 hex inverters from Mouser, which showed-up within two days. Everything went together without any problems, and this is the loaded board:

The idea here is that each LED is connected directly to the output of a fuse. When a fuse has power and is not blown, the LEDs show green. When a fuse is not getting power or the fuse is blown, the LED shows red. The LEDs are arranged in the exact same orientation and position as the fuses in the fuse box. I am thinking about making my main fuse box accessible in flight via something like a pull down panel, or a removable door, so it would be important to be able to easily identify which fuse is blown.

With all inputs connected to a power source, the LEDs look like this.

When you disconnect an input, simulating a blown fuse, you see this.

And if things are starting to go really bad, you might see several LEDs go red, like so.

This board is simple, and it does exactly what I want. The only problem is that with all of the green LEDs on, this might be a bit much at night, so I have a little experimenting to do. I can try higher resistance to reduce the light output, or, I might try to use a front plate where the whole LED does not stick through the panel. Also, I might try to find different LEDs. It's hard to get good pictures of LEDs when they are on, and these do not look as bad as the pictures show, but I would rather have darker reds and darker greens, and maybe smaller LEDs.

Also, I am still thinking about the super duper enunciator panel with a micro processor and a few switches, so I can blink the LEDs, disable them, and play a message when a fuse blows. My next thing to play with are those chips that can store and play back audio. I'm still working on this...