ECE 2010 Laboratory # 3 J.P.O Rourke

Size: px

Start display at page:

Download "ECE 2010 Laboratory # 3 J.P.O Rourke"

Brent Cobb
6 years ago
Views:

1 ECE Laboratory # 3 J.P.O Rourke Prelab: Simulate the circuits in Part through 4 of this Laboratory. Use the given Multisim circuits that follow Lab 3 on the Website. You do not need to look up any sensors in the Multisim library. Record the simulated results for each part of the lab. Your Prelab is due at the beginning of lab and will be checked off by the TA's after the lab starts. Purpose: The objective of this laboratory is to investigate the properties of a Potentiometer(Pot), Thermistor, Photocell and Flex sensor. The last three components are Sensors and will handed out in lab along with a switch and must be returned at the end of the lab period. Part : The use of a potentiometer is quite common when it is desired to vary a voltage to another circuit. However it should be noted that the effectiveness of this is dependent on the load the circuit presents to the Pot output. If it desired to have a linear variation then the choice of the Pot's ohmic value and wattage is critical. The wattage value will not be evaluated here but in general can be calculated from the Pot and load resistance and the applied voltage. Set up the following circuit using the K potentiometer and ohm resistor in your kit and a switch that will be handed out with the Sensors. This circuit will be used to investigate the loading effects on a k pot for a load of ohms and K ohms. The results should show that linear operation is a function of the load resistor. V V.kΩ 5% J 3 Key = Space Ω Load Resistor Start off with a ohm load resistor and use a DVM to measure the Pot output voltage. With switch J open, use the DVM to adjust the Pot for volts output. This condition is when the Pot is unloaded since the switch is open. Now close the switch J and measure the output again with the DVM. This new voltage value shows the effects of Pot loading if it is a factor. For the volt setting, there should be little change. However it is necessary to take more measurements at other unloaded output voltages to

determine any loading effects. To do this, reduce the Pot output voltage in volt increments while measuring with the DVM for J open and closed for each increment.

2 determine any loading effects. To do this, reduce the Pot output voltage in volt increments while measuring with the DVM for J open and closed for each increment. Repeat the above all over again for a K ohm load. Use the following table to record all these measurements for both load resistor values. Shut off the Supply when finished. Load Resistor = Switch J open Load Resistor = Switch J closed Load Resistor = K Switch J open Load Resistor = K Switch J closed Using the results from the above table to Plot a curve of loaded Pot voltage verses no load voltage for both resistor loads. Note the simulated results can be automated. Check out the Power Point Hints # and Circuit fileon my class web page.

3 7. Part : In this part and the next two the properties of three sensors will be briefly investigated. The first is the Thermistor. There are basically two types, a PTC(Positive Temperature Coefficient) Thermistor and a NTC(Negative Temperature Coefficient) Thermistor. In this Lab the NTC type is being used. Thermistors are temperature variable resistors used to detect a temperature or its variation. Set up the following Lab bench Test Circuit to investigate the Thermistors operation. 5kΩ kω 4% 3 V V Lab NTC Thermistor Rt Thermistor (NTC) Test Circuit Note the center variable tap on the potentiometer, is jumpered to one of its sides which turns it into a variable resistor who's value can be varied from zero to K ohms.

Once you have completed the circuit set the DVM to measure DC Volts. Also, set the Power Supply for volts, turn up the Current Limit off of zero and then turn on the supply.

4 Once you have completed the circuit set the DVM to measure DC Volts. Also, set the Power Supply for volts, turn up the Current Limit off of zero and then turn on the supply. Now adjust the variable resistor, until the DVM reads an integer voltage somewhere between 3 and 7 volts. This will be the Reference Voltage of the Thermistor at room temperature which will assume to be 7 degrees F. Next pinch the Thermistor tightly between your thumb and index finger for about a minute then record the DVM reading as you did before. This is approximately equal to 85 degrees F. Finally try to breathe a hot breath from deep in your lungs, record the lowest Thermistor voltage reading after a few attempts. Shut off the Supply when finished then disconnect the positive lead from the circuit. Then use the Ohmmeter function of the DVM to measure the resistance of the Pot, be careful not to touch 's adjustment control since we don't want its value to change. Room Temp. Finger Temp. Breath Temp. across Thermistor Temp. Degrees F 7 85 Pot 's Resistance Calculate Rt XXXXXXXXXXXXXXX XXXXXXXXXXXXXXX Plot the first two readings, Voltage verses Temperature. Assume the graph is linear, use it to estimate the temperature of your breath.

