“This is a simplest EM field detector you can find on the internet. I designed it myself and it’s explained how it works in the next step.
Basically what you will need, are two transistors some resistors, antenna for example made from a copper wire like mine, 3v coin battery, PCB, LEDs and a switch.
NPN transistor BC547 x2
100 Ω resistor x2
Antenna (mine is from copper wire)
3v coin battery
Coin battery holder (mine is 3d printed)
Any color led x2
optionally a cover”