I had a sudden need for a logic probe. Here’s how to build one in 15 minutes.
Get an Element 14 pen. IT MUST BE FROM ELEMENT 14. No other normal biro will work.
Cut off the clicker end with a junior hacksaw and discard the useless innards.
Drill a 2mm hole in the side. Stick a black wire through and terminate with a crocodile clip.
Cut a single header off a header strip. Solder on a red wire.
Pick a soldering iron you don’t particularly care for. Melt the plastic of the header until it fits in the end of the pen.
Solder an LED and appropriately valued resistor onto the red and black wires. Use heatshrink to cover up your soldering crimes.
Use polymorph to secure the LED into the end of the pen.
Hey presto! Cheap-and-dirty logic probe.