This sound meter or sonometer was found in a Popular Electronics from the 80's. The circuit can be easily implemented today, and the microphone is made of electret or ceramic.
This circuit that uses only transistors indicates when the voltage in the circuit has the voltage...
This power supply makes use of a capacitance multiplier. C2 can be 100 uF which with the action of...
Found in an English publication from the 70's, this radio uses a germanium transistor and requires...