LM75B – Temperature Sensor

lm75b_pic    lm75b_ol

lm75bd_on_board_pic

The LM75B is a temperature-to-digital converter using an on-chip band gap temperature sensor and Sigma-Delta A-to-D conversion technique with an over temperature detection output.

This device will measure the ambient temperature with the specified accuracy assuming that the host PCB is at the same temperature as the surrounding air.

The current version (Version A) of the WiSense wireless sensor node uses this sensor.

This sensor is relatively inexpensive (less than $ 0.5 @ 100).

We have written a simple library for this sensor. Refer to the files listed below.

  • “sensor_fw/pltfrm/src/lm75b.c”
  • “sensor_fw/pltfrm/inc/lm75b.h”.
  • “sensor_fw/app/src/app.c”

Let us walk through a simple use case. On a battery operated node, the sensor should mostly stay in the “SHUTDOWN” mode in which it will consume less than 1 microampere of current. Periodically, the node’s microcontoller will wake up from deep sleep and change the sensor’s operating mode to “NORMAL”. This will trigger a single conversion. The microcontroller should wait for at least 10 milli-seconds (the conversion time specified in the spec) after which it can read the temperature register to get the latest value of the temperature “sensed” by the device. After reading this value, the sensor should be put back into “SHUTDOWN” mode. If this temperature value needs to be reported to some application within or outside the sensor network, the node should wake up the radio (cc2520), send out a message over the air, shutdown the radio (enter cc2520 low power mode LPM2) and put the microcontroller back into low power mode (LPM3 on MSP430).

Details

  • Manufacturer: NXP
  • Part Number: LM75DBP
  • Package: TSSOP8 (Surface-mount Device)
  • Dimensions: 3 mm x 5.1 mm
  • Interface: Two Wire I2C Bus
  • Output format: 11 bit two’s complement (in units of .125 Deg C)
  • Range: -55 Deg C to +125 Deg C
  • Accuracy: +/- 3 Deg C
  • Power supply: 2.8 Volts to 5.5 Volts
  • Temperature to data conversion time:  10 milli-seconds

Current consumption

  • Shutdown mode (< 1 microamperes)
  • Normal mode (< 300 microamperes)

I2C specific:

  • I2C address configuration pins: 3  (Allows up to 8 such devices on one bus)
  • I2C 4-bit address prefix: 1001b
  • Frequency range 20 Hz to 400 kHz. The lower limit is important since we have implemented the I2C interface in software on the WiSense sensor nodes. This rules out single stepping through the I2C code. Refer to section 7.2.1 (Bus fault time-out) in the device spec. In any case, you should not have to debug the code since we have already tested it.

Operating modes:

  • Normal mode (continuous conversion). In this mode, the temp-to-digital conversion is executed every 100 milli-seconds. The conversion itself takes only 10 milli-seconds. The device automatically enters and stays in low power consumption mode for the remaining 90 milli-seconds.
  •  Shutdown mode (conversion suspended).  This is low power consumption mode. Only the I2C interface remains active allowing register write/read operation to be performed.

Other features:

  • The device also includes an open-drain output (OS) which becomes active when the temperature exceeds the programmed limits.

Data sheet:  http://www.nxp.com/documents/data_sheet/LM75B.pdf

Coming soon …..

I will post some graphs (from the WiSense GUI) on this sensor’s response time to changing conditions. I will be using two or more WiSense nodes and the fridge at home.

Posted on December 2, 2013, in Uncategorized. Bookmark the permalink. Leave a comment.

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