This primitive solar radio was found in a Gernsback publication from 1960. The transistor is made of germanium of the time and the earphone is magnetic with high impedance.
The simple optical intercom we show is from a 1995 documentation. The LEDs can be infrared or...
With this circuit it is possible to monitor a signal that is being recorded on an old tape...
An oscillator that can be frequency controlled by a resistive sensor is shown in the figure. The...