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I recently acquired an old electro-cautery unit. Think it is from 1910s or 1920s. Has a big old multi tapped torodial transformer running to a couple of connectors/poles. It is by F. Geiger of Philadelphia. Wood case is labeled 110V. 60 CY. and 4V. 100A. Seriously old school cool and a little horrifying.

Insulation on power cord was brittle as was insulation from secondary to pole. Both have been replaced. I used crimp connections on the primary wires (didn't measure gauge but it's on the order of 20 AWG). I was in a hurry to test and will eventually solder it properly. I added a switch to the new power cord.

It works with output voltage varying from 0.1V to 4V. It gets a woodburning tip glowing orange at setting 4 (2 volts or so). Should make a great woodburner and be useful if I ever need to interrogate someone.

There isn't really a way to ground it and the screw on the control knob is electrically connected to the secondary. I have a gfci adapter that I could plug it into. Basically, if the transformer shorts, 110V could be exposed on the face.

There are two holes on the face plate, the power cord is running through the largest and the other is empty. It may have had a foot pedal.

I want to add a RGB indicator light for voltage level using the small hole. It's a little tricky to tell when the tap is lined up and you're getting voltage. A green - yellow -red fade and maybe white for NC. I have a spare Arduino Nano clone, but am not sure what circuit to rectify the AC with for the ADC. Maybe an Op amp follower? Power supply would be a 110-5v usb charger board. I would prefer to avoid a dual rail supply or higher than 5v.

Thoughts and suggestions appreciated!