Car Rpm Out Problem .

[MaxiBoost]

Hello everybody ,
I want use a rpm tachometer with shift light our car ,
New tacho run only one coil normally .
But I can see only Rev/4 signal on tacho . Because car has 4 coil (multi coil) ignition ,for example when real rev is 4000 rpm I can see 1000 rpm on tacho .I must make a circuit for Rev X 4 and use it .
DO you have any idea ? Can I make it use with 555 ?

*I cant use cars tacho signal ,its digital signal (alfaromeo 147) ,
I must connect new tachometer to coil 1 .

 

[Russlk]

A diode OR circuit is the easiest way:

 

[MaxiBoost]

Interesting ,
Im trying it before (diode) but no working ! ,

Detail :
Im connect only two coil out to new tacho with diode than I see on the tacho 0 (zero) rpm . I think if one coil out rev/4 rpm on the tacho ,two coil out must be read on the tacho rev/2 rpm .
Im connect 3. and 4. cylindir out .

 

[MaxiBoost]

Have anybody frequency multiply circuit for this problem ?
Thank you .

 

[Nigel Goodwin]

Have anybody frequency multiply circuit for this problem ?
Thank you .

It's not trivial to try and multiply a frequency over such a wide range, far easier to do as Russlk suggested and combine pulses from all four coils. You will require some method of picking the pulses up, but you would to feed a frequency multiplier as well.

Another possibility is to modify the tachometer, these are usually just a simple monostable - you could always replace the insides with a simple 555 timer design.

If you want to try a multiplier, you could try a 4046 PLL with a divide by four circuit in the feedback loop - you could overcome the engine stopped problem by muting the output from the 'PLL locked' pin.

 

[ante]

Hi.
Is it possible to use your normal tacho and build a separate circuit for the shift light?
Then the shift light could be trigged by any pulse frequency available and set according to your normal tacho.

Ante :roll: