This space mark demodulator for FSK signals was found in an American telecommunication documentation from the 1980s. The circuit operates with 2025 Hz and 225 Hz mark space signals.
The current accross the LED as function of the resistor value is given by the formulas. The...
This small transmitter appeared in the March 1989 issue of Radio Electronics magazine. The...
This circuit was found in a 1967 Electronic Design. The FET is from that time. Capacitor C is...