Room Thermometer using Multi meter. Sensor Circuit 13

Temperature sensor IC LM34/35, are used in various applications to detect temperature variations. These devices can switch off / on circuits when the temperature varies from the normal set level. LM 34 and LM 35 Integrated Circuits are precision temperature sensors widely used in temperature sensing applications. LM 34 senses temperature in Fahrenheit range while LM 35 senses temperature in Celsius (Centigrade) range. The output voltage of these sensors is linearly proportional to the temperature. LM 34 and LM35 sensors are too sensitive and the static electricity while handling may damage the device. You can make a Room Thermometer using Multi meter. If you use a Digital display unit, you can make a permanent set up.

LM 34

It is the Fahrenheit sensor and the output voltage is linear to the temperature in the Fahrenheit range. It can operate between -50 to + 300 F range and requires a single supply. The LM 34 has low output impedance, linear output and precise inherent calibration feature. Its draws minimum current of 75 uA during operation and has very low self heating property of less than 0.2 F at normal air temperature. The improved version LM 34 C is designed to sense temperature of -40 to + 230 Fahrenheit. It operates between 5 to 30 Volts DC.

LM 35

It is the precision temperature sensor with output linearly proportional to the Celsius (Centigrade) temperature. Like LM 34, it also operates in single supply and draws only 60 uA current. Its self heating is as low as 0.1 degree at still air. LM 35 is rated to operate between -55 to +150 Degree Celsius. LM 35 C operates between -40 to +110 Degree Celsius. Supply voltage required for LM 35 ranges between 4 to 30 Volts DC

Check before connecting

Before using the Sensor, test its performance using a digital multi meter. Connect 6 / 9 volt battery to the + and Ground pins. Then connect the Black probe of Multi meter to Ground pin and Red probe to the Output pin of Sensor and set the Meter to 20 volt range. In LM 34 and LM 35, the meter will show 0.25 volts at 25 degree room temperature. Then press the plastic case of the Sensor using fingers. You can see a rise in voltage in the Meter.


Connect the Temperature sensor as shown in the diagram. Set the DMM in the 20 volt level. At 25 degree it will be 250 milli volt Adjust pot to get Zero or a milli volt reading at room temperature. Thus with each degree rise in temperature, display will show the reading. Using a thermometer, calibrate for the exact reading, since the reading shows milli volts. For example at 37 degree, it is 0 and then 38 degree…..note the reading and 30 degree etc.

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