With the configuration shown in the figure, we have the relay being activated when the microphone picks up a sound and shutdown when the X1 reset sensor is activated.
This circuit of a 1978 American publication monitors a telephone line by triggering a relay. The...
This interesting FET-tester circuit was obtained from an old American documentation. The bipolar...
This old VHF TV booster that also serves FM was found in Radio Electronics magazine from April...