I recently got 2 old PCs (Windows XP Era) from work. They were throwing them out, and I grabbed them. I really just wanted the power supplies, but as I took them apart, I realized just how much useful stuff they have inside
Lets start with the power supply. 35A-50A @ 5v… You could charge A LOT of USB devices at once with that. You could also use the 12v side to power things made for cars. Such as: Car Stereos, A Car Amplifier, CB Radio, Ham Radio, LED Lights, Headlights for a super bright work light. Hobby RC Battery Chargers. You could even put in a current limiting resistor and use it to charge your car battery…
Then you have the CPU heat sync. That is a match made in heaven for one of those flat super high-power LEDs you can get from ebay… You could also carefully cut it into pieces, and use it for TO-220 heat syncs.
The CPU Fan, and optional Case Fan could be turned into a small working hover craft.
The hard drive contains a few super strong Neodymium magnets. A brush-less motor, and usually a small solenoid. Not to mention the case is made of solid aluminum that you can melt down to cast your own stuff.
If it’s old enough to have a floppy drive, there is another stepper motor for you.
The metal case provides some sheet metal you could use to bend your own enclosures if you get a brake. You can make a shelf out of it (even I’ll admit that’s kind of ghetto…)
The old IDE and Floppy cables make great connectors to breakout the GPIO header on a Rasp Pi or other SOC computers.
Some computers have a tiny speaker attached to the motherboard pins for that annoying BIOS diagnostic beep on boot. Others have a larger speaker in the front grill.
From the motherboard, you could de-solder the usb connectors for the aforementioned device charger. The audio ports could also be used to connect a portable device into the car amp you’re powering from the power supply as well.
If you were really adventurous, you could use the onboard DVI and HDMI connectors to make your own HDMI to DVI adapter. Here is a schematic to get you started.
And all the other random bits are great for a day at the range :)