Found in a Popular Electronics from the 90s, this circuit can act as a transmitter for an audio link. The output transistor admits a higher power equivalent, depending on the LED.
In this circuit of a 1991 publication, we can use the 2N2646 unijunction transistor and the sensor...
Measuring capacitances and resistance this simple instrument was found in Radio Electronics...
Found in a 90’s 303 Circuits, this circuit uses components that are still common and has a common...