# Temperature Sensor help

#### zaid

Jul 14, 2013
15
i am looking for a circuit to measure difference in temperature (+ or -) on the human body.
the distance is between two thermal sensors is about 5 cm, and the reading range is between 0 -50 Celsius.
i have a circuit but i need to make it more sensible. i was thinking of adding differential amplifier.
any one has got an idea of how to improve that

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#### zaid

Jul 14, 2013
15
Hi Dave
I am using 2 sensors because I would like to measure the difference in temperature between 2 points therefore it was nessesaty to use 2 sensors.
i just would like to knowhow to cariblate the sensors

#### zaid

Jul 14, 2013
15
I just wanted to know where to put the differential amplifier in the cercuit.
any help

#### duke37

Jan 9, 2011
5,364
The meter in your circuit is connected in a differential way. This is where a differential amplifier should be connected if you wish to provide an output referenced to ground.

Two thermocouples connected back to back will provide a differential output but may need considerable amplification to feed your meter.

#### zaid

Jul 14, 2013
15
Hi duke 37
How would th he circuit would be then after considerable amplification

zaid

#### duke37

Jan 9, 2011
5,364
Thermocouples produce a voltage between a cold and hot junction. A chromel/alumel junction will produce 41μV/degC. If you can measure this, you will not need amplification. I do not understand what you mean by what circuit would be after amplification, you will get a voltage to deal with as you wish.

Some meters have a thermocouple input, one of these should not be used since it contains a cold junction that will give wrong answers.

You could amplify so that a display shows 1 for 1degC differential for convenience.

#### zaid

Jul 14, 2013
15
hi
this diagram shows how the reading will be.
when you connect one sensor it will read eight negative or positive
when you connect both it will be in the middle is there is no difference in the temperature between those two points
i need to have amplifier in order to sense the small difference between the two point
can you put that in a circuit diagram please

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#### duke37

Jan 9, 2011
5,364
I have looked up the data for the LM35 and LM335. It appears that they give an output of 10mV/degC so if you want a range of 10deg, you will need a meter to measure the resulting 0.1V, connected as in your diagram #1. If you can measure the 0.1V then you will not need an amplifier.

#### zaid

Jul 14, 2013
15
hi
i would be very grateful if you could sketch the final diagram for the circuit please

#### zaid

Jul 14, 2013
15
hi
do you think the final diagram will be like this
but i don't know how to connect the differential amplifier to the cercuit

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#### duke37

Jan 9, 2011
5,364
That looks like it will work. The meter is connected in a differential way, that is, it is measuring the voltage between the two sensors.
The sensors give 10mV/degC and the meter should be capable of showing this as a sensible number, if it is not sensitive enough, then an amplifier should be added between the sensors and the meter.

#### zaid

Jul 14, 2013
15
thanks
what changes do i need to make to get sensible number and which pin of the amplifier goes to and which one goes to sensors.
can you help me and sketch that for me please

#### zaid

Jul 14, 2013
15
hi duke 37
is that what you meant?

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• circuit 002.jpg
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#### duke37

Jan 9, 2011
5,364
The output of the sensors is 10mV/degC, your amplifier has a gain of 10 so you now have 0.1V/degC, is this better for your display? If you use a gain of 100 then you will get 1V/degC, a voltmeter with a range of +/-10V will then show the temperature directly with the decimal point in the right place.

The amplifier must not have the inputs shorted, you will get nothing out!
You may be able to run less than 15V and may be able to run on +9V only (connect p4 to ground).

#### zaid

Jul 14, 2013
15
i am stuck with the circuit because i don't know how to connect the temperature sensor because it has 3 pins, and i don't know which one is going to the human body
i will be very grateful if anyone can redraw the circuit for me and i don't know whether the circuit will work or not.
and this what i wanted to create and i am not sure if this circuit is the same
http://www.autospeed.com/cms/article.html?&title=Differential-Temp-Meter-Part-2&A=111357

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Last edited:

#### duke37

Jan 9, 2011
5,364
The LM135 gives an output depending on its temperature. It does not need to be electrically connected to the human body and in fact any such connection could pick up electrical noise and give a faulty reading. To measure the temperature accurately you will need to sleeve the LM135 and press it against the body with a thermally insulating pad. It will take a few seconds for the temperature to stabilise.

The article you have referenced seems to be well thought out and written. It also describes a suitable display. Why not follow the construction as described there?

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