The signal is taken from the Jacobs + distributor trigger.

Result of VR test

With no cap installed, and the engine still, the ECU reads many thousands of RPM (it varies) and triggers the injectors. With a 10K electrolytic cap in series it seems to read OK when triggered manually (opening the points rapidly with a screwdriver) BUT before or afterwards when stopped with ignition on, the ECU again reads many thousands of RPM and fires the injectors.

Tried it with a 22 uF BP electrolytic cap, and also with 10k to 20k of R in series. The scope shows a signal of .4V peak-to-peak, about 20ms period - this without the engine running. Sometimes it was fine and read 0 RPM with a clean waveform, but when noisy the RPM was all over the place again.

Basically, it works but there's an issue with a noisey or unstable signal, and the LM1815 is triggering at a lower than desirable voltage.

Plan A:

Here's the 0.76 Volt square-wave signal:


How to read this safely?

0.76V is a bit too low for a HALL setup. It might work, with some experimentation:

If this does not help (try to play with the values, eg. try several values from 680 Ohm to 4k7 instead of the 1k resistor).

Shaping the signal with active components (transistor, inverter, amplifier) is probably not justified.

VR setup

VR does not require high amplitude, but requires the signal to go under 0V. A series cap is necessary. Min 10uF 25V