In the figure we have a basic shield circuit with TTL to TTL optical isolator. For the emitting LED the voltage can be different from 5 V.
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...