0.3 Led thermometer lab

Led thermometer lab using national instruments low cost data acquisition

Introduction

One common measurement that scientists and engineers like to make is temperature. For example, it may benecessary to record the temperature of a biology experiment, a heat testing chamber for electronics, or a jet engine.

In order to convert temperature information into electrical form, a transducer is needed. A temperaturetransducer is simply a sensor that will allow the user a mechanism to detect a temperature change (increasing or decreasing voltage,current, resistance, etc).

During this exercise, the experimenters will use the integrated circuit LM34CZ temperature sensor to output alinear, temperature dependent voltage. This voltage will be digitized by a Low Cost USB DAQ so that measurements can be takenon a computer. Finally, a LabVIEW virtual instrument (VI) will be created in order to visualize sampled data and control severalLEDs.

Pre-lab assignment

1) Many types of temperature sensors exist, including RTDs, thermocouples, thermistors, and ICs. Use anyresources available (textbooks, the web, etc) to research each type of transducer and prepare a short written summary of each.

2) Find and print the datasheet for the LM34CZ temperature sensor using Google. By reading the datasheet, can youdetermine how we will measure temperature with this IC? Write a formula relating temperature and sensor output voltage (this shouldbe fairly simple).

3) Suppose we were using the LM35 temperature sensor instead. Obtain the datasheet and write a second formularelating temperature and sensor output voltage for the LM35.

4) Become familiar with the National Instruments USB 6008 and 6009 data acquisition devices. Thesedatasheets are available at www.ni.com .

Theory

One method to measure temperature involves using an integrated circuit transducer such as the LM34CZ. This ICwill generate a voltage that is linearly proportional to the measured temperature. Compared to other measurement methods, ICsprovide an extremely linear output with very little error. They are also one of the simplest temperature sensors to connect anduse.

Below are several views of the LM34CZ in a TO-92 package:

As the experimenter can see, the LM34CZ is a three-terminal device. Two pins are used for power (+Vs and GND),while the third pin is used to output a temperature dependent voltage.

Since the LM34CZ outputs a voltage that is linearly dependent with temperature, it is easy to compute measuredtemperature from sensor output voltage or vice versa. In fact, the LM34CZ ideally outputs exactly 10mV / deg F.

To visualize this relationship, a graph sensor output voltage vs. temperature can be plotted using LabVIEW:

Hardware and software required

• (4) red LEDs
• (4) 220 Ohm resistors
• LM34CZ temperature sensor IC
• National Instruments Low Cost USB DAQ
• LabVIEW 8.20 software (LabVIEW 7.1 or 8.0 will work as well)

Laboratory exercise

During this exercise, the experimenter will construct an LED thermometer that resembles the following:

1) In order to create the LED thermometer output, Connect LEDs and resistors to the digital output lines ofthe Low Cost USB DAQ as shown below. Each LED will be programmed to light up when the temperature exceeds a certain threshold.

2) To measure the temperature, connect the LM34CZ IC to the Low Cost USB DAQ as shown below. Note that the +5Vsupply needed by the IC can be obtained directly from the National Instruments USB 6008 or 6009.

3) Using the DAQ Assistant Express VI, create the following LabVIEW block diagram. In this virtual instrument(VI), temperature data is acquired using the Low Cost USB DAQ, scaled, and displayed on a chart. The temperature measurement isalso compared with several thresholds. Once the temperature exceeds each threshold, a corresponding physical LED and front panel LEDwill light up.

4) Run the VI and adjust the threshold levels so that several LEDs light up. Warm up the temperature sensor bysqueezing it with your fingers and observe the rise in measured temperature on the front panel chart. You have now succeeded increating an LED thermometer!

Post-lab questions

1) Did the measured temperature levels match what you expected? If the temperature is warmer or cooler than youexpected, elaborate on possible sources of error.

2) Look at the LED Thermometer VI createdabove, and determine how often temperature measurements are being taken. Why wouldn’t you want to take room temperature measurementsat a faster rate such as 1 MHz?

3) If you were incorporating the LM34CZ temperature sensor into a shipping product, what would you do tocalibrate the sensor? Outline the process you would go through to make your temperature measurements more accurate.

4) How could you modify the LabVIEW virtual instrument above to output a Centigrade or Farenheit temperaturedepending on user input? Assume you are using the same LM34CZ temperature sensor.