This 5.1 MHz circuit was found in a 1980s documentation. The frequency is given by the crystal and general purpose transistors can be used.
This circuit shows how to use an LED as a photodiode controlling a signaling LED. As the LEDs can...
This circuit was obtained in a 1969 Mullard manual. The transistors are from that time. The input...
This alarm does not interrupt its operation when the power is cut. A battery is kept at a constant...