This circuit with the ULN3784B appeared in the October 1984 issue of Radio Electronics magazine. It can typically be powered from 9 to 24 V.
Frequencies up to just over 100 kHz can be obtained with an oscillator based on one of the four...
This circuit, found in a Popular Electronics from 1992, is based on an oscillator and a crystal...
This is a discrete equivalent version of an open collector TTL gate. What we have is an inverter...