Jinx

I just need to know how much material left on the reel to prevent the
problem that I start a long printing without knowing if the material is
going to finish before the print is done.
the resolution I need is not that accurate (about ~100-500mm) so any
mechanical wheel with a pulser will be enough.

TTYL
Tal

-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On
Behalf Of Jinx
Sent: Wednesday, September 10, 2003 2:35 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: Length measurement


> If someone know about a circuit that I can build I will appreciate
> it, I need it too urgently to involve with long development

Tal, I'd have thought something like a small pram or pushchair
wheel would be OK as the base for an encoder (but I don't think
you said what resolution measurement is required or what type
of surface the paper has so maybe not). They have soft grippy
synthetic grey rubber tyres that shouldn't slip. You could detect
the rotation several ways. eg Hall effect and magnet, holes thru
the wheel or an attached disk for an opto interrupter, or reflected
opto (paint the wheel matt black. Use strips of adhesive aluminium
foil and an opto-reflector. Tried this, it works well. Or print out a
Gray scale). Then you'd just need to calibrate it against a known
length. Which, even if you did happen to find something already
made, would be a good idea anyway. Assuming you did this and
had a wheel circumference of 250mm, make as many sensor
indicators as needed. Perhaps 5 to get 50mm resolution or whatever.
Then simply count pulses and use a bit of maths eg "multiply/divide
by a constant" to convert to a distance

http://www.piclist.com/techref/piclist/codegen/constdivmul.htm

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