Senseur Température

Présentation

Introduction

An analog temperature sensor is pretty easy to explain, its 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), bimetalic 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 consistant between sensors and readings. Moreover they are very inexpensive and quite easy to use.

Quelques informations

These stats are for the temperature sensor in the Adafruit shop, the Analog Devices TMP36 (-40 to 150C). Its very similar to the LM35/TMP35 (celsius output) and LM34/TMP34 (farenheit output). The reason we went with the '36 instead of the '35 or '34 is that this sensor has a very wide range and doensn't require a negative voltage to read sub-zero temperatures. Otherwise, the functionality is basically the same.

• Taille: boitier TO-92 à 3 broches (similaire à un transistor)
• Prix: ~2.50 eur disponible chez MCHobby
• Gamme de température: -40°C a 150°C / -40°F a 302°F
• Tension de sortie: 0.1V (-40°C) to 2.0V (150°C) mais la précision diminue après 125°C
• Tension d'alimentation: 2.7V a 5.5V
• Courant de charge: 0.05 mA
• Fiche technique

Comment mesurer la température

Comment calculer la température Il faut donc convertir la tension analogique en degré. Comme le TMP36 permet de mesurer des température négatives, le 0 degré Celsius est placé à une offset de 500 milliVolts. Ainsi, toute mesure inférieur à 500 mv correspondra à une température négative.

La formule est la suivante pour le TMP36:

Temp en °C = ( Tension_de_sortie_en_milliVolts - 500) / 10

Donc, si la tension de sortie est de 1 Volts, la température correspondante est de
(1000 - 500)/10

Soit 50 degrés Celcius.

si vous utilisez un LM35 ou similaire, la température se calcule comme suit (utiliser la ligne 'a' sur le graphique):

Temp en °C = ( Tension_de_sortie_en_millivolts) / 10

Utilisation de plusieurs senseurs

Vous pourriez rencontrer des problèmes si vous utilisez de multiples senseurs analogiques. Problems you may encounter with multiple sensors: If, when adding more sensors, you find that the temperature is inconsistant, this indicates that the sensors are interfering with each other when switching the analog reading circuit from one pin to the other. You can fix this by doing two delayed readings and tossing out the first one

Vous obtiendrez plus d'information en lisant cet article (en anglais)

Tester le senseur

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°C), the voltage should be about 0.75V. Note that if you're using a LM35, the voltage will be 0.25V

The sensor is indicating that the temperature is 26.3°C also known as 79.3°F

You can change the voltage range by pressing the plastic case of the sensor with your fingers, you will see the temperature/voltage rise.

With my fingers on the sensor, heating it up a little, the temperature reading is now 29.7°C / 85.5°F

Or you can touch the sensor with an ice cube, perferrably in a plastic bag so it doesn't get water on your circuit, and see the temperature/voltage drop.

I pressed an ice-cube against the sensor, to bring the temperature down to 18.6°C / 65.5°F

Source: AdaFruit

Traduit avec l'autorisation d'AdaFruit Industries - Translated with the permission from Adafruit Industries - www.adafruit.com

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