This circuit was originally recommended to give the call signal on intercoms. However, it can be used with other applications. The circuit is from 1978 documentation. The transistor may be the BC548.
Found in an Electronic Design from the 80's, this circuit monitors the charge of a 12 V battery....
Found in an EEE from 1970, this circuit supplies a voltage of 12 V DC x 10 mA current from the...
Using old germanium transistors like the CK722, this circuit has good gain. We found it in a...