This circuit for an optical audio link using LED or LASER was found in a 90s QST magazine. The circuit can use operational amplifiers like the 741.
This circuit with an industrial standard 4-20 mA interface uses a KPZ20G pressure sensor but can...
This simple receiver with germanium transistor and listening on a crystal phone was discovered in a...
This circuit is from Motorola technical documentation from the 70s, but it can be easily assembled...