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 voltage-controlled oscillator or VCO was found in a Radio Electronics magazine from July...
This signal follower, from a publication from the 1970s, uses old germanium transistors such as...
This watch uses a liquid crystal display. The circuit is from the 80s and the module used is...