This circuit can be used in an experimental link for educational purposes. based on an integrated 4093. The signal consists of an audio tone of approximately 1 kHz that can be coded in Morse with the connection of a manipulator, for example between pin 1 of the integrated and the positive of the supply. From pin 1 to ground, to keep the CMOS input at a low level with the manipulator, a 10 k to 1 M resistor must be connected open. The transistor can be exchanged for equivalent and the 75 ohms resistor determines the intensity of the emission. Note that when driving the transistor, the emission level is zero, as the system operates by shorting the LED. This technique allows obtaining a waveform closer to the rectangular one, which may be interesting for the detection system used.
