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PICList Thread
'Optical pickup'
1999\10\26@004218 by Geoff Child

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<x-flowed>I am making a no contact tachometer using a pic for a model airplane and am
trying to find a reflective sensor for the pickup. The ones that I have
looked at have to be to close ~.150 inches and I would like to be several
inches away. Does any one know of any any sensors?

Thanks

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</x-flowed>

1999\10\26@164015 by Andy Kunz

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At 11:37 PM 10/25/1999 -0500, you wrote:
>I am making a no contact tachometer using a pic for a model airplane and am
>trying to find a reflective sensor for the pickup. The ones that I have
>looked at have to be to close ~.150 inches and I would like to be several
>inches away. Does any one know of any any sensors?

Did one already.  You don't need to be reflective.  You can simply use a
photo diode (or an LED - somebody will explain how this works) to sense the
light.

The diode simply gets connected across an op amp, and voltage changes
according to light.

This is why, btw, things like the Tower tach tell you NOT to use them
indoors with flourescent lights.  They see the lights pulsing and report
THAT as the RPM (try it - it works!)

Ask private if you have any questions.  I can't share everything, but the
basic ideas are good.  You also want an AGC on the output (a cap &
resistor) to sense the changes.

Andy

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1999\10\26@190127 by Brian Kraut

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I remember from my RC days that the commercial tachs for airplanes could be
used a few inches away.  They also read light through the prop, not reflected
from it.

Geoff Child wrote:

> I am making a no contact tachometer using a pic for a model airplane and am
> trying to find a reflective sensor for the pickup. The ones that I have
> looked at have to be to close ~.150 inches and I would like to be several
> inches away. Does any one know of any any sensors?
>
> Thanks
>
> ______________________________________________________
> Get Your Private, Free Email at http://www.hotmail.com

1999\10\26@194931 by Wagner Lipnharski

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You can use a reflective label on the prop, or just reading the light
intensity  difference through the prop. To improve that technique, just
attach to the prop a label half white, half black, so it will helps to
create a discrimination of light / dark even in different ambient light
conditions.

Wagner.

Brian Kraut wrote:
{Quote hidden}

--
--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm

1999\10\26@212927 by Gennette Bruce

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'Look' at the problem a different way - you are shining a light onto a
reflector and measuring when you get a little / lot back.  In ambient
conditions there is already a *LOT* of light around so you have to get close
to to be able to pick up the *SMALL* increase when the reflector passes.

So - how can you get more light?
 1. use different light to the ambient - ultra violet is good (otherwise
your TV remote wouldn't work in a brightly lit room).
 2. increase the concentration of the light onto the reflector with a
focusing system (don't forget to re-calculate the focal length of your
selected lens from 'natural' light to UV wavelengths).
 3. eliminate the ambient light by collimation (put a shade tube over your
reciever to make it highly directional).

Bye.

> {Original Message removed}

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