These circuits were found in Exar documentation from the 90s. The circuits use both a small speaker and a piezoelectric transducer. The integrated circuit is not common today.
This circuit is from an English publication from 1973, but can be easily assembled today. 2N718 can...
The integrated circuits used in this 1989 project are uncommon today, including the IF coil that...
We found this receiver for the 144 MHz band in a European publication from the 1970s. The circuit...