This filter for receiving audio in telegraphy was found in a magazine 73 for radio amateurs, in an edition of the 80s. The circuit is tuned to 700 Hz and has a 200 Hz bandpass.
Using an integrated circuit from the time, this circuit was found in Radio Electronics from the...
This shield circuit is from old documentation. The operational amplifier used is not common today...
This circuit powers a small 5 W lamp that goes out when power is lost. The circuit can use the...