I have made digital RPM meter, digital CNG meter and temperature meter and battery voltage meter for my Alto. The display can be selected (RPM,CNG,temperature and battery voltage)with push button installed on display.
Regarding accuracy of measurement:
For CNG:
I didn't calibrated/verified CNG and RPM measurement yet but i think it should be close to be accurate. For CNG, i used same sensor to measure that used to show CNG on default (4-led point) on conversion switch. 0-200 bar pressure corresponds to 0-100 % on digital display. This reading , 89 % matched with CNG pressure on gauge installed under bonnet when i first installed. I will verify further readings when CNG pressure will be vary in cylinder (75, 50 25 bars).
For RPM:
I used ignition coil pulses to read RPM of engine. As Alto engine is 4 cylinder, so I think it gives 2 pulses/rev from primary of ignition coil . These pulses are directly measured by micro controller in about 0.5 sec time window after interface circuit, that remove noise and spikes. so i think RPM displayed is almost accurate.
The RPM interface circuit with ignition coil is given for interested ones.
Note:
The large digit is reserved for gear display. I will add this functionality later in this project (if succeeded).
