This circuit is from an American documentation from 1975. The single junction transistors can be 2N2646 and the signal must be applied to the input of a good amplifier.
This sensor circuit from a 1967 Ferranti documentation uses components from the era. The circuit...
This is the receiver for the previous circuit transmitter. Similarly, no information about the...
Found in a 90s Popular Electronics, this circuit uses two PLL tone recognizers to trigger by touch an...