We found this circuit in a spy book from the 90s. Transistors admit equivalents and the coil can be designed as in the CB19425E project. Power is provided by cells or 9 V battery.
Found in Popular Electronics from the 90s, this circuit uses a reed switch on the rails as a...
This low input impedance amplifier stage was found in a 1963 documentation. The circuit employs a...
This 2003 circuit uses two types of sensors: by reed-switch and by infrared. If the magnetic...