I added a transistor in line wirh the voltage reference on a 24 to 12 volt power supply. With a zener diode and resistor that progressively brought the output up with input between 22 and 28V, I made it into a solar charge controller for a 60 cell 240 Watt panel for the motorhome. It worked great and closely matched the panel peak power curve keeping the panel voltage high for any input power. It has been running trouble free for a couple years now.
Not electronics, but related. Converted disposable Freon tanks into high power t shirt cannons for an engineering challenge. www.youtube.com/watch?v=9Klxqav_6NM Free tanks counted as part of the bill of materials, where cost was part of the contest. I'm in the blue shirt with the initial prototype at the end of the video.
Salvaged a 0.001Mhz crystal osc to use in an electronics shop. Used it for a reference for adjusting tape decks for speed and wow and flutter. A free crystal audio reference was much better than a reference CD with a short tone track. It was mush more stable than any shop function generator we had at the time. In a pinch it doubled as a stable square wace source to use for TDR with a scope.
A Hall sensor from a broken PC fan coupled with a 9V battery and a couple LEDs made a quick magnet sensor to check relay states in equipment for quick troubleshooting. Coupled with a scope, doubled as a tach for brushless DC motors.There is more I can't think of at the moment.