David:
I found I had to unravel the switch positions in order to find out how the circuit worked, but it didn’t make sense. Usually in tachometer circuits, the incoming pulse triggers a monostable to give a fixed length pulse (the ON condition). That is followed by a variable time (the OFF condition) until the next pulse arrives. The ratio of ON:OFF times is a measure of the speed that the tach is measuring.
I expected to find Q1 and Q2 as the monostable, but cannot see how it works – unless I have misread the switch connections. Perhaps Ken Moffett or one of the other experienced regulars can shed some light on it.
However, it appears that the tach is powered by positive pulses from the coil when the contact breaker opens (not negative as I had assumed). Assuming a max speed of 10000RPM and 6 cylinders, the max pulse width must be less than 1ms so in my circuit I have made the 555 pulses 600us wide. The pulses are clamped at 6V in the tach by D4 so to protect it I have added R5 in my circuit. I forgot to label the transistor. Any small signal NPN will do.
I think there is a fair chance it will work.
- Ignition Pulse generator.GIF (4.86 KiB) Viewed 53926 times