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Digital-Numeric Air/Fuel Gauge

If you want to accurately tune you engine, you need an Air/Fuel gauge, among other things. There are two basic types; those that use narrow-band sensors and those that use wide-band sensors. Narrow-band types use your stock oxygen sensor and read most accurately around an A/F of 14.7:1 (stoichiometric). Wide-band types use a controller to drive a special sensor and read accurately from very rich (7:1) to very lean (22:1). This page discusses the construction of a digital-numeric Air/Fuel gauge to read from your stock narrow-band sensor.


3.5 digit LCD display with green LED backlight (Lascar Electronics SP-400)
Enclosure (Newark Electronics 93F9932)
1% Tolerance Resistors (Newark Electronics 220k ohm - 84N1845, 470k ohm - 84N1885, 100k ohm - 84N1805)
Stranded 22 AWG wire, 3 colors (Radio Shack)
Circuit board (Radio Shack 276-148)
Voltage regulator (Radio Shack 276-1770)
Heat sink (Radio Shack 276-1368)
4-40 Screw to secure regulator to heat sink (Hardware store)
Heat sink compound (Radio Shack 276-1372)
Foam double stick tape (Hardware store)
1/8" Shrink tubing (Hardware store)
Small zip ties (Hardware store)


Soldering iron (Radio Shack 64-2802)
60/40 rosin core solder (Radio Shack 64-008)
Solder flux (Radio Shack 64-021)
Wire strippers/cutters(Radio Shack 64-409)
12VDC power supply (an old computer power supply will work)
Voltage divider circuit (use same resistors and circuit board as in A/F Gauge)
Multi-meter (Radio Shack 22-810, 22-806, 22-809)
Dremel tool with cutting bit


Fit the display and heat sink to the enclosure. You will have to cut the front and rear panels of the enclosure. Measure the display or heat sink against the panel and mark where to cut. Score the front of the panel with a knife and break it at the cut. You will also want to drill a 3/16" hole in the rear panel to allow the wires to exit. A dremel tool will be necessary to fit the display to the main enclosure body. Just carve out some of the outer lip to accept the display. It will be a tight fit.

Cut wire into the following lengths: one 3" green, two 3" red, four 3" black, one 1' black, one 1' green, and one 1' red. Strip the end of each wire. Add flux to each stripped end and dab some solder on each one. The short wires connect items within the enclosure. The short green wire goes to INH. Short red wires go to V+ and DP3. Short black wires go to GND, BL-, INL, and regulator ground. The long wires run out of the gauge. The long green wire is for the O2 sensor. The long red wire is for the 12V power. The long black wire is for the gauge ground.

Use a knife to score the circuit board and break it into 4 equal pieces.

Wipe heat sink compound on the back of the voltage regulator and screw it to the heat sink.

Carefully push the leads of the resistors through the holes in the circuit board in the pattern shown in Figure 1. Twist the leads together, apply flux, solder, and cut the excess wire off.

Wire the display, voltage regulator, and circuit together as shown in Figure 1.

Figure 1.

Use foam tape to secure the heat sink to the enclosure so that the fins protrude through the rear of the enclosure. Tape over the circuit board to prevent short circuits. Fit the circuit board and display into the enclosure. You may wish to coil the wires some to prevent it from kinking. Slide shrink tubing over the long red, green, and black wires. Position it so that it will form a strain relief when the rear panel is slid over the wires later. Use the lighter to carefully heat the tubing and shrink it over the wires. Allow it to cool and slide the wires through the holes in the rear panel. Attach the zip tie to the wires on the inside of the rear panel. When it is tightened and trimmed, it will prevent the wires from being pulled out when the gauge is installed in the car. Before you screw the enclosure together, you want to calibrate the gauge.

Build the divider circuit exactly like you did for the gauge. This will step down the voltage from the 12VDC power supply and create a signal around 1 volt for the A/F gauge to read. Hook up the multimeter, A/F gauge, power supply, and divider circuit as shown in Figure 2.

Figure 2.

Turn on the power supply and the A/F gauge should turn on. The backlight should glow green. The display should show a number around .8 to 1 volt. Use a tiny screwdriver to turn the potentiometer on the back of the display until the A/F gauge display matches the multimeter. They may fluctuate +/- 0.001V, so don't worry about getting them perfect. Just get them to match within 0.002V.

When it is calibrated, unhook the wires and screw the enclosure together. When the A/G gauge is fully assembled, I suggest you hook it up to the test rig again and run it for 20 minutes or so. This will make sure that it will run reliably in the car. If you get any odd results, you should open up the enclosure and check your wiring. Be sure that all wires are securely soldered.

Contributed by Corbin

Cars Modifications Power Tuning Digital-Numeric Air/Fuel Gauge

Document statistics: Last modified on 2009-01-11 17:06:46 by Corbin

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