It’s one of the most popular articles ever published in AutoSpeed: the
Mixture Meter covered in
Cheaply Monitoring Air/Fuel Ratios
has now been built in the thousands. As a simple and easily fitted indicator of
the rich/lean status of the engine, it’s waaaaay better than having no
instrument at all. (And of course, it’s not nearly as good as having an
expensive professional wide-band air/fuel meter on the dash, either!)
But while it’s been a very successful design, we thought that it could be
improved upon. Specifically the new Smart Mixture Meter covered here:
better protection of the main IC, resulting in a much lower chance of damage
when power surges occur
the LEDs to be auto-dimmed at night
an audible alarm that sounds when there are lean mixtures present at high
If you’ve ever done any electronics soldering and you can recognise some
components, the AUD$25 kit is easy to build. (And if you don’t want to build it,
the module is available fully built and tested for AUD$70.)
This week we’ll cover the design and next week we’ll install it step-by-step
in a car.
The Background Story
The Smart Mixture Meter was developed and designed by www.siliconchip.com.au electronics
magazine. It is one of a number of projects that will be covered in a unique
Silicon Chip publication - High
Performance Electronic Projects for Cars - which will be available from
newsagents in Australia and
New Zealand, or
online through the AutoSpeed shop. Additionally, it has also been published
online at A Smart Mixture Display For Your Car.
The electronics design of the Smart Mixture Meter was carried
out by electronics engineer John Clarke, while I came up with the concept.
(During this period I wore a different hat to an AutoSpeed contributor, working
for Silicon Chip Publications as a freelance contributor.) AutoSpeed’s Michael
Knowling did the on-car testing.
So by no means should the Smart Mixture Meter be seen as an
AutoSpeed-developed project, at the same time I am happy that AutoSpeed endorses
it and promotes it.
The Smart Mixture Meter
As with the previous design, the Smart Mixture Meter uses 10 LEDs arranged in
a moving dot bargraph display. There are two yellow LEDs (indicative of rich
mixtures), six green LEDs (mixtures of ‘middle’ strength) and two red LEDs
(these show lean mixtures – remember, red is for danger). The Smart Mixture
Meter is designed to work with zirconia oxygen sensors, that output a range
that varies from about 0 – 1 volt. When the mixtures are rich, the sensor
outputs a higher voltage (eg above 0.8 volts); when the mixtures are lean, the
sensor outputs a low voltage (eg below 0.2 volts).
If you think of each LED lighting up in turn as the output voltage of the oxy
sensor changes from 0 to 1 volt, you’ll be pretty close to understanding how the
Oxygen sensors are used by the engine management system for feedback – the
Electronic Control Unit (ECU) uses the signal to keep mixtures around what’s
called ‘stoichiometric’, or an air/fuel ratio of about 14.7:1. From the
designers’ point of view this is a desirable air/fuel ratio because the cat
converter works best at this ratio, so emissions are able to be kept low.
The zirconia oxy sensor is particularly suited to this application because
its output voltage rapidly changes as the air/fuel ratio goes through 14.7:1. At
air/fuel ratios leaner than this the voltage is low, then it switches very
quickly to a higher voltage as the air/fuel ratio becomes richer than
stoichiometric. IOTW, the output voltage of the oxygen sensor is very non-linear
compared to air/fuel ratio. (See The Technology of Oxygen Sensors
for more tech on sensors.)
But you want an English translation of all that? - the raw voltage output of
the oxy sensor isn’t really great for monitoring mixtures.
However, people who don’t go any further with this idea often therefore
assume that a mixture meter of this sort is useless. After all, if it can’t tell
you whether your car is running 11:1 or 12:1 air/fuel ratio, what use is it? In
fact, in a car the Smart Mixture Meter is extremely functional. This is
If the engine suddenly starts to run badly, by looking at the Smart Mixture
Meter you can immediately tell if it’s an air/fuel ratio problem. In fact, have
a Smart Mixture Meter in the car and if the engine stumbles, the first
instrument you look at is the LED display. Is the engine suddenly running
ultra-lean? If so, perhaps the fuel pump or fuel pressure regulator has died. Or
is the engine running badly because the mixtures are over-rich? That might be
indicative of an airflow meter or MAP sensor problem. Without having the
immediate and graphic indication on the dash, it’s just so much harder to find
what the problem is.
2. Oxygen Sensor
You can clearly see the health of the oxygen sensor. The ECU doesn’t use the
signal from the oxy sensor all the time to control mixtures to 14.7:1. In most
cars, at full throttle and on the over-run the ECU instead adopts what’s called
an ‘open loop’ strategy – it ignores the output of the oxy sensor and goes just
on its internally programmed maps. (When the oxy sensor signal is being used by
the ECU the car is said to be in ‘closed loop’.)
All this is really easy to see on the Smart Mixture Meter. By using the
output of the factory sensor the high-speed fluctuations in mixture strength are
easily able to be seen. So when the ECU is in closed loop, you’ll see the middle
LEDs flashing back and forth as the ECU rapidly corrects the mixture strength to
keep the average air/fuel ratio at
14.7:1. When the ECU uses open-loop control, only a single LED will be
illuminated as the mixture strength is kept constant.
