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 transistors of this circuit found in an English 1980s documentation may be the modern BC548...
This circuit was suggested by a Radio Shack publication from the 1980s. The purpose of this...
This circuit was published in a 1977 Radio Shack catalog. The PNP transistor may be BD135 and NPN...