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CoachLab probeware systems have built in sensor profiles for all CMA, TI, and Vernier brand sensors that allow you to avoid calibration and save you time in the lab. However with CoachLab hardware and software it is also easy to make any sensor work with a computer or with TI-CBL, CBL2, or Vernier LabPro. The ability to make your own sensor is useful because:
| The CoachLab II Interface Panel is easy to use as a base to create your own sensors because it has two inputs that have 4mm female banana jack connectors. These inputs are found on the left and right sides of the interface and are made up of three jacks each. A +5 volt DC output (red), an analog input (yellow), and a ground (black). These ports may be used as a + 5 volt power supply for simple experiments, as a zero to 5 volt DC voltmeter, or as an input for analog sensors of your own design. Additionally inputs number 1 and 2 that use the British Telecom style connector may also be used in order to create your own sensor via an adapter plug available from most Coach dealers. All inputs are protected against normal fault conditions, however do not connect voltages above +/- 10 volts or A.C. power voltages to these connectors. | |
This article will detail the steps necessary to connect your own sensor to a CoachLab II Interface Panel. You may also make your own sensors for use with CoachLab I, ULAB, and TI/Vernier interfaces.
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Plus... if you wish to calibrate the sensor manually
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A thermistor is simply a resistor that changes its resistance value in response to a change in its temperature. By applying a 5 volt power supply to one side of the thermistor and reading the voltage drop across the thermistor it is possible to determine the temperature of whatever the thermistor is in contact with.
The simplest way to build our circuit is to make a voltage divider network out of a low value resistor and the thermistor.
| Cut the connecting cable into a length of at least 4 feet and strip the wires on both ends. Solder the male red banana jack to one wire, the male yellow banana jack to the other wire, and the black male banana jack to the ground wire (or third wire). Connect one end of the thermistor to the wire with the red jack, connect the other side of the thermistor both to the other resistor and to the wire with the yellow jack, and connect the other side of the other resistor to the black jack. It may be advisable to use electrical tape or heat shrink tubing to insulate the various connections and exposed wires in this circuit. (See Circuit Diagram at Right) |  |
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CoachLab II acts as a +5 volt D.C. power source and causes a current to flow through the combination of Thermistor and 10 Ohm resistor to ground. The total circuit uses 5 volts. Since the resistors are wired in series the total voltage drop of the circuit is the sum of the voltage drop through each resistor. By ohm's law we know the current is a a constant in this circuit (again because it is a series circuit) therefore the amount of voltage dropped by each device is the equal to the current flowing through that device multiplied by its resistance. Since the thermistor changes its resistance value based on external temperature it is possible to correlate a voltage value at the analog in port of Coach to a temperature value in the real world.
Thermistors have a fairly linear relationship between resistance and current, and so it is possible to calibrate the sensor by locating a few points (such as boiling point and freezing point).
If you have the specification sheet that came with the thermistor then it should already contain a graph relating temperature (probably in Celsius) to the resistance value at that temperature. You can use the details of your actual circuit and Ohm's law in order to build a table of Voltage outputs to Temperature in order to calibrate the sensor more accurately.
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When you create your own sensor it is often useful to create calibration data for the sensor by empirical means. Temperature sensors are very easy to calibrate.
First, use graph paper to draw a graph. Place voltage observed along the X-axis and label that axis in 1-volt increments up to 5 volts. Place temperature observed along the Y-axis and label that axis in 1 degree Celsius increments from a few degrees below zero up to a few degrees above boiling.
Connect your sensor circuit to a + 5 volt supply (as shown in the circuit above). Set your volt meter to an appropriate scale for reading up to five volts and connect the meter's positive lead to the analog output of your sensor, and the negative lead to the ground lead in the circuit.
Place a thermometer (preferably digital) next to the thermistor casing and note the temperature and voltage measurements. (record these values on your graph. Next place the thermistor and thermometer into a boiling (or very hot) water bath. Allow the sensor to stay in the bath for about one minute and then note the voltage and temperature readings on your graph. Finally place the sensor and thermometer into an ice water bath and note the voltage and temperature readings.
Use a ruler to draw a best fit curve between the three points you collected. The line should pass through the zero volts point on the X axis.
By using the graph you just created you can now translate the voltage coming from the sensor into any temperature reading within this range. Keep this data for use in setting up the sensor with Coach software.
