I am going to make a barcode reader.  This is the chronicle of that journey.  Step 1.  Create a simple test circuit to read barcodes.  The basic Idea I am going to use is an LED will be shined onto a small spot, and the reflected light will be sent back to a photo transistor.  The output of the phototransistor will be fed into an amplifier (probably a Schmitt trigger) and viewed on a logic analyzer.  This initial test will help me to develop a feel for how the barcode reader will function, and what ideas work and don't work.

10/23/05

So I built a simple "wand" barcode reader.  Here is the basic idea: A LED provides a bright source of "focused" light.  The light is then bounced off a small portion of the barcode into a phototransistor which registers the small changes in light and dark by the amount of reflected light received.  One of the challenges was how to get the light to focus on a tiny area and to get the reflected light back to the photo-transistor without having the source swamp the receiver.  This was my simple solution:

So I grabbed one my children's wooden blocks (sheepish grin) and set to work on fabricating a prototype out of wood to see if the idea was even "half baked" so to speak.  After about 10 minutes, I had this:

That's 1 infrared LED and a phototransistor.  The alligator clips give it (and the test barcode) some scale.  The cube is approx. 1 inch on each side.  To test the theory, I rigged up this super simple test circuit:

I then ran the barcode reader over the barcode and watched the scope trace.  JOY OF ALL JOYS IT WORKS!!!  Here is a picture of the scope trace superimposed over the barcode it read.  You can clearly see the difference between the thick and thin bars.

But I was not happy with the result, this barcode above was HUGE and a regular barcode will be much finer.  Therefore I decided to redo the barcode reader (because a small mistake had made the reader aperture much larger than I had wanted).  I made a new reader with another wooden block (another sheepish grin, "What!  they have a whole bucket and I am short on building supplies") and went online to find a real barcode.  I printed one off (about 3 1/2 inches long and 2 inches tall, still not perfect, but compared to my homemade barcode, it was a major difference) and ran the old barcode reader over the barcode.  This was the result:

The scope trace is backwards from the barcode because I swiped the reader from right to left, but if you look closely, there is no distinction in the first two humps which are a scan of 2 thick lines, 4 thin lines, a thick line and one more thin line, with varying spaces between.  This simply will not do.  I then modified the new readers aperture to an opening of approx. 1 mm. using electrical tape (very high tech)

After the modification, I was surprised to see that the reader still worked (wow) but more importantly, it worked well!  below is a superimposed composite scope trace on top of the horizontally flipped barcode (because I again read it from right to left)

Its not perfect, but it could work.  The Volts / Div setting on my scope was 1 V/Div so that's actually a pretty significant wiggle.  With some better circuitry, It could probably provide a TTL output for Light / Dark.