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For a pet project, I am trying to fly a kite using my computer. I need to measure how far a cord extends from a device. I also need to somehow read out the results on my computer. So I need to connect this to my pc, preferably using something standard like USB.
Since the budget is very small, it would be best if I could get it out of old home appliances or build it myself.
What technology do I need to make this measurement?
Sure, here are a couple of choices for you:
For high end, you can look at a 200 counts per revolution rotary encoder like this one:
$30 Sparkfun 200 Counts Per Revolution Rotary Encoder
You'll need a microcontroller like an Arduino to count the rotations, there's some sample code to play with.
$5 Adafruit 24 Counts Per Revolution Rotary Encoder
Cheaper, not quite as much community support in comments.
For a little more fun, you can combine the rotary encoder with a gearmotor so you could control the kite string with one piece of equipment.
Maybe you could take one from an old computer mouse, and count the light pulses directly from the mouse board so you could tell the length of the cord.
I like @kramer65's answer but because you indicated you might like to make an encoder, I thought I'd share some information on that.
http://www.societyofrobots.com/sensors_encoder.shtml shows a very common way of making a rotary encoder. You attach a disk with evenly spaced holes or reflective surfaces depending on implementation to your shaft. You align a light sensor with the holes and count the number of holes that pass to determine how much the shaft has turned. This has the disadvantage of not being able to determine direction but there was ways to modify the design to add that feature.
More information can be found in these places:
