The post explains how to make an LM35 circuit by understanding its pinouts and other technical specifications.
By: SS kopparthy
LM35 Main Specifications
The IC LM35 is a temperature measuring device that looks like a transistor (most popular package is TO-92 package).
This device is found in most of the circuits that need to measure temperature as this device is a low-cost, reliable and with an accuracy as high as +-3/4 degrees Celsius.
The low cost of the sensor is because of its wafer-level trimming and calibration.
This IC is much better than the thermistor due to the accuracy in temperature measurement.
As you can see in the above figure, the LM35 IC consists of three pins out of which two are for powering the sensor and the other one is the output signal pin. The sensor can work anywhere from -55 to 150 degrees Celsius.
The output temperature is directly proportional to the temperature change in Celsius. Also the other variant LM35C is available and its temperature range is -40 to 110 degrees Celsius.
Technical Specifications and Features
This device gives out 10mV per degree Celsius rise in temperature.
This device consumes just 60µA current. So it does not drain much of power from the battery or power source.
Also, due to this low current, self heating of the device is as low as 0.1°C.
Other variants of this device that have different packages are also available such as TO-46 and TO-220.
These work same as the traditional one but differ in their usage areas and feasibility for a particular application.
For example, TO-46 metal package can be used to measure surface temperature as the metal package can be directly kept in contact with the surface whose temperature needs to be measured, while the TO-92 package could not as the device measures the temperature mostly based on the temperature of the terminals of the device as metal terminals conduct more temperature than the plastic casing.
Hence the TO-92 packaged LM35 is used to measure air temperatures in most of the circuits.
Block Diagram and Internal Functioning
The above image shows the internal block diagram of the IC LM35. Here we can see that the IC is internally configured around a couple of opamps A1 and A2. The first opamp A1 is configured as an accurate temperature sensor through a feedback loop formed by a couple of BJTs configured as a current mirror.
The current mirror ensures a perfectly linear and a stabilized rate of temperature detection and prevents false triggering or inaccurate temperature readings at the output.
The sensed temperature is produced at the emitter side of the current mirror at a rate of 8.8mV per degree Celsius.
The output is applied to a buffer stage using another opamp A2 which is configured as a high impedance voltage follower.
This A2 stage acts as a buffer to reinforce the temperature to voltage conversion, and presents it at the final output pin of the IC via another high impedance BJT stage configured as an emitter follower.
The final output thus becomes highly isolated from the actual temperature sensor stage and delivers highly accurate temperature sensing response, which can be used by the user with an external switching stage such as a relay driver stage or a triac.
Using a Heatsink
The LM35 sensor IC can also be soldered to a heatsink fin to increase the accuracy and to decrease the sensing and response time in slowly moving air.
To understand much better, let’s take a look at the following circuit that uses LM35 to give an indication when the temperature is above the specified level:
Temperature Detector Circuit Using LM35 IC
The LM35 circuit uses an op-amp IC741 as comparator. The op-amp is configured as an non-inverting amplifier.
That means when the LM35 detects high temperature, the output of the op-amp becomes +ve and the red LED lights up and the temperature falls below the specified level, the output of op-amp becomes –ve and the green LED lights up.
The high temperature level can be set with the help of preset in the circuit.
To set the temperature level for which the red LED lights up, you need to know the actual temperature where the circuit is being tested. For that you can use a multimeter.
As we know that the LM35’s output voltage increases 10mV per degree Celsius rise in temperature, we can use the multimeter to measure the output voltage and for example the voltage is 322mV, then the temperature at the place is 32.2°C.
You can even test the IC if it’s working or not using the above procedure. You can measure the actual temperature using an atmospheric thermometer and compare it with the values obtained with the LM35. You might not get the exact values but you should get close values.
After understanding how the above LM35 circuit works, you should have understood how LM35 works practically in circuits.