# Help Using the LM35

#### aut0pilot101

Joined Feb 23, 2009
8
I want to read voltages from this temperature sensor. However, I need to hook up a resistor to the device's output to read temperatures below 0 (see bottom of page 1 at the link provided). The other end of this resistor needs to connect to -Vs, where Vs is my supply voltage (5 or 12 V). What's the easiest way to generate this negative voltage?

#### SgtWookie

Joined Jul 17, 2007
22,201
Look on page 7, Figure 7. It shows a simple way to use the LM35 with a single supply.
[eta]
You can also use a 1N4148 switching diode; they are considered basically equivalent to the 1N914.
If you don't have any, if you're in the USA, your local Radio Shack carries them in 10-packs for a couple of bucks. Expensive, but convenient.

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#### aut0pilot101

Joined Feb 23, 2009
8
Thanks! I totally missed that diagram. So do you think that will work with both Vs=+5 and Vs=+12 (with the 18k resistor)?

#### SgtWookie

Joined Jul 17, 2007
22,201
Depends upon the range of temps you want to measure.

Did you read about what the mV per degree ratio was? If not, go find it and post back.

It helps a lot to read through the datasheets a few times; it's often that you'll miss things on the first scan-through - or perhaps just not understand what it's trying to say. Then that lightbulb will come on...

#### aut0pilot101

Joined Feb 23, 2009
8
It says 10.0 mV / degree Celsius. I will be measuring -25 to 50 degrees, approximately. How does this effect the value of the resistor? Thanks for responding so promptly, BTW! It's crunch time and you've been very helpful.

#### SgtWookie

Joined Jul 17, 2007
22,201
It says 10.0 mV / degree Celsius. I will be measuring -25 to 50 degrees, approximately.
OK, so multiply 10mV by -25. What's your result?
Multiply 10mV by 50. What's your result?

Add the absolute value of the two together. Result?

What do you think the Vs range minimum is after those calculations?

How does this effect the value of the resistor?
The resistor won't change if you use 1N914/1N4148 diodes. It's already calculated for you; 18k +/- 10%. You could use two 9.1k resistors in series if you wanted to.
Thanks for responding so promptly, BTW! It's crunch time and you've been very helpful.
You just lucked out to get a reply so quickly; this isn't really an interactive board. It can be hours or days before one gets a reply, depending on the question and who's around.