This circuit digitally counts input pulses. The circuit from a November 1970 Radio Electronics uses unijunction programmable transistors. Note the counted pulse duration.
This is a combined circuit of tone generator and preamplifier. We found it in a 1975...
We found this simple alarm circuit in a 1986 documentation. The circuit uses sensors of the NA...
We found this circuit in the Linear Applications Handbook by National Semiconductor. The base is...