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ellizard
02-24-2007, 04:05 PM
HI people,

I'm looking for a non touch system to read temperature of an object,
I'm thinking at using an IR fototransistor, with it's sensible area aimed at the object.

It's this feasible?

any idea?

Regards
Stefano

Beau Schwabe (Parallax)
02-24-2007, 05:40 PM
Stefano,

Forest Mimms has an example of doing this using the reflector from a flashlight and substituting the bulb with an IR phototransistor.

If I remember right, the circuit actually used two IR phototransistors configured in a whetstone bridge where the local or ambient
temperature was used as a reference to calculate the remote temperature based on the result of an Op-Amp output.

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

ellizard
02-24-2007, 05:59 PM
HI Beau,

Thanks for the prompt reply,

Any more links?
I have no access to Mimms book and a search on the net gave me only references to the book

Regards
Stefano

Beau Schwabe (Parallax)
02-25-2007, 01:25 AM
Stefano,

I was going from memory, I'll see if I can find something...

Edit


There are a few links here that use a thermister as the sensing element rather than a IR photo transistor in a wheatstone (whetstone?) bridge configuration.

www.thinking.com.tw/db/pdf/ntc/Application.pdf (http://www.thinking.com.tw/db/pdf/ntc/Application.pdf)
www.bbc.co.uk/scotland/education/bitesize/higher/physics/elect/analogue3_rev.shtml (http://www.bbc.co.uk/scotland/education/bitesize/higher/physics/elect/analogue3_rev.shtml)
www.selcoproducts.com/PDF/ntc_pdf/ntc_appl_notes.pdf (http://www.selcoproducts.com/PDF/ntc_pdf/ntc_appl_notes.pdf)


As far as focusing IR radiation, this can be done in a similar way that you would focus visible light.

www.glolab.com/focusdevices/focus.html (http://www.glolab.com/focusdevices/focus.html)


Note: Something I might point out. Most wheatstone (whetstone?) bridge configurations don't account for any local ambient temperature that might affect the
temperature reading. A simple way to compensate for this is to use two sensors, one in the top and one in the bottom leg of the bridge and allow only
one sensor to be exposed or focused to the remote IR interference you want to measure..

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 2/24/2007 7:55:29 PM GMT

Franklin
02-25-2007, 03:13 AM
Take a look at www.melexis.com (http://www.melexis.com) one of their devices was used in an article in the Feb 2005 Circuit Cellar magazine.

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- Stephen