electronic pulse converter for automotive tachometer

electronic pulse multiplyer for automotive tachometer

I have a old points style single cylinder lawn tractor engine that I took the magnito off of and replaced with a 1970 chevy c-10 coil for a 350cid V8. I also have a 4/6/8 cylinder electronic automotive tachometer. I need a custom circuit schematic to multiply the one pulse per revalution coming from the negitive side of the coil into four pulses for the signal input of the tachometer. I have a good knowlage and understanding of how to read electrical circuit schematics and building circuits from scratch,but I know nothing about designing custom circuits from scratch. If someone could figure out what componets I need and draw up a detailed schematic, I would be very greatful.

edit: is it possible to create a circuit using a 555 timer?

Well, I don't have time to draw a detailed schematic for you but I can suggest a way to do the pulse generation. Increasing the frequency of pulses is somewhat more difficult that reducing the frequency.

One relatively simple way is to input the pulse to both inputs of a 2-input exclusive-OR gate, such as the CD4030B, where one of the inputs has an RC delay. That will generate an output pulse on both the rising edge and falling edge of the input pulse with pulse width equal to the delay time. Connect those pulses to another similarly configured XOR gate to give the four times pulse frequency you need.

I would suggest a delay of about 5ms for the first XOR gate (50kΩ in series with 0.1µF to ground), and a delay of 0.5ms for the second XOR (5kΩ in series with 0.1µF to ground). That should be good up to at least 6000rpm.

Make sure you use transient protection circuit (clamp or zener and RC circuit), at the output of the coil to protect from the voltage spike when the points open.

Perhaps someone else in the forum can give you a more detailed circuit, if you need it.