Active audio filter (CB21770E)

This circuit was found in a 1977 documentation for radio amateurs. Integrated circuits are common operational amplifiers.

Audio crossover filter (CB23743E)
We found this active crossover in a Radio Electronics magazine from June 1961. Note the different...
Output indicator 555 (CB20967E)
Found in Popular Electronics from the 90s, this circuit serves to indicate the output level of a...
Serial Driver RS232S (CB17022E)
If you are on an old PC that you use for experiments, control or other applications, this circuit is...