This circuit was obtained from an 80's EDN. The components used are still common. The circuit can be used as a shield and the frequency is approximately 775 Hz.
This circuit from Popular Electronics powers a small speaker. The transistor can be of general use...
When the sensor X1 is opened, the relay is energized. When waiting, the current drained is very...
This circuit for measuring bridges was found in the October 1995 issue of Electronics Now...