This circuit digitally counts input pulses. The circuit from a November 1970 Radio Electronics uses unijunction programmable transistors. Note the counted pulse duration.
In fact, we can say that this circuit found in documentation from the 90s, consists of an intercom...
The basic integrated circuits of this 1988 multimeter are no longer very common today, but if...
This circuit is from an old publication of the 70's. It can be implemented with modern transistors like the...