Guy needs help filtering a tach signal for race car

This is from @setenta9 I've been helping with the code part, but I'm not good enough with circuits to help with the wiring part. He has a tach signal and needs it cleaned up enough to use as a interrupt signal to get engine RPM.

corte (2.0 MB)

The signal you see is the one the car delivers from its own rpm signal. That signal that I show you is obtained directly from the cable that the car has for the tachometer and comes directly from the original ignition coil of the car. Unfortunately, it is a very noisy signal.

The E part is the ignition coil
The A part is the conector for the tacho
The P part is the crank sensor
The crank sensor gives the correct ignition signal to the coil

Hopefully someone can help straighten him out on what he'll need to turn this into something he can use.

Manu thanks in advance!!

-jim lee

Try this, I'm not 100% sure of the R1 and R2 values, you might have to vary them depending on you signal.


I think you need to determine the peak voltage we are dealing with.

Hi guys. Thank you very much for your help.

According to the oscilloscope, it seems that the maximum peak is around 100v. That's possible? Even if it's just a peak, it seems like a lot to me.

In the diagram it seems that the top left line would be the tacho signal and the bottom line would be negative. It's right?

Thank you all

There is a lot of inductance being charged and rapidly discharged. Any current induced into the coil secondary is reflected back to the primary because it's a transformer.

Yes, During the time the points are closed and current is running through the primary of the coil. During this time the current is (kinda) converted into magnetic energy.
The Coil gets really angry if the current suddenly changes. It will create a voltage itself in an attempt to keep the current going. Hence the negative voltage.
At the same time this voltage times the coil winding ratio created another voltage on the secondary trying too to keep the current going.
In theory these voltages can go to infinity. The limiting factors are:

  1. The condenser across the points
  2. the breakdown of the vapor in the spark gap of the sparkplug.

I'd expect a higher peak, maybe in the 300 - 350 range. But I don't know what system you have. My experience is with automotive ignitions systems (old ones :slight_smile: )

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