LM34 PRECISION TEMPERATURE SENSOR

The LM34 is a linear Fahrenheit temperature sensor with an output of 10mV / °F.

Typical accuracy is +/- 0.5 °F at room temperature and +/- 1.5 °F over a range of -50 °F to +300 °F. The LM34C is rated for a range of -40 °F to +230 °F and the LM34D is rated for +32 °F to +212 °F.

It is possible to measure the output from this sensor directly with a voltmeter. Since the output is a linear 10mV / °F, a temperature of 75 °F would be shown on a voltmeter as 750 mV.

However, adding a scaling circuit that can be interfaced to a microcontroller has a couple of advantages. Having a zero and gain potentiometer allows us to use the entire 0-5 VDC range of the microcontrollers ADC input. It also allows the temperature display to be calibrated at both the low and high ends of the temperature range.

The circuit can be simplified using only a gain pot if the desired temperature range does not have to go to zero or less than zero.

Shown below are three circuits that I have tested. The first two circuits require only a gain pot. The first has a temperature range of +5.0 °F to +99.5 °F and has a 0.1 °F resolution. The second has a range of +5 °F to 240 °F. The third circuit has a temperature range of -20 °F to +120 °F and has both a zero and a gain pot.

When using a sensor such as this, it is important to insure that the sensor is firmly attached to the surface that is being measured and also is insulated from the ambient air temperature. Also adding a small thermal mass to the sensor will make the sensor more immune to random air currents and provide a steadier display reading.

Three conductor shielded cable is recommended for remote applications.

The microcontroller circuit consists of a FREESCALE MC9S08QD4 MCU used as an analog to digital converter and also provides a bit-banged SPI output to control a FREESCALE MC14489 to drive the LED displays.

99.5 DEGREE TEMPERATURE  CIRCUIT

CLICK ON FOR LARGER IMAGE







 

240 DEGREE TEMPERATURE CIRCUIT

 

CLICK ON FOR LARGER IMAGE



 

-20 TO +120 DEGREE TEMPERATURE CIRCUIT



CLICK ON FOR LARGER IMAGE



This picture shows the prototype that was used to test the three circuits described.

This picture shows a close-up of the temperature scaling circuits. To the left is the circuit that can be used for either the 5.0 °F to 99.5 °F or the 5 °F to 240 °F temperature readout.  It requires a gain potentiometer only. A jumper is used to change the gain resistor value.

On the right side is the circuit used for the -20 °F to +120 °F temperature readout. This circuit has both a zero and gain potentiometer.