have to build a tacho for my college project, as it is very difficult for me to take an oscilloscope in the car to monitor the type of signal from the black wire of the coil(this is where most tachos are connected, i guess), so can anyone please tell me the type of signal.. i mean voltage level etc...
thanks

At a guess. Square wave, ~12V. Frequency half the speed of the engine. Also quite noisy.

I built a shift light using a PIC 16F84. I had to condition the input signal quite a lot. I found a suitable circuit on the web.

Cheers

[Edited on 12/9/07 by SeaBass]

http://www.niksula.hut.fi/~mdobruck/siililand/mini/diy/alien/tacho/tacho.html

or something like that is the usual thing...

ie a Hz-V converter then a V to bargraph IC...

if you have as scope, all ypu need to do is park near an electricity supply and use an extension lead.

the LT side of the coil is switched - the coil being indictive will produce a big spike - up to 200 V - so never try to hold the lead with your hands.

there will be ringing on the waveform too; I have seen a trace somewhere on the net.

IIRC Rev counters are based on a diode pump using the pulsesd to charge a "leaky" capacitor; a voltmeter in effect reads the voltage on the capacitor to disply RPM.

Again, I have seen the internal circuit of one of these - seeing this may help give some clues about the incoming waveform.

so after that rambling, it is 12 V sqwuare-ish with a big spike.

The input signal on the negative side of the coil is at low revs a square signal.

12volts when coil is fireing / idle and 0volts when coil is charging

sow i would measure the time between every rise to 12volts!

wy?because on high rpm the time needed to charge the coil fully is bigger then the time between 2 revs sow the only pulse wich you will see is the short release pulse off the coil.

you then measure the time between 2 fireing events on one piston. |--------|-------| at high rpm. at low ||||||--------||||||-----|||-----||||------||||| hope it makes sense.

so on a 4 stroke you measure the duration of 2 revs.