5 7. Below is an optional circuit simulation based on a Thermistor Sub-Circuit which I created from measured lab data. The Sub-Circuit is available on the class web-site if you are interested. 5kΩ kω 4% HB/NTC V V HB/Temp_Ctrl Batt. Volts= Temp F V3 7 V HB Temp_Ctrl JOR NTC NTC NTC Thermistor (NTC) Test Circuit Part 3: The next sensor that well be investigated will be a Photocell or LDR(Light Dependant Resistor). The Photocell or also known as a Photo-Resistor is basically a resistor who's resistance decreases as the intensity of light falling on it increases. The response is not linear but exponential. Use the same circuit arrangement as in part except replace with a K resistor, with a K Pot connected as a variable resistor and the Thermistor with a Photocell(LDR) Rp. kω kω 4% 3 V V Lab LDR Photocell Rp Photocell (LDR) Test Circuit

6 Turn on the Supply and set it to volts, DC if it was changed. Bend the Photocell's leads down so that it lays on the protoboard. Then Cover the Photocell with palm of your hand putting the cell in complete darkness. Adjust until the DVM reads something between 7 and 9 volts. This voltage will be the reference. Now expose the Photocell to room light and record this new voltage with the DVM. Record the data in the table below.. Turn off the Supply when you are finished. Then use the Ohmmeter function to measure the resistance of the Pot, be careful not to touch 's adjustment control since we don't want its value to change. Same procedure as with the Thermistor. LDR in Total Darkness LDR in room Light Pot 's Resistance Calculate Rp XXXXXXXXXXXXXXXX The schematic on the next page is another optional circuit simulation based on a Photocell Sub-Circuit which I created from measured lab data. The Sub-Circuit is also available on the class web-site if you are interested.

7 7. kω kω 6% V5 V Key = Space Room Dark J HB/P Room Lit HB/C C P P HB/P HB LD LD LDR SC HB/LD Part 4: Photocell (LDR) Test Circuit The final sensor to be investigated as part of this lab is the Flex Sensor(Strain Gage sensor). In the previous circuit, replace the K resistor with a 5K and the Photocell with the Flex Sensor. 5kΩ kω 4% 3 V V Lab Flex Sensor. Rf Flex Sensor Test Circuit Turn on the supply and adjusted it to volts as before. Then adjust the variable resistor till the DVM reads something between 3.5 and 6 volts. Do not touch the sensor while making this measurement. This will be the reference voltage for the unloaded Flex sensor. Next, carefully press with a finger on the cross-hatched area of the Flex sensor. While pressing down(represents the sensor under load), record the new DVM reading in the table below. Turn off the supply when finished. As before, use the Ohmmeter function to measure the resistance of the Pot, be careful not to touch 's adjustment control since we don't want its value to change.

DVM reading in volts Pot s resistance Calculate Rf Flex sensor no load Flex sensor Loaded XXXXXXXXXXXXXXXXXXX The schematic below is another optional

8 DVM reading in volts Pot s resistance Calculate Rf Flex sensor no load Flex sensor Loaded XXXXXXXXXXXXXXXXXXX The schematic below is another optional circuit simulation that maybe used for test purposes in Multisim. V V 5kΩ kω % 3 J Key = B 4 5 Not Flexed R3 3kΩ R4 3.5kΩ Flexed Flex Sensor Test Circuit

9 JOR. Explain why the Potentiometers output voltage is not linear under certain load conditions?. What general rule can you make that would apply for a load resistor/potentiometer combination that would allow for essentially linear operation. 3. From the measured value of the Potentiometers resistance, calculate the circuit current. Using the measured DVM voltages, calculate the Thermistor resistances for both test conditions. Hint for finding the current and resistance. Note, I=(V-V3)/(+)=( - Vdvm)/(5K + m) m=measured value of. Rt=Vdvm/I or Rt=(5K + m)(vdvm)/( - Vdvm) Rt = (5K + m)/((/vdvm) - ) 4. Now that you have calculated the two resistances for the defined temperatures. Assume the curve of Temperature vs Resistance is linear, what would be the resistance at 93 degrees F? 5. Using the same procedure as the thermistor section, Calculate the two resistance of the Photocell for the light settings defined? 6. Find the two Flex sensor resistances using the same procedure as above. JOR

ECE 2010 Laboratory # 3 J.P.O Rourke

ECE 2010 Laboratory # 3 J.P.O Rourke ECE 21 Laboratory # 3 J.P.O Rourke Prelab: Simulate all the circuits in this Laboratory. Record the simulated results for each part of the lab. Your Prelab is due at the beginning of lab and will be checked