Moving cable sensing

[ACDC]

Does anyone have a good method or idea on how to detect a moving small diameter cable (0.050 inch)? <p>The cable raises a window blind and goes through
pulleys so nothing can be attached to the cable.<p>Basically I want to trip two limit switches at full open or full close of the blinds by sensing a black and white colored transion on the moving
cable. Csd's may be too slow of a response time.
Tried a IR photo diode and dectector but not sure why it did not work. <p>Thanks in advanced.

[haklesup]

I think you are on the right track by putting black and white contrasting bands on the cord. This is essentially a linear optical encoder.<p>The IR LED/Detector depends on line of sight between the two parts (transmitted light). Since the cord blocks the beam all the time, it would not work. You need to use a detector in the visible optical band and use reflected light. so the the light source and detector would be on the same side.

[ACDC]

Thanks for your comments. I am using a IR detector and emitter oriented at 90 degrees
and "bouncing" off the wire.

[haklesup]

It's difficult to tell how much contrast two colors will have when viewed in IR light. The paint you used may not be good enough. Try glitter nail polish or some other paint with high reflective qualities. Or use a carbon black for high contrast. Use a lighter to make soot on a piece of Al foil then rub the soot on the cord<p>Get one of those infrared sensor cards from radio shack (276-1099) so that you can see the beam and confirm that it is present. It has a phosphor spot that glows when hit by an IR beam.<p>Another thought had to do with the width of the light beam vs. the width of the cord and what kind of background there was. These factors would influence how much light reached the sensor as it was reflected by the cord vs. how much light reaches the sensor from other scattered light sources (the signal to noise ratio SNR). You may need to put an oscilloscope on the sensor output as the cord passes by to see how much signal you have. After that you can adjust the sensitivity of the sensor circuit ( a comparator would do the job) to match the SNR.<p>You can either have no background (open frame)or enclose the sensor in a dark box. Use black felt or carbon black on the inside of the box to minimize scattered light from the source getting to the sensor.<p>Chris