What is a temperature sensor?
An analog temperature sensor is pretty easy to explain, it's a chip that tells you what the ambient temperature is!
These sensors use a solid-state technique to determine the temperature. That is to say, they don't use mercury (like old thermometers), bimetallic strips (like in some home thermometers or stoves), nor do they use thermistors (temperature sensitive resistors). Instead, they use the fact as temperature increases, the voltage across a diode increases at a known rate. (Technically, this is actually the voltage drop between the base and emitter - the Vbe - of a transistor. By precisely amplifying the voltage change, it is easy to generate an analog signal that is directly proportional to temperature. There have been some improvements on the technique but, essentially that is how temperature is measured.
Because these sensors have no moving parts, they are precise, never wear out, don't need calibration, work under many environmental conditions, and are consistent between sensors and readings. Moreover they are very inexpensive and quite easy to use.
Some basic stats
These stats are for the temperature in the Adafruit shop, the Analog Devices TMP36 (-40 to 150C). Its very similar to the LM35/TMP35 (Celsius output) and LM34/TMP34 (Fahrenheit output). The reason we went with the '36 instead of the '35 or '34 is that this sensor has a very wide range and doesn't require a negative voltage to read sub-zero temperatures. Otherwise, the functionality is basically the same.
- Size: TO-92 package (about 0.2" x 0.2" x 0.2") with three leads
- Price: $2.00 at the Adafruit shop
- Temperature range: -40 degrees C to 150 degrees C / -40 degrees F to 302 degrees F
- Output range: 0.1V (-40 degrees C) to 2.0V (150 degrees C) but accuracy decreases after 125 degrees C
- Power supply: 2.7V to 5.5V only, 0.05 mA current draw
Step 1: How to use a temperature sensor
How to measure temperature!
Using the TMP36 is easy, simply connect the left pin to power (2.7-5.5V) and the right pin to ground. Then the middle pin will have an analog voltage that is directly proportional (linear) to the temperature. The analog voltage is independent of the power supply.
To convert the voltage to temperature, simply use the basic formula:
Temp in Celsius = [(Vout in mV) - 500] / 10
So for example, if the voltage out is 1V that means that the temperature is ((1000 mV - 500) / 10) = 50 degrees Celsius
If you're using a LM35 or similar, use line 'a' in the image above and the formula: Temp in Celsius = (Vout in mV) / 10
Testing your temperature sensor
Testing these sensors is pretty easy but you'll need a battery pack or power supply.
Connect a 2.7-5.5V power supply (2-4 AA batteries work fantastic) so that ground is connected to pin 3 (right pin), and power is connected to pin 1 (left pin)
Then connect your multimeter in DC voltage mode to ground and the remaining pin 2 (middle). If you've got a TMP36 and its about room temperature (25 degrees C), the voltage should be about 0.75V. Note that if you're using a LM35, the voltage will be 0.25V
(See image below)
You can change the voltage range by pressing the plastic case of the sensor with your fingers, you will see the temperature/voltage rise.
(See image below)
Or you can touch the sensor with an ice cube, preferably in a plastic bag so it doesn't get water on your circuit, and see the temperature/voltage drop.
(See image below)
Connecting to your temperature sensor
These sensors have little chips in them and while they're not that delicate, they do need to be handled properly. Be careful of static electricity when handling them and make sure the power supply is connected up correctly and is between 2.7 and 5.5V DC - so don't try to use a 9V battery!
breadboarded to-92 -
They come in a "TO-92" package which means the chip is housed in a plastic semi-cylinder with three legs. The legs can be bent easily to allow the sensor to be plugged into a breadboard. You can also solder to the pins to connect long wires. If you need to waterproof the sensor, you can see the next step for an Instructable for how to make an excellent case.