Use an eight bit (isa) card witht eh parallel port or connect it to a microcontroller.
Computer case coat hanger.(and a variation for pants.
Memory from old personal computers can be used with the Arduino.
Thermistor from a computer fan or old dryer could be used as part of a temperature sensor.
Arduino equivalent from the past.(1990's) ports 16 out 12 in 4 analog 2 async etc etc can be used by qbasic.
Use old mice for robotic wheel encoders and the balls as part of a 3/4 " pvc based check valves for home made pumps.
Build your own private plug in play internet network with an old router and access point..
Turn an old pxe laptop into a pxebootable thin client.
Use that old composite monitor with your Arduino using the TV.out software.
Use an old AT (not atx) power supply to power your microcontroller projects
On some older legacy machines you could upgrade the cpu to a faster speed and in some caseddifferent brands (socket 7) On pentium 2's and 3's you could switch out faster processers, but they had to be the same brand. Amd socket a's also would let you do the same thing. we always kept spare cpu's for just such an occasion.
Use parts from and old scanner to make an cnc machine.
Learning electronics or like to do prototyping. instead of spending 10 -20 dollars for a jumper wire kit, just use the wires from an old discarded ethernet cable.
Simple piece of pvc pipe and a nut makes camera amd tripod more valuable.
This old network hub (predecessor of the switch) can be reused for spare parts, (voltage regulator. leds, and etc.)
Build your own computer from scratch i.e. http://chaokhun.kmitl.ac.th/~kswichit/68k/68k.html
Old case for bench testing computer parts before final installlation.
Yet another use of the usb backplane adapter tp make a powered usb cable.
Convert a dual usb backplane adapter to a female to female usb gender changer. This cable helped me with my Nexus 7 to add a keybioard via a homemade otg cable.
Printer into stoarage area.
Yet another hd clock.
Switches can always be resued. Contact switch would be good for a robot to detect a collision.
Battery holers can be used to power light electronics projects.
Harvested connector from dead power supply probable saved a bit of money and time.
Old drive frames can be used with new home made cases or testing boards.
Keeping an old pc around just for the parallel port is a good thing for programming micro-controllers.
Used the case from the scanner you pulled the parts for a cnc as a coffee table.
Use an old P! as diskless thin client. (yes it needs a case!)
Use an old sega dramcast to learn nix via netbsd.
Maximize resources Upgrade one machine and then take the old parts to enhance other equipment.
Run the latest Slackware linux on an old P1-233 computer.
Battery holders from dead motherboards are great for using with micro-controllers.
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