A sick oxygen sensor will show up as the closed loop cycling back and forth
of the LED will be slow. And a dead oxy sensor? Well, you won’t have any LED
lit, will you!
3. Audible Alarm
In addition to its LED display, the Smart Mixture Meter has another important
function. It monitors engine load by taking a feed from the airflow meter. When
the load is high and the air/fuel
ratio is lean, the alarm sounds. Since these two conditions should never occur
together, you should never hear the alarm (apart from when setting the system
This feature is a real plus because it will immediately warn you of a
catastrophic lean-out. These can occur when you get fuel surge when cornering
hard (I lost an engine once that way) or when accelerating at full power and a
fuel reg hose falls off. In both cases, you won’t normally be watching the LED
So you can see that despite the fact that the Smart Mixture Meter doesn’t
give you an exact numerical read-out of air/fuel ratio, it’s actually a bloody
useful display to have running in a car. (And don’t think that it’s impossible
to tune by the output of a standard-type oxy sensor – see Real World Air/Fuel Ratio Tuning.)
The Signals You Need
For the Smart Mixture Meter to work you’ll
need a car equipped with a zirconia-type oxygen sensor that outputs a signal in
the 0-1 volt range. To use the lean-out alarm feature you’ll also need to access
a voltage within the 0-5V range that rises with engine load. Most typically,
that’ll be an airflow meter output, but in a turbo car you can probably also use
a MAP sensor output.
Rather than being disappointed after building the kit (or after buying the
preassembled version), first use a multimeter to measure the outputs of these sensors
to make sure that you’ll have the right signals available.
The oxy sensor will be mounted in the exhaust manifold or first section of
the exhaust pipe. In some turbo cars it is mounted in front of the turbo but in
most cases, it is immediately after. Many cars these days use multiple oxygen
sensors – eg four sensors aren’t uncommon on a V6 or V8. However, you’ll need to
tap into the output of only one, which should be close to the engine.
Older cars use oxy sensors with just one output wire. That’s pretty easy –
just use a multimeter to measure its output voltage when the engine is fully
warmed up. It should be around 0.5 volts (ie 500 millivolts) at idle (probably
with lots of up/down movement) and rise to 0.8 – 0.9 volts (800 – 900mV) under
(To do this, set the multimeter to Volts DC and connect the black lead to the
car’s body. Start the car and let it warm up. With the other multimeter input, back-probe
the working oxygen sensor.)
If you’re dealing with a multi-wire oxy sensor, just probe each wire in turn
until you find a voltage that behaves in the way described above. Other wires are likely to have 0 volts
(ie ground) and battery voltage on them.
The connection to the signal sensor output wire can be made either under the
bonnet or at the ECU, which is usually mounted inside the cabin. The ECU
connection is better (there’s less chance of picking up funny signals) but
either way usually works fine.
Finding this signal is done in much the same way, but this time you’ll be
backprobing the plug at the airflow meter. What you’re looking for is a signal
that varies within the range of 0-5 volts. At idle expect to find about 1-1.5
volts, while sharp blipping of the throttle should take this up to 3 volts or
so. Under full load on the road the voltage should rise to about 4.5 – 5 volts.
There may be other voltage signals that vary a bit when the throttle is blipped,
so don’t just do a cursory test.
The Smart Mixture Meter has no less than five adjustable pots – which seems a
helluva lot! However, when they’re broken down into their different functions it
VR3 (the pot on the far right) simply adjusts how much the LEDs auto dim at
night. Clockwise will give you a brighter display at night.
Because not all oxy sensors work over the complete 0-1 volt range,
adjustments are needed so that on a properly running car the lit LED isn’t
always up one end or the other. Pot VR1 allows you to adjust the range so that
the end yellow LED lights when mixtures are fully rich. This should occur at
full throttle, high rpm. Pot VR2 is the adjustment for the other end of the
range – you should set this pot so that when the car is in closed loop, the
up/down movement is centred around the middle LEDs.
Adjusting the Lean-Out
There are two adjustments for the lean-out alarm – the load and the LED. Pot
VR5 sets the Load Threshold – adjust this until LED 12 (the bottom LED on the
PCB) comes on only at reasonably heavy loads. Then adjust pot VR4 until LED 11
(the other red LED on the PCB) comes on at what would be a lean mixture for that
For example, in a turbo car set pot VR5 so that LED 12 lights when you’re
just coming onto boost. Then set VR4 so that LED 11 lights when the last green
bargraph LED before the red is on.
Depending on the operating system of the car you may need to do some tweaking
of these adjustments before there aren’t any false alarms in normal driving.
(But set them so that there are just no false alarms, if you see what I mean!)
Once you have installed a LED Mixture Meter on your dashboard, it is very
hard to go back to having no mixture indication at all. Not only do you feel
blind without it, you can also rest easy knowing that you’ll be instantly warned
of a catastrophic lean-out.
Next week: the step-by-step fitting
of the Smart Mixture Meter to a car.
Buy the Smart Mixture Meter: AutoSpeed Shop