Step 1: Materials
Breadboard (You could make a PCB if you want)
Any NPN transistor
10k resistor ( Or trimpot for variable sensivity)
A LED ( You choose the color)
2X AA battery pack or any 3v battery
Step 2: Build the Circuit
Build the circuit on the breadboard!
Step 3: Done!
Done! Now turn off the lights and it should light up.
Step 4: Stripboard Version
JaiS5 made it!