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 was found in a Popular Electronics from the 90s. The circuit has temperature...
Found in old American documentation, this receiver transmits signals via the power grid. We can...
This circuit is from the LM317's own manual obtained from an old documentation. The IC should be...