Lambda sensor based Air-fuel ratio indicator [AFR] 1

Once I bought a new car with a fuel injection I started to think how to improve my fuel efficiency. I know that engine breaking is efficient, I also sometimes tried to explain this technic to my mates. To help myself I designed little circuit which "steals" voltage from lambda sensor and uses two-color LED to show you if drive efficiently or not. (to show if engine runs on lean, stoichiometric or rich mixture)


What does it do?
When you drive in a steady conditions, engine warm, no strong acceleration engine computer controls amount of fuel to produce clean exhaust, this can be observed on voltage from lambda sensor as values ranging from 200mV to 800mV

In such case this circuit will light both colors of LED producing yellow light as a information for driver that everything is normal, AFR is close o to 1.
When you do engine braking - it means release gas pedal engine computer will stop fuel flow - engine does not consume fuel, it can be observed as voltage from lambda sensor close to 0V. Presented circuit will light up green LED to show you how clean and cheap you driving is at that moment.
Right after turning on cold engine and when you try to accelerate quick engine computer will let some extra fuel to pistons - your car consumes much more fuel than in cruising conditions. This device will light up red LED to warn you that you should expect huge fuel consumption.


How does it work?
Lambda sensor is connected to engine controll unit like this:

Signal from this sensor informs engine controll unit about mixture richness, I decided to measure this signal like this:

I did some measurements with my good old Tektronix scope just to makse sure that theory of lamba sensor applies to my car and than I draw something like this:

On the left side is a main connector, two terminals are connected to lambda sensor and other two to +12V and to ground. Signal from lambda sensor is fed to differential amplifier IC3A. This might seem to be not necesary because most of lambda sensors have got grounded terminal. But I wasn't certain if my ECU does it. I also didn't want to damage ECU or sensor when I tinker with this vital signal in my car so I used differential amplifier as separation stage. +12V is regulated by good old-fashined 78L05 to supply microcontroler. I used PIC12F510, mostly because of its low price. Two colour LED is conntroled by microcontroller.

How to build one?
I draw a small one-sided printed circuit board for all components, this is a result:

minimalistic, isn't it?