🚀 go-pugleaf

On Wed, 18 Jun 2003 11:53:17 +0200, "Iman Habib"
<imanhabibREMOVETHIS@eml.cc> wrote:

>Hi there.
>
>I have just started out on a small project and at this moment I am at the
>point where I need a linear and rotational sensor with high accuracy
>and the sensor should not be prone to wear down after a short while.
>And the sensor would somehow be connected to my PIC chip
>that would be continuously reading from the sensor and storing the
>most recent value in the memory and later sending it of to wherever
>I need it.
>
>First I was thinking about the obvious choices.
>A variable potentiometer. But high accuracy pots with long usage
>lifetime costs way to much.
>A IR diode & IR transistor in an light isolated slider tube. But that would
>not give me linear results directly. And I think the that there would be
>an accuracy problem.
>
>After some thinking I came to the conclusion that capacitors would be
>an excellent choice. The idea was mostly due to the digital calipers
>I have. It has an accuracy of 0.01 mm and I read somewhere that
>the sensor is essentially a linear capacitor strip running inside the
>calipers slider. And I thought the actual electronics shouldn't be that
>expensive as the caliper itself costs about 30$.
>
>Also capacitor would  give me a more linear values than anything else
>as long as the surface geometry of the variable capacitor would
>increase linearly.
>
>Essentially what I want is a primitive form of a capacitor meter.
>
>Now the problem is that I have never done this before and I am not that
>sure of how it would be done and I thought some of the guys here
>may have come across similar problems.
>
>I imagine that I could use a multivibrator (555 based circuit) and generate
>and square pulse depending on a variable capacitor. And on the PIC
>circuit just increment a number from 0 to N depending on the time it takes
>between the pulse peaks that I get from the 555 circuit. Thus the resolution
>of the meter would be up to the operating frequency of the PIC (?).
>
>Would this be the way to go ?
>
>I'm unsure of how accurate it would become.
>Where are the primary places where the accuracy
>could be lost ? Maybe in the 555 circuit ?
>
>Could anyone here give me some pointers and
>help out a newbie here... please....
>
>thanks
>//iman
>
>ps. sorry for the crosspost.

What sort of resolution do you need?, as that may well dictate the
options available.
A capacitor meter using a 555 is fairly easy, and has been used in
several capacitor meter adaptors in SC and EA projects over the years.
1% accuracy is easily achievable.  Search the back issues and you'll
find some projects. 

Regards
Dave :)
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