[aklug] Re: Making a temperature sensor

On Sat, 21 Nov 2009, Bob at Estiminc wrote:

> I'm interested in making up my own temperature=20
> sensors using National Semiconductor LM34 IC's.=20
> It looks like a transistor, a small can with=20
> three wires coming out of it. Connect one wire to=20
> ground, the 2nd to 5VDC and the third puts out a linear +10.0 mV/=B0F.
>
> spec sheet: http://www.national.com/mpf/LM/LM34.html#Overview
>
> My idea is to solder up the cable connections,=20
> shrink some heat tube around each leg, slip it=20
> all into a short length of 1/4" copper tube and=20
> seal it all up with some sort of potting compound.
>
> I have a couple of questions. First of all, does=20
> this sound like a good approach? Any better ideas? Hints? Tips? Tricks?
>
> Secondly, I'm looking for recommendations or=20
> thoughts on a good potting compound -- something=20
> thermally conductive but electrically insulating.=20
> Is RTV silicon a suitable candidate?
>
> Bob Crosby

You might want to take a look at the Dallas one wire devices. Serial
interface, bus capable. As I recall you can chain multiple units off of a
bus and interface with the bus via simple interface to a std serial port.

The Dallas devices report a numeric value via serial but, no worries about
using an ADC, or calabration/voltage drop.

As to the potting, silicone is not a great thermal conductor... You could
mix in powdered metal (like they do with epoxy), but your need to be
careful as it is then conductive.

RTV also releases acids while setting, it can corrode things.

One thing I have seen... Aluminium or copper bonded to the sensor, and
passing _through_ the potting compound. Think about those old transtor
heatsinks that were just a metal strip, with one end wrapped around the
device body. Keeping in mind, the more mass, the slower the responce will
be.
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Received on Sun Nov 22 13:38:01 2009