Found in a 1964 publication, this circuit makes use of transistors from the time. It operates on frequencies up to 3 kHz.
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...