piclist 1999\10\09\012322a >
Thread: PIC temperature sensor
www.piclist.com/techref/microchip/ios.htm?key=temperature
picon face BY : Russell McMahon email (remove spam text)



Octavio,

From: Octavio Nogueira <RemoveMEnogueiraEraseMEspamKILLspamAJATO.COM.BR>
>I'm developing a temperature controller with two sensors,
>- I though of using  ....
>Any other though?
>The range is from 50F to 150F and need to be cheap.


A cheap and "good" accuracy temperature sensor can be made at moderately low
cost if you are prepared to allocate an analog input and a control pin.
OR you could use it to drive a VCO and then use a digital ionput pin to read
frequency.

The voltage drop across a diode is related to its absolute temperature.
A calibration factor is involved.

BUT

The difference in voltage drop across a diode at two known currents is also
related to temperature and is independent of anything except the physical
properties of the junction (ie - all silicon diodes give the same result). A
practical circuit will involve a switchable current source and an amplifier.
A quad package of low cost 324 opamps would very probably suffice for all
the active electronics.

I don't have a practical circuit to hand but various magazines have done
devices based on this principal or you could derive it directly. Many
libraries have collections of circuits in book form and these are nearly
certain to include a practical circuit.

I'm probably going to be doing something based on this in the next few
months but not just yet.





     Russell McMahon
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