D.I.Y. air/fuel ratio meter 

I’m sure you have seen many air/fuel ratio meters for sale. In many cases this is small gauge with many colored leds. If you have little experience in welding small parts you can make this useful gauge yourself.

D.I.Y. a/f ratio meter budget is incredible small. You will spend only about 7 Euro on main parts. You must buy 10 diode leds (6 red, 2 yellow and 2 green), 1 diode (code: 1N4007), 2 resistors (any kind 2 kohm), chip (code: LM 3914N), some wires and universal plate. If you have any experience in welding electronic parts this easy job won’t take long. You should print electronic scheme (first picture on right top) and weld all parts. I suggest to weld chip first of all and after weld 10 led diodes, wires, resistors and diode.

Why you need a/f ratio meter? Usually it’s used for measuring air to fuel ratio in your fuel mixture (by measuring exhaust gases). Of course you need to check if oxygen sensor (or lambda probe) is installed into your cars exhaust system. Air/fuel meter is directly connected to lambda probes signal wire.

Originally oxygen sensor was developed for road cars to measure a/f ratio to meet emissions requirements. Common lambda sensor will send signal varying from 0 to 1 volt. Factory installed ECU is programmed to understand that 0.2 volts (in our case 2 red lights from left) means lean mixture (ECU doses more fuel to the mixture) or 0.8 volts (green light from right) means rich mixture. Typically ECU is not looking ant lambda probe then accelerator is flat.

Signal also could be measured using simple multi-meter, but as sometimes signal changes 10 times in second it’s too hard to watch it. That’s why colored gauge is used. Looking at what color lights burn you can understand what is happening inside your cars engine. If mixture becomes too lean it could result in melted pistons.

You must understand that using stock lambda probe you won’t get accurate air to fuel mixture readings. For engine tuning special wide band lambda probes are used.

This simple device will guide you about what fuel mixture is created at various engine loads and speeds.

Connecting home made a/f ratio meter is also easy. First of all find lambda signal wire and connect it to a/f meters signal input wire, also connect ground wire and +12 volts wire should be connected to ignition.

In next special feature article i’ll write about another usefull device called “knock sebsor” and how to D.I.Y. knock gauge.