Build Your Own Air/Fuel Gauge

You can spend a lot, or you can spend a little. But if the results are the same, I personally prefer to spend a little. If you have any electronics experience (or know someone who can help you), build your own!

Parts Required:

• LM3914 - Bar-Dot display chip
• 10-seg LED Bar display (or 10 of any LEDs, or just the top 3, or 4, or whatever)
• 1k resistor
• 1 or 2 x 10k trimpot

I got all of these parts from a place called "Active Components", for just a few dollars. Don't bother going to Radio Shack, they've dropped all these pieces. I recommend that you seek out an electronics wholesaler in your area, the prices will be lower than retail stores (if they grumble about the quantities, try the magic word "prototyping". Convince them that you may need thousands of these in the next few weeks).

You do NOT need a voltage regulator, precise 1% resistors, or anything else expensive. You might want to get a small box to put it in, or you could somehow integrate the LEDs into your dash so they don't intrude.

I used reasonable quality multi-turn trimpots. You only need one for R_HI. "R_LO" is optional and lets you determine the lower range for the meter. If you just want the lower range to be 0.0, just connect pin 4 to ground.

Here's the schematic... use this to determine which pins connect to where:

To summarize the LED connections: each LED has one side that connects to +, and the other connects to the LM3914. Be aware that LEDs are polarized, and will not light if connected backward. The pins to connect to from 1 to 10 are:

1, 18, 17, 16, 15, 14, 13, 12, 11, 10

It is possible to solder the chip right to the back of the 10-segment LED display. Bend the + pins of the display inward if you do this, then solder them all to a single wire. The resistor fits in between the display and chip. I also filled the whole sandwich with silicone seal, which ought to be good mechanical protection.

Note that you can use a 4.7k resistor instead of 1k for the "brite"  if you want the brightness to match your dash lights. This will be too dim for daytime use, though. Consider putting a switch on the + wire, so you can turn it off at night when you're just motoring around (voice of experience!).

Once you've built it, adjust the R_HI trimpot so that the voltage at the output (pin 6 of the chip) is exactly 1.00 (use a digital meter for this!!) If you used the R_LO, I recommend you try 0.50 for a starting point, which will give you one light for each .05 volts.

Notes:

• The chip can work with voltages up to 25, and can withstand surges over 40... ideal for automotive use. It is especially important that you get a good, clean ground. Connect the "input" to the odd-colored wire on your O2 sensor (ie. 2 wires are black, one is white - connect to the white). I simply used a small wire that I jammed into the O2 sensor socket... this is NOT a permanent connection, but actually works quite well.
• The output from pin 7 is always 1.25v. This is a regulated output, no matter how the input voltage changes this output will be the same. We connect the trimpots to this and ground, so that the trimpot reference settings will always be accurate - in spite of voltage or temperature. Some other A/F gauge designs don't use this internal reference voltage, adding complexity by using more components.
• If the electrical system in your vehicle is not in stock form, ie. you've removed the coil cap or are using certain types of performance ignition wires, you could be getting regular surges in supply voltage that will affect this gauge. In the event that your display flickers or seems grossly inaccurate, you will need to add a capacitor. Use an electrolytic capacitor, 2200uf or higher, 16v rating or higher. Connect its - to ground, and the + to your power wire. Connect this capacitor as close to your gauge as possible. It acts like a tiny battery, smoothing your car's voltage.