Detecting Black area on White Paper

Hello,

I have 300 foot rolls of printing paper on a motorized roller. I am using a 12VDC motor to advance the paper onto a take-up roller. Every time I hit the run button I would like the paper to advance 48 inches and automatically stop at that point until the next time the run button is pushed for another advance of 48 inches.

My question is; If I have a wide black line on the paper roll every 48 inches, is there a photo sensor on the market that can detect that black line on the paper to stop the motor at that point?

I thought about using a simple light sensing photo cell but am afraid it will now work because of ambient light. Perhaps that would work if I put the sensor in a tube to block light and set the tube tight against my paper. Not sure.

I would appreciate any helpful information.

Thank you,

Bob
Adroit Manufacturing

If your motors/gears/etc are pretty accurate..ie...not alot of slippage, what about using a magnet on the shaft of the roller and use a hall sensor to count the number of rotations. You may have to use more than one sensor or magnet and calibrate it, but then you don't have to worry about ambient light affecting the reading. Also you would probably be better off using a IR tx/rx pair (Fairchild makes some among others) that are designed for this type of application. The amount of attenuation by the color will let you know when its dark vs white. Your stripe will have to be wide enough to detect it as well.

A way to get around the ambient light issue is to modulate the light source at a specific frequency and use a bandpass filter in the detector tuned to that frequency.

For example, turn an LED on and off at a 10 kHz rate and incorporate a 10 kHz bandpass filter in the detector circuit.

The modulation frequency needs to be coordinated with the black line width and the roller speed. This is so the detector bandpass filter detects a sufficient number of cycles to respond. Let's say the line width is 0.06" and the roller moves at 6 inches per second. The time to detect the line is 10 milliseconds. If we want 100 cycles in 10 milliseconds the modulation frequency is 10 kHz.

Looking at Digikey I see many photodetectors that could work.

http://www.digikey.com/product-search/en/sens...nsors-reflective-analog-output/1967052?k=opto

The Honeywell HLC1395 is an example of a short range sensor. It is designed to be placed 0.04" from the surface. I think it could work well with a 0.05" wide black line.

http://sensing.honeywell.com/index.php/ci_id/50389/la_id/1/document/1/re_id/0

Another example of an infrared sensor with daylight blocking, which will not be affected by ambient light:
TCRT1000 or TCRT5000 Reflective Optical Sensor

These are currently 10 for a (GB) pound from China on Ebay if you want some to experiment with, but a supplier in your own country will get them to you quicker. (Delivery from the far east can take a few weeks.)
You can very quickly google up the datasheet.