Found in an Electronic Design and 1970s, this circuit uses transistors of the time. The transistors can be replaced with equivalents and the relay is 12 V depending on the power supply.
This circuit of a 1978 American publication monitors a telephone line by triggering a relay. The...
Found in 1965 General Electric technical documentation, this half-wave circuit controls shunt type...
The power supply shown in the figure provides voltages between 0 and approximately 15 volts, with...