This shield circuit is from an American documentation from 1969. The components can be modified according to the relay and power, and the coupler can be of a modern type.
In this circuit found in a 1991 publication we can use the 2N2646 unijunction transistor and the...
This microphone preamp allows the use of high impedance crystal microphones with amplifiers that have...
This listening transmitter uses an integrated circuit that is uncommon today. The circuit was...