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From: email@example.com (john jardine)
Subject: Re: Designing a barcode reader
Date: 28 Sep 2002 10:37:44 -0700
NNTP-Posting-Date: 28 Sep 2002 17:37:44 GMT
"Philip Pemberton" wrote in message news:...
> I'm currently thinking about building a simple (Code39 only) barcode
> reader. Has anyone got some information on suitable methods for detecting
> and decoding said barcode? I was thinking about using a high-brightness red
> LED to illuminate the barcode. A lens will focus the resulting barcode onto
> a phototransistor with a metal sheet wrapped around it with a small pinhole
> in the middle. Is this how most barcode wands work, or do they use
> fiberoptics to route the light around?
> I've pulled a Cuecat to pieces before and I've seen some of
> Digitalconvergence's patents (thank you EspaceNet) but I'm still thinking
> about how I could decode the data. Would a PIC16F84 RISC MCU running 10MHz
> with 64 bytes of RAM be able to do it or will I need some more RAM?
They work pretty much that way although the pinhole isn't necessary.
Slow analogue AGC is needed to offset ambient light level variations.
Some of the pens enclose the photo tranny/diode with a outer ring of
maybe 5 red LEDs.
You should be able to decode effectively with the PIC's 64 bytes of
ram at 10MHz.
Main programming problem to watch out for is varying stripe timing as
the pen is tracked across the label. Can lead to 2:1 black/white
timing variations over even just one character (wrist accel/deccel,
muscle tremor ect). Programming wise it is easier to collect all the
data first, (just watch for the start/stop sentinels) then do the
statistics to get the stripe timings before decoding the actual chars.
(requires external memory though). For prototyping, your algorithm
can be checked out easily by using one of your PC's, LPT digital port
Manufacturers of wands with built in decoders, do their timing and
decoding 'on the fly' also finding the correct type of code to use, as
the label is being read. Most use a nothing special, single chip uP.
The projects complexity is in the machine code, decoding algorithms.
There are about 5 patented decoding methods out there. They are
pretty much all the same and of course designed to be incomprehensible
to anyone reading them.
Hewlett Packard used to have a fair application note on the actual
data decoding problem but it was withdrawn about 12 years ago. They
should still though have a couple of decent notes on the different
If after all this, you cant meet a >95% first swipe, read rate, then
you have done something wrong!.
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