AN2278

LM135 temperature sensor

3/9

1

LM135 temperature sensor

LM135 is a precision temperature sensor and can be easily calibrated. It operates as a 2-

terminal zener and the break-down voltage is directly proportional to the absolute

temperature at 10 mV/K. The temperature measurement range of the LM135 is -55 °C to

+150 °C.

Temperature calculation in Celsius:

The ST72325 microcontroller has a 10-bit ADC, so the maximum digital value it is capable of

holding is 0x3FF. The maximum voltage available at the V+ terminal of the LM135 (refer to

Figure 4) is approximately 4800 mV (after taking into account a voltage drop of 200 mV).

Lets say x is the ADC value corresponding to an unknown temperature value.

Therefore the temperature value in kelvin is k= [(x * 4800) / (0x3FF)]/10; as zener

breakdown voltage for ST LM135 changes by 10 mV for each degree Kelvin change. The

temperature value in Celsius is C=(k - 273) / 100.

2

Temperature sensor application

This temperature monitoring application senses and logs the temperature value into an

EEPROM. If the sensed temperature is beyond the threshold limits, specified by the user, an

alarm signal is indicated by toggling the LED.

The application flow is as follows:

1.

On starting the application, it enters configuration mode and the user is prompted to set

the lower and upper bounds for the normal temperature values.

2.

Once the threshold values are set, the application enters normal mode. An average of

16 sample values, taken over a period of 1 second, is displayed by the Windows

Hyperterminal application via the on-chip SCI interface.

Note:

It is advisable that number of samples to be recorded should be of order 2^n, where n is an

integer greater than 0. This will help in calculating the average temperature value using a

‘right shift(>>)’ operator.

3.

When the temperature crosses the threshold values, the application shows a ‘Critical

value’ message in the hyperterminal window and the LEDs on the board toggle.

4.

The application keeps account of the minimum and maximum temperature values over

a span of 1 hour. At the end of an hour these minimum and maximum temperature

5.

You can re-enter configuration mode and change the threshold values by pressing

Button5 on the board. Since Button5 is not connected to one of the microcontroller

interrupt pins, you have to keep the button pressed until the firmware polls it.

6.

To view the log of temperature values stored in the EEPROM, you have to press

Button6 on the board. The temperature values (in Celsius) are displayed in the

hyperterminal window. Since Button6 is not connected to one of the microcontroller

interrupt pins, you have to keep the button pressed until the firmware polls it.