This circuit is from the time when cars used contact breakers. This 80's version measures the dwell angle of the contact breaker for dwell adjustment using an era integrated circuit, the LM3917.
This circuit is from an old publication from the 90s, but it can be adapted to operate with LEDs....
This squelch circuit used in transmitters was found in a magazine for radio amateurs 73 in the...
daptation of a common multimeter for the measurement of high voltages with high input resistance,...