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Home > Technical Articles > Building an In-Dash Air-Fuel Monitor

Guest Technical Article
Building an In-Dash
Air-Fuel Monitor

Gabriel Pennella

[Click on Photo]

Figure 1:
Voltage Curve of Lambda Sensor

Figure 2:
Air Fuel Mixture Curve

Figure 3:
Construction Schematics

Forward from Wayne:

Gabriel offers us this great tip on building an in dash air-fuel monitor.  This can be potentially used to monitor your car's performance, and may also allow mixture adjustments while driving.  Gabriel has just recently begun to learn English, so I have edited the text for better readability.


The lambda sensor outputs a voltage signal that represents the instantaneous composition of the air-fuel mixture, from near 0 Volt (too lean) to 1 V ( too rich). See Figure 1.

The sensor must have reached a temperature of above 350C before it outputs a reliable signal.  In my case, my Porsche 911 has 1974 headers, and I installed the O2 sensor approximately 15cm before the muffler connection.  Don't install the O2 sensor too close to the exhaust valve cover, otherwise, you will have to remove the o2 sensor every time you do a valve adjusting job.  A generic O2 sensor has a M18x1.5 threads, but I didn't use them.  I found it more practical to measure the sensor diameter (this is measured on the electrode end, approx 10.25mm), then drill a hole with the same or smaller diameter and install it using a "pressure fit". Then, to completely constrain it I used a wire type bracket. 

I used (for more accuracy) a four wire (internally heated) O2 sensor.  There are two signal cables (wire it to the cockpit) and two for the heating element (connect one to ground and other to +12V).  In my NGK O2 sensor, the heating wires are white. You can check it with a multimeter, as the heating element has aprox. 6 ohm resistance. The other two (black & gray on mine) are the signal cables.

When you have the signal wires routed near your dashboard, you can connect a generic digital voltimeter. However, as you can see in Figure 1, voltage from the O2 sensor is not a linear function; it rises quickly near lambda factor 1 (14.7:1 air/fuel) and slowly at rich & lean ends, so you have to compensate this if you wish more precise readings. I used two LM3915 units to drive 20 leds, and to compensate for the lambda curves at the same time. See Figure 3 for the schematics.


  • 2 LM3915
  • 2  180 OHM RESISTOR
  • 1 22K OHM PRESET
  • 1 4.7uF 16v capacitor
  • 1 7805 (5v regulator) with heat sink
  • 1 pcb

LED board:
You can place the LEDs as I do, 
from left to right, LED 1 to 20
              00      <- MAX POWER (LED 15-16)
            00  00
           0      0
          0        0
0      0
 00  00

Less than (to the left of) LED 5      = TOO LEAN
Greater than (to the right of) LED 15 = TOO RICH

   From 1 to 5:    Red
   From 5 to 15:   Green
   From 16 to 20 : Red

The following information was obtained by me by playing with this A/F Meter.  You can start with this information and practice.  If you have found anything  wrong or incorrect in this article, please email me.

Adjusting P1:

  • Apply 0.92 Volt (check it w/ a digital voltimeter) to the signal input and adjust P1 until led 16 just comes on.


  • Turn on your engine and wait about 15 or 20 minutes to warm up the O2 sensor. If you have CIS ( K-jetronic fuel system) just apply light pressure on the sensor plate with your finger until the engine reach maxium RPM. At this point, led 16 should be on, if not, adjust P1.

On carburated engines, move the idle mixture adjusting screw to obtain maxium rpm and move P1 to turn led 16 on.

DRIVING WITH IT (also see Figure 2):

On Idle: Values between led 6 to 15 are acceptable.
You will have smoother idle around led 11.

Partial throttle: Between LED 6 to 11

Full throttle: LED 16

Suggestions & questions: write to:

Gabriel Pennella


John Francis Duncan wrote:

People who already have an o2 sensor installed in their car can use the existing o2 sensor by tapping into the signal lead.  Yes, you'll have to find the correct wires.   Some lambda sensora have one, two, three or four wires; it depends if it has a heating element or not.  I don't know exactly what kind of o2 sensor Porsches use (I think, 1981 year onwards), but I hope that somebody can provide more info about that.

John Rodgers ( has the following hints and tips on making this Air-Fuel Monitor:

  • 20 LEDs take a lot of room, plan accordingly for a case 4 to 5 inches x 1 to 2 inches so they can go across the face nicely.
  • The 4.7 microfarad capacitor may not be necessary. According to National Semi-conductor it is needed if LED wires are over 6 inches in length. I left it out and it works okay.
  • The 22kohm preset was too low in value. I ended up using a 47k preset set to full resistance. to make light 16 come on at 0.92 volts signal.
  • Use old/wornout 1.5 volt batts to supply the signal voltage or make a variable DC power supply as I did, but it is hard to find one that goes below 1.25 volts.  Adding a large pot across the signal connections will drag the signal voltage down less than 1 volt so tou can calibrate it.
  • The long wire on the LED is the Positive side, won't work if backwards.
  • Use 18 pin sockets to mount to the pcb and insert the ICs after all soldering is done.
  • Use the minimum heat to solder or a heat-sink, I overheated some parts and had to redo them.

Dick Lague ( adds this additional thought:

I read the tech piece on installing an oxygen sensor. I have used such a setup in my 1955 Speedster. It is a great tool for tuning. There are at least two commercially available units. K&N [the air filter people] and C.B. Performance make air/fuel Ratio meters.

I used the C.B. Unit in the Speedster. It is a 2" round analog electrical gauge. It is heavily dampened and reads from 12:1 to 17:1. You can either buy it as a complete kit of the gauge only.

Gauge only part#2910
Complete kit 2905

C.B. Performance
1715 N. Farmersville Road
Farmersville, CA 93223

K&N makes a gauge with 10 LEDS. It is also a 2" round instrument. The part # for the complete kit, which includes the gauge, oxygen sensor and weld on sensor adapter is 85-2441. I believe it is around $100.00

The gauge is also available separately.

K&N engineering
Riverside CA 92502

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