LM35

M

Midnight Sun

New Member
Hi,

I came across a box of LM35 temperature sensors. I would like to use them to control the temperature of a freezer in the range -5 to 20 C, since I got them for next to nothing. The positive temps are easy to implement, but it doesn't seem possible with the minus ones. :confused:Is this so?
In case anybody is wondering why such odd temps, it's for storing and brewing beer.:D

Cheers
Midnight Sun
 
M

marcos.placona

Senior Member
Not really sure if I understood it, but how come you can handle positive temperatures but not negative?

The datasheet says it can worn on a range of −55° to +150°C, so -5 should be easy peasy.

Do you get any sort of results at all when the temperature is negative?
 
M

Midnight Sun

New Member
The problem lies with the ADC in the PIC which should read the input voltage generated by the LM35. I don't think it can read negative voltages, but I'm not sure.
 
hippy

hippy

Ex-Staff (retired)
Also take a look in the datasheet. Figure 7 on page 7 shows how to offset the LM35 so that it becomes above 0V biased so negative output is raised upwards to being positive.

http://www.national.com/ds/LM/LM35.pdf

That circuit generates two signals although you may be able to get away with just the one from the LM35.

If you want accuracy, which I expect you do, you'll (probably) have to go to having a negative supply rail and an op-amp inverter. While the LM35 may be so cheap they're free, it doesn't mean they're the best option. Given the effort, time and cost, plus having to calibrate what you have, I'd say spend a few $, use a DS18B20, get it done quickly and get on with another project. Keep the LM35 for situations where they can be used without a lot of aditional complexity.
 
M

Midnight Sun

New Member
Yes Hippy, I have come to the same conclusion after looking at the datasheet. By the time I have added everthing else to make it work I might as well use a DS18B20. Something to do with design elegance, or was it the KISS principle.

Thanks everbody for the help and suggestions. And a good weekend to all
 
L

lahaye

Member
I noticed that one too...3 minutes later: GONE :D

Lots of thanks to the moderators. I don't know whether this happens on a daily basis or even multiple times a day but I barley notice.

BTW: ridiculous grammar; can't they show some effort?

@ midnightsun
What kind of beer are you brewing? Just curious. If the alcohol taxes in Norway are anything like in Sweden these tempsensors will pay them self back with a couple of zips.
Skol
(I just know some [barley any] Swedish but I hope that works for Norwegians too)

Florian
 
M

Midnight Sun

New Member
Hi Florian

To a Norwegian Sweden looks like paradise. The booze is taxed in Norway according to the alcohol content, with something approximating an exponetial scale. So if you buy normal beer, you save about 20% in Sweden. But if you buy a belgian Orval at 6,5% alc it costs 42 NOK in Norway, and 18 NOK in Sweden. If you buy something stronger it gets much more expensive.

I brew just about anything, british real ale, stout and porter, german ale, weiss, bock and märzen, american IPA and APA, saison, biere du gard and belgian beer. What I don't brew is lager or pils(ner).

At least once a year I hold a beer fest for my friends with 3 different beers on tap and one keg in reserve. I brew for weddings, baptisms, birthdays and parties and for any other excuse too. I dread the thought of having to go and buy some insipid swill in a shop.

I tend to get garralous when I start talking about my hobby, so it's time to stop.

cheers
Hein
 
M

Midnight Sun

New Member
Thanks a lot Eclectic, it's just what the doctor ordered! Prevents me from re-inventing the wheel, although I could have done with the programing practise, I haven't done it for years.
If you're ever in Oslo, give me a shout, you're good for a homebrew or several.

Cheers Hein
 
You must log in or register to reply here.
Top