Embedded Lab

There are quite a variety of active analog temperature sensor ICs that provide an output voltage proportional to the temperature. They usually don’t require any external calibration and signal conditioning, and as such their output can be directly fed to the input of an ADC for digital processing. A few examples of such sensors are LM34, LM35, TMP35/36/37, and MCP9701. If you are having any trouble using any of these sensors in your project, here is a quick way to test if your sensor is working or not.

The technique is very simple. These sensors give analog output voltage proportional to the temperature. You can use a multimeter as a low range voltmeter and measure the output to see if it is giving you the right output voltage corresponding to the room temperature, as specified in their datasheets. You can also give some of your body heat by putting a finger over the sensor and see if the output voltage increases accordingly. Lets take an example of the LM34 sensor, whose output voltage is proportional to the Fahrenheit temperature. The temperature-to-voltage conversion factor is +10.0 mV/°F, which means if the surrounding temperature is 75 °F, the LM34 output will be 750 mV. LM34 operates from 5 to 30V, so you can power it directly with a 9V battery. The figure below shows the setup for testing the LM34 sensor using a multimeter. The multimeter is setup as a 0-2000 mV range voltmeter. Whatever displayed on the meter, if divided by 10, will give you the Fahrenheit temperature. Isn’t this one of the simplest ways of constructing a digital thermometer?

Here are some snapshots that I took while testing the LM34 and LM35 sensors.

Now place your finger over the sensor and watch is the output goes up due to your body temperature, which is higher than the room temperature. If it does then the sensor is doing its job correctly.

The output voltage of LM35 will be lower because they are calibrated in Celsius scale with a conversion factor of +10 mV/°C.

If you want to try this with other sensors, please confirm their supply voltage range. Not all analog temperature sensors operate at such a wide supply range.