NOTE: Obviously more data points will give you a more accurate calibration curve. Also, the data table that comes with a thermistor may indicate a non linear relationship between resistance value and temperature value. If this is so then you can pick the "most linear" area of the relationship in order to use the sensor. This linearity will limit the accuracy of the sensor at its lowest and highest ranges.
TIP: Make several sensors at one time and compare their values to each other. (use a different color pencil when plotting all the points on the same graph). Then do a best fit curve among all of your sensors in order to build an "average" calibration curve that will give consistent data from all the sensors you will build.
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Start Coach Software in Author mode. Open a project and create a new activity, choose the CoachLab II interface panel, and set up the activity as you would any other CoachLab activity.
Right Click on an empty sensor block in the sensor palette and choose "Create New"
A Sensor Profile window will then appear on screen that will let you set up the various parameters of your new sensor.
This Sensor Properties Window is divided into four main sections
The Name section is used in order to give your sensor Name, and Brand information. This information is used to locate and load the sensor once it is saved to the list of sensors available on your computer. You can also pick the Icon that will appear in the sensor palette. Kind can always be left at "General" unless you are working to create a "Lego" sensor.
The Signal Type section is used to identify to the computer what kind of output to expect from the sensor. Most sensors you will create will be an analog sensor (i.e. a device that outputs some continuously changing voltage). You can also set up sensors as counters (like light gates). Counters simply increment a counter for every change of any kind in a sensor. Digital sensors are also possible, but more complicated and so will not be discussed in this article.
Scaling and Display effect how data that is taken from the sensor is displayed in the Coach software interface. This includes the minimum and maximum values for graphs and digital values, the type of quantity you are measuring, unit of measure, what kind of meter display to use when reading the sensor as a multimeter, and the input range of the sensor. Input range can be -10 to +10 volts, or 0 to +5 volts (and a few other combinations).
The Calibration section lets you tell the computer what input voltage corresponds to what quantity/unit of measure. It is possible to do a simple linear two-point calibration as well as a more advanced calibration for sensors that do not have linear response curves.
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Since this sensor is a temperature sensor choose "Temperature Sensor" from the name section. Pick "Self Made" for brand, and leave the settings for "Short Name" and "Kind" as they are.
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Set up the following fields:
Min: Set to the lowest temperature you wish to read with your thermistor sensor
Max: Set to the highest temperature you wish to read with your thermistor sensor
Decimals: Set to the maximum number of decimal places that Coach should calculate for Temperature
Quantity: (since you picked a Temperature Sensor Coach picked "T" for temperature by default)
Unit: (Since you picked Temperature Sensor Coach picked "degrees Celsius" by default)
Display: Set to your choice for multimeter display. I picked "Vertical Bar" see below.
Finally set up the input range. This sensor will use Coach's own 5 volt supply and never output a voltage over 5 volts so set this scale to 0..5 V.
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Since the temperature sensor is an analog device that has a fairly linear response of Voltage vs. Temperature it is possible to set up Coach using a two point (or linear) calibration curve.
Click on "Linear" in the calibration section and enter two wide spread data points from the calibration data you collected when you built your sensor. X0 is the lowest voltage value, Y0 is the lowest temperature value, X1 is the highest voltage value, and Y1 is the highest temperature value.
If you want to have a really accurate sensor that takes advantage of the full range of output voltages/temperatures that your thermistor can respond to you can use the "Advanced" calibration window. Choose "Advanced" from the calibration choices and then click on the "Calibrate" button that appears.
For the advanced calibration it is possible to enter several individual data points taken from your calibration data, or you can also use linear or quadratic functions in order to build the relationship between Voltage and Temperature (or whatever value your home made sensor works with).
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Once you have completed all these steps click on "OK" in the sensor profile window and save your activity. This will save the sensor profile you have just created to your computer. It may be useful to test your new sensor by measuring temperature and comparing to a known temperature source, or by comparing to a thermometer.
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With Coach Software it is possible to turn any device that outputs a voltage between -10 to +10 or 0 to + 5 volts DC into an accurate sensor for use in science or technology experiments. This is useful for teachers who wish to adapt older sensors they already own for use with Coach, as well as for technology teachers who wish to teach their students how sensors work. This example used a simple temperature sensor, but many other home-built sensors are possible including photo gates for use in timing races or measuring velocity, wind speed indicators for use in weather testing, strain gauges for use in materials testing, and many more.
Creating a sensor in a Physics or Math class is a great way to teach and reinforce math concepts and graphing skills.
The Help Button located on the "Sensor Properties" window has details of each option and setting for creating your own sensors. Take advantage of this help file.
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