This circuit retriggers when a new pulse arrives. It does this by discharging the capacitor. The circuit is from a documentation from the 80s. The transistor can be any general purpose one.
Varactor diodes are our varicaps of today. This circuit for switching signals in an RF circuit was found...
This Hughes circuit is from 1968. Equivalent modern components can be used. The input impedance is...
This circuit digitally counts input pulses. The circuit from a November 1970 Radio Electronics uses...