This circuit from a 1967 documentation generates 2.5 Hz signals depending on the capacitors used, in this case 160 uF. For 4 6700 pF the signal will be 20 kHz.
This source was found in a 1982 American publication, but can be easily assembled using 2N2995 (PNP...
This receiver can operate with the transmitters shown in previous articles and even others...
We got this circuit in an old 1983 publication, but it can be fitted with equivalent transistors. For Q1...