Almost all my doodads produce debug output on a serial port, but it's a pain in the ass to set up a computer with the right cable, etc., so I made a battery-powered serial monitor with two wires (RX and GND) and a level shifter with optional signal inversion so it should work with any serial signal. This could be much smaller and prettier with a custom PCB but I'm not willing to pay the bastards $45 to ship me a 50¢ board. Anyway here it is hooked up to my nixie clock.