This circuit was found in a Hands On Electronics from the 90s. It turns on an LED to indicate that the phone is in use.
For the 600 kHz range coil is formed by 35 turns of 28 AWG wire on a ferrite core with a diameter...
Found in a 1970’s documentation, this circuit uses components of the time, uncommon today. |...
This 2003 circuit uses two types of sensors: by reed-switch and by infrared. If the magnetic...