Construction is simple.
All the components are fixed in a plastic case with a hole cut in the bottom and top cover to allow the tool to be magnetized or demagnetized placed within the coil. The 150 ohm resistor is used to limit the current in the LED's to a safe value, while the two 1000 PIV diodes are installed because the inverse peak volts which can be very high. A hot glue gun is very handy to fix and insulate the small components even the coil can be fixed that way or you can used an epoxy resin cement. Switch the switch to the diode section and one LED will light and the tool will be magnetized. To demagnetize, switch to the straight through position and both LED's will light and the tool will be demagnetized.