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Transcript
THERMOMETER KIT: PART NO. 2191242 Time Required: 1 to 2 hours | Beginner Level We always use thermometers and hear about what the temperature is, but often times we do not think about how we actually know the temperature. Many thermometers use different means of measuring temperature (like a mercury thermometer), but many new thermometers we use today measure a change in resistance within a circuit. This is how the thermometer in this kit works. How does this circuit work? This circuit works based off of a variable resistor called a Thermistor. As temperature increases or decreases, the resistance of the Thermistor changes. In this circuit, the Thermistor has a negative temperature coefficient (NTC). This means that as the temperature rises, the resistance of the Thermistor decreases by a certain percentage every unit of temperature. This is useful because, in our circuit, we have a potentiometer in series with the Thermistor and, as the resistance of the Thermistor drops, the voltage drop of the potentiometer rises in relation to the voltage drop of the Thermistor. This is because of a certain give and take between components in a series configuration. In series, the components together have a voltage drop equal to the power sources voltage. However, each component drops only its share based off of resistance of the voltage. For example, our potentiometer and Thermistor have a combined voltage drop of 9v from the battery. When the potentiometer is at 5,000 Ohms and the Thermistor is at 5,000 Ohms, each component drops 4.5v. Now if the temperature increases, the Thermistor’s resistance may drop to 2,500 Ohms while the potentiometer is still 5,000 Ohms. This means that the potentiometer will now drop 6v, a 1.5v increase. We can then measure the voltage drop across the potentiometer to see changes in temperature. The LED array is built into a voltmeter circuit using the LM3914. The two potentiometers can be used to calibrate the readout and sensitivity of the LED array. The green LED is simply used to state if the circuit is on or off. The schematic for our thermometer circuit is: What is in this kit? In this kit, you will receive all of the components needed to build the circuit and a PCB designed for this circuit. The components include 2 potentiometers, 12 resistors, an LED array, a LM3914 driver, a green LED, a Thermistor, a 9v battery, and a 9v battery plug. The PCB is about 1 inch by 5 inches and is the rough shape of a normal oral thermometer (but do not use this circuit as one). What else will you need? RequiredThis circuit requires a lot of soldering. It is necessary to have a soldering iron with a fine tip to get into the small spaces. Other than soldering supplies, you will need a pair of wire cutters to clip the excess ends from the leads of components after soldering. Examples of the soldering equipment and wire cutters you will need are: Description (P/N) Qty. 1 Soldering Iron 116572 1 2 Soldering Iron Stand 36329 1 3 Solder 170457 1 4 Wire Cutters 34658 1 RecommendedIn the PCB directly above the two potentiometers, we added a small notch to keep a rubber band from slipping if you decide to use one to hold your battery. We have noticed that a small rubber band or other way of securing your battery to the PCB makes the thermometer much more usable. We also recommend having a multimeter on hand because it is handy if trouble shooting is necessary. One multimeter we recommend is Jameco P/N 1928476. Helpful Tips 1. LEDs have a specific direction they go (long side to positive). This is important because if the components are installed backwards, the circuit will not work and pieces may fail. 2. Do not trim leads before board is soldered and tested. 3. Be very careful of what is touching and what should be touching. 4. Keep you soldering nice and neat, act deliberately. Soldering is a skill mastered over many years. Don’t get frustrated it your soldering is not a piece of art just yet. 5. Solder the smallest components first. This is much easier than soldering larger components in first and then squeezing in the small pieces. 6. Don't solder things outside of the PCB first. I know this is obvious, but if you accidentally damage a component, the whole circuit will not work as designed and the component may not fit into the PCB. What can you measure with this thermometer? While you can measure anything with temperature, this thermometer works best when being exposed to room temperature and then something that is much different in temperature (either hot or cold). Keep in mind that the Thermistor takes a few seconds to reset, so you will see a gradual changing in the display when new temperatures are presented. In testing, the quickest ways we have been able to change the output of the LEDs is to pinch the Thermistor in our fingers or set the entire thermometer in the refrigerator or freezer. Maybe you can keep yours near a nice hot (or cold) cup of coffee or hot chocolate to know when the temperature is just right? Remember to be creative with how you use your thermometer, but always remember what is safe to do with the thermometer. To be safe, do not use the thermometer above 50 degrees Celsius (122 degrees Fahrenheit). Also, like all circuits, keep the thermometer dry at all times. Directions It may become useful to refer to the pattern for the PCB after you have started soldering: *NOTE: The silkscreen pattern for the LED array is reversed (the notch should be on the opposite side). Install the LED array with the notch near the outside of the PCB. Step 1Check to make sure you have all of the pieces for the kit. You should have 12 small resistors, 2 potentiometers, a Thermistor, a green LED, a LM3914 chip, a 10 LED array, a DIP switch, a 9v battery clip, and a PCB. Step 2Insert the components and solder them into place one by one. Insert the components on the top of the PCB and solder on the bottom. The placements for the components on the PCB are: BAT1 9v Clip Leads D1 Green LED 100k Ohm Potentiometer R1 R2 10k Ohm Potentiometer R3 220 Ohm Resistor R4 220 Ohm Resistor R5 220 Ohm Resistor R6 220 Ohm Resistor R7 220 Ohm Resistor R8 220 Ohm Resistor R9 220 Ohm Resistor R10 220 Ohm Resistor R11 220 Ohm Resistor R12 220 Ohm Resistor R13 1k Ohm Resistor R14 330 Ohm Resistor R15 5k Ohm Thermistor U1 LM3914 U2* LED Array U3 DIP Switch Note: Make sure you have the LM3914, LED, LED Array, and battery placed in the PCB in the correct direction to their polarities. The positive side (anode) of an LED is the side with the longer wire lead. The positive side of the LED array is the long side with the notch taken out of the corner. The red wire from the battery pack is positive. To identify the pins of a LM3914, look at it with the pins facing away from you and the side with writing facing you. Rotate the LM3914 so that the notch in the plastic is at the top of the component. The pins will be numbered counter clockwise as follows. When mounting in the PCB, note that the notch in the LM3914 is marked in its outline on the PCB. *REVISION 1 NOTE: The PCB silkscreen pattern for the LED Array is incorrect. The notch indicating the side with pin 1 should be on the other side (toward the sensing end of the thermometer). 1 2 3 4 5 6 7 8 9 LM3914 18 17 16 15 14 13 12 11 10 Note: The different values of the resistors can be noted by the colored bands on their bodies. The 330 Ohm resistor has bands: ORANGE, ORANGE, BROWN, GOLD. The 220 Ohm resistors have bands: RED, RED, BRONW, SILVER. The 1000 Ohm resistor has bands: BROWN, BLACK, ORANGE, YELLOW. Step 3Clip excess length from the leads of the components on the back of the PCB. Only do this after the pieces are soldered in securely. Step 4Connect the 9v battery once everything is soldered to the PCB. Using the DIP switch on the PCB, turn on the thermometer. The green LED should light. By adjusting the potentiometers, you can calibrate the thermometer.