Fuel consumption meter

I received a e-mail from Gábor Grátzl from Hungary. He owns a Suzuki Wagon R+, in which he replaced the original display with a Triple Info Display of a Opel Astra. He made a fuel consumption meter, using the protocol information on my site.

The meter measures the speed of the car and the fuel consumption to show you how many liters you use per kilometer (Or 100 km). The fuel consumption is measured by measuring the time during which the injector is "on" per second (Or other fixed time interval). The "on"-time is proportional to the amount of fuel used.

The speed is measured by measuring the amount of revolutions per second that the speed sensor provides. Or more precisely by measuring the time between two pulses. Next the calculation is done:

( Injector_"on"-time / Fixed_time_interval ) * Revolution time * calibration_constant = L/km.

Of course the Fixed_time_interval is chosen to be "1" to avoid the division. This can be corrected with the Calibration_Constant. The answer of this calculation is written to the screen using the protocol I described. If the car is not driving the fuel consumption is very high and tends to go to infinity. Than he writes "L/Km --.-" to the display.

He is also using a PIC16F84, He changed the sendi2c routine to calculate the Parity bit automatically. Gábor was so friendly to also send me the source of his project. You can download the source here Fogy.zip. The zip-file also includes drawing of the schematic. You can read his e-mail and see the photo's below:

Hi!

I'm Grátzl Gábor from Hungary. I have a Suzuki WagonR+. In this car there isn't outdoor termomether, so I bought an used Astra display. (The backlight color is same the Opel and the Wagon.) After I istald the display, I thought I will make a consume(?) meter, but my biggest problem was the display programming. I found nothing on Internet except your site. I look trought your description and your program and I made my program and HW. The display operated on the first time. I was very happy. I use also PIC16F84, but I use the on/off wire of display. (which goes to radio's antenna output) So the consume is endless, the controller turn off the display, therefore the display show the date. (for example the car is standing whit running motor) In short: I wrote this mail, that's whay I thanks for your site. That helped me.

Grátzl Gábor

P.S.: Do you know, waht is the function of the 12 pin of display. The service book write: Frequency signal wire. What is this?

Reply from Eelke: "I think that, pin 12 is probably for a speed signal (Km/hour). Which is used to eliminate the error in temperature that will arise from the driving wind cooling the temperature sensor."

In this picture, the voltage of the zener-diodes reads 47V This should be 4.7V.