Logic probes are great for checking signal levels on traces, component pins. They may be packed with more features if desired. Anyway here is a simple probe designed by matseng. It is based on PIC10F202 6-pin microcontroller. The PCB takes only 5mmx5cm space which nicely fits in to ballpoint pen.
Logic probe has 5 LEDs where one is power and four for indicating purposes. Probe input is protected with double diode clamps. Power supply is protected from reverse polarity using MOSFET. Interesting thing, that probe doesn’t have any buttons for selecting functions. Author plans to use probe tip for command input by simply taping it to power/ground.