There’s almost enough info in the image to reverse engineer this thing. The TO92 at the 1 o’clock position looks like a 5v linear regulator. A 78L05 to knock down the 12v battery to 5v for the Atmel controller. Q1 is a little MOSFET or BJT to boost up the coil signal to something friendly for the Atmel input.

The “program” is just reading an input and assigning it to an output.

I looked at this design back in the day and wondered why he even used the microcontroller. He’s basically using it as a wire using a line of code to do it. Kind of a waste...but, it probably is priceless to control the diagnostic lights. Those will save you a tech support call. It did seem like overkill and it adds $3 to the design + programming (burning).

I think an easier way to do it is use a dual op amp to threshold the input with some hysteresis and then buffer the output signal to the tach. My weakness is how to make an amp input for the coil but some googling with make that clear.

I used to be an embedded systems designer ten years ago.

Is this really important to recreate? I just touched up the solder on mine (82 Sd) in 2010 and went on with life.

Looks like these amps are NLA now?

Edit - ok like usual I spoke too soon without doing my home work. The LM1815 that MB used is a tricky variable hysteresis adaptive feedback amplifier. I bet the Atmel risc microcontroller is sampling the signal with its adc and doing adaptive digital thresholding on the signal. The problem with out crank sensor is it puts out a different voltage depending on speed. This needs to be compensated. I was thinking you could just take the input, amplify and threshold and POW, you have a signal you can feed into the tach. If it were that easy MB would have done that.

So it looks like Techguy used a diode clamped BJT transistor input (three pin device at 6 o’clock) to get a signal the microcontroller could accept. Probably cribbed the circuit from the original unit. You can kind of see it on the reverse engineered schematics. It looks the same.

Then he samples using the ADC and does thresholding with some code. He adds adaptive hysteresis based on the inputs to get rid of noise.

It may be easier to just lay out a circuit with the LM1815 and be done with it.

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