Thermopile Sensor

A thermopile is a device that converts thermal energy into electrical voltage.  It's what you find in those in-ear infrared thermometers or remote temperature probes used in the food industry.

there are lots of cool applications they can be used for including motion sensors, temperature probes, fire alarms, heat flow detectors, robot sensors, forge temperature monitors or low resolution thermal imaging to name a few ideas.

This Instructable will describe the practical application of a thermopile (in particular a TS118-3) and show how to get a readout using an Arduino.

I'm fairly new to the world of electronics and I found experimenting with this to be great way to learn.  I have attempted to write this Instructable so it is easy to understand by beginners and useful to experts.  Please feel free to leave constructive comments!

I will publish another Instructable soon describing how to build a practical circuit using a thermopile for a Heat Activated Soldering Fan.

Here is what you need.  Except for the thermopile which I had to buy the choice of other components was based on what I had to hand.

• TS118-3 Non-contact Infrared Sensor Infrared Temperature Sensor (the thermopile)
  • I got mine from Ebay
• LM358 Dual Op-Amp
• Resistors:
  • 3 x 1K
  • 2 x 8K2
  • 1 x 47K
  • 1 x 68K
  • 1 x 1M
• 1 x 100nF Capacitor
• Arduino
• breadboard

This is the beastie this project is all about. 
This small component contains a thermopile.  Essentially it converts the difference between the ambient temperature and the object being measured temperature into a voltage using the Seebeck Effect.  It requires no outside power to work and can measure a temperature range between 0 and 100 degrees Celsius.

Because it uses temperature difference you also need to know the ambient temperature if you want to accurately measure actual temperature.  The TS118-3 also contains a thermistor just for this purpose. 

Pins 1 and 3 are for the thermopile itself.  Pins 2 and 4 are for the thermistor.

The voltage that the thermopile outputs is measured in only milli-volts.  In fact, its only produces about 4.4mV at maximum.  This isn't very useful so we need to boost the voltage using an op-amp.

This document provides lots of useful technical information on thermopiles, how to calibrate them, factors affecting accuracy, and how to perform ambient temperature compensation with an op-amp.

The more you find out about thermopiles the trickier and more complex they seem!  Hopefully this Instructable will make using these simpler.

The op-amp I used for this project is an LM358 which actually contains two op-amps within the same chip.  It requires external power to work.
Check out the circuit diagram for how the op-amp boosts the voltage of the thermopile. 

The two resistors R1 and R2 provide feedback to the op-amp.  The amount voltage boost is set by the ratio between them.  In our case R1 is 1MOhm (1000KOhm) and R2 is 1KOhm.  1000K/1K is 1000, so the voltage is boosted 1000 times.
The maximum voltage from the thermopile is about 4.4 milli-volts which is way too small.  A 1000 times boost now makes it 4.4 volts.  This is a practical voltage for us to use.
R3 affects the sensitvity of the thermopile up to a point.

Experiment with changing these resistors to see what affect it makes.

A thermistor is a resistor that changes its resistance depending on the surrounding temperature.  It is useful to measure the ambient temperature.

The thermistor in the sensor is apparently a Ni1000 which is important to know to calculate the temperature from its resistance.

To read the thermistor output we use a voltage divider  (See the circuit diagram).  When we read the output voltage we can work backwards to calculate the resistance and hence the temperature.  We can use a lookup table or use an equation to convert resistance to temperature.  The one in this data sheet seems to be reasonably accurate.  See the picture above for the formula.

Note that the reference temperature is 0^oC at 1000 Ohms.

I hope the maths doesn't scare you off.  It took me ages to find this out so I'm writing it down here to save you the effort!

I hooked up the sensor to an Arduino to do some testing. 

I quickly realised I had a problem.  The analog pins on the Arduino can read positive voltages between 0 and 5V.  But the sensor produces a negative voltage below 25^oC.  That makes our readings wrong!  The solution is to shift the voltage.  The sensor reads about 4.4mV at 100^oC and about -0.6mV at 0^oC.  That's a difference of 5mV.  Now that's handy!  If we add 0.6V to the output of the op-amp we get a range of 0-5V to send to the Arduino. Perfect!

Do do this we use a summing amplifier circuit.  The second amp in the LM358 comes in handy after all.

In the pictures of the bread board you'll see I actually used an additional LM358 to make it less confusing to wire up and view.

The sketch code to run is attached.
The program is simple and just outputs the readings from the thermopile and the thermistor via serial port.

Note:
The circuit is very sensitive due to the sensor only outputing milli-volts.  The temperature may appear to fluctuate wildly with any interference.
The circuit itself seems to add about 0.6mV to the thermopile raw reading.  I account for this in the sketch with a constant called verr.
The sensor reads the temperature over the entire area it sees.  The sensor must only see the object being measured and nothing else to get an accurate temperature reading.

That's a thermopile in a nutshell.  Stay tuned for another Instructable using this sensor to build a Heat Activated Soldering Fan.