LED Thermometer V2 (Fahrenheit/Celsius/±1 ) PART NO. 2244754 After completing this great starter kit, users will have a nice interactive LED thermometer. You will learn one principle how temperature can be measured in simple but precise way. It is a very good design using low cost industry standard parts. Thermometer Specifications: Indication: 40 LEDs Power Supply: 9V (Recomended 9V battery OR 9V from power adapter - dont use both at the same time) Supply Connectors: 9V Battery snap and 2.1mm Male DC Power Jack (Adapter included in kit) Switches: Tactile switch for momentary temperature indication and Slide Switch for permanent indication Dimensions: 2.7cm x 17cm (1.063" x 6.6929") This is an improved version of the previous LED Thermometer kit project (Part no.: 2260261). Main improvements are increased number of LEDs and therefore better temperature indication with one Celsius degree increments. PCB dimensions for this LED thermometer are same as for previous version (Part no.: 2260261) but this one works with 9V and comes with power adapter. Further improvement is that temperature will be labeled both in Celsius and Fahrenheit. Time Required: 3-5 Hours depending on experience Experience Level: Beginner Required tools and parts: Soldering iron Soldering iron stand Solder Wire cutters Multimeter (for calibration) Bill of Materials: Qty Jameco SKU Component Name 4 300003 U1, U2, U3, U4-10 Segment LED Driver 1 155740 U5 - Temperature Sensor 1 120862 U6 - General Purpose Dual OP Amp 1 254028 RV1 - Potentiometer 50K Ohm 1 691008 R1 - Resistor Carbon Film 3.9k Ohm 1 690865 R2 - Resistor Carbon Film 1k Ohm 1 690945 R3 - Resistor Carbon Film 2.2k Ohm 4 691171 R4, R7, R8, R9 - Resistor Carbon Film 20k Ohm 1 690988 R5 - Resistor Carbon Film 3.3k Ohm 1 691024 R6 - Resistor Carbon Film 4.7k Ohm
3 94001 C1, C2, C3-2.2 uf 16 Volt Tantalum Capacitor 1 109171 Slide Switch Single Pole Double Throw 1 153252 Tactile Pushbutton Switch 1 2168114 2.5mm Male DC Power Jack 1 109154 9V Battery Snap 1 2227428 9 Volt 1 Amp 9 Watt Regulated Linear Wall Adapter 40 2168421 3mm Blue LED with Diffused Lens 1 --- Custom printed circuit board (included) Step 1 - Schematic This design uses low cost industry standard parts. LEDs have a large viewing angle in order to be seen from all directions.
Step 2 - PCB 3D model Two layer PCB with components on top and copper on bottom and top layer. PCB dimensions are 1.063" x 6.6929". There are small mounting holes at the edges of the PCB. Step 3 - How does this circuit work? Temperature sensor used is LM35, which produces voltage of 10 mv per one degree Celsius at its output pin. In this circuit, this small voltage is then amplified in order to be measured with four LM3914 integrated circuits. This small voltage is amplified by a factor of 12.5. Reference voltage of the first LM3914 is 1.25V, so when 1.25V signal is applied to its input, 10 LEDs will turn ON. The second LM3914 has a reference voltage of 2.5V. Two LM3914 reference voltage sources are connected in series, so that is the reason why the second LM3914 has a 2.5V reference since its ground is not at 0V. Instead it is at 1.25V (Rlo output is also not on ground but on 1.25V), so that ensures that its LEDs are turned OFF at input voltages below 1.25V. Same principle works on other two LM3914 integrated circuits. Let's give an example. Ambient temperature is 20 degree Celsius. Question is how many LEDs are turned ON in that case? At 20 degree Celsius, output voltage of LM35 temperature sensor is 200 mv. This voltage is then amplified by a factor of 12.5 and now at the output of the amplifier there is 2500 mv which is 2.5 V. Total reference voltage of two LM3914 is 2.5V and that means that each LED is at 0.125V voltage level. So 2.5V/0.125V equals 20, which means that 20 LEDs will be turned ON at 20 degree Celsius. In order to calibrate circuit, only one potentiometer is used. This potentiometer changes amplification factor. In order to get 12.5 amplification factor, potentiometer resistance should be set to nominal 44.85k Ohms. This circuit works with 9V power supply. Temperature is indicated with 3mm blue LEDs with a large viewing angle and each LED corresponds to 1 degree Celsius.
Step 4 - 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 your soldering nice and neat, act deliberately. Soldering is a skill mastered over many years. Don't get frustrated if 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. 7. It is advised to read these kit instructions carefully. Step 5 - Polar Components You should take care of the orientation of some components. Four LM3914 (U1, U2, U3 and U4), temperature sensor LM35 (U5) operational amplifier LM358N (U6), capacitors (C1, C2, C3), blue LEDs (D1 - D40) and battery holder are all polar. When mounting on the PCB, note that the notch on the LM3914 is marked in its outline on the PCB. Same is with LM358N operational amplifier you should rotate it so that notch on component package aligns with notch on PCB. Temperature sensor LM35 should be placed on PCB, so that its body is aligned with pattern on the PCB. Tantalum capacitor is also polar component and you should take care of how you place it on PCB. There is one line on its body marked with + at the end, as you can assume this is + lead of capacitor which is then placed through hole with + written near it on PCB. The positive side (anode) of an LED is the side with the longer wire lead. Those longer leads are placed in holes closer to edge of PCB. The red wire from the battery pack is positive. Tactile switch and Power jack cannot be placed incorrectly since there is only one way how they can be placed on PCB. Slide switch can be placed in either direction. Step 6 - Resistors There are nine resistors and one potentiometer, which need to be soldered on the PCB. R1 is 3.9k designated with: ORANGE WHITE RED R2 is 1k designated with: BROWN BLACK RED GOLD R3 is 2.2k designated with: RED RED RED GOLD R4, R7, R8, R9 are 20k designated with: RED BLACK ORANGE GOLD R5 is 3.3k designated with: ORANGE ORANGE RED R6 is 4.7k designated with: YELLOW VIOLET RED Potentiometer RV1 is 50k Ohm and it can be placed only in one direction in PCB, this potentiometer should be set to 44.85k Ohm nominal value. Resistors are non-polar components.
Step 7 - Temperature Measurement Calibration In order to correctly measure temperature, this circuit needs to be calibrated. Calibration is done by setting resistance of 50k potentiometer RV1. Calculated resistance value which needs to be set is 44.85k. It should be set after you solder potentiometer on the PCB. To correctly set 44.85k, you can use multimeter to measure resistance at nodes shown on the image below. If you don't have multimeter, you can set potentiometer slider to be at 90% of total scale value. By changing resistance value, you are actually changing amplification factor of amplifier which is calculated with following formula: Vout = Vin * (1 + RV1/R1) R1 is 3.9k resistor and Vin is voltage from temperature sensor (10 mv per one degree Celsius) At 20 degree Celsius, Vin is 200 mv and we need to get 2.5V at amplifier output to turn ON 20 LEDs so: Vout = 0.2 * (1 + 44.85k/3.9k) = 2.5V A = (1 + RV1/R1) = should be as close as possible to 12.5 Sometimes calculated values can deviate from real world ones. That is because each component has its own tolerance. In that case, there is one simple practical way how you can calibrate your thermometer. Simply use standard thermometer you already have and read its temperature indication. After that, force LED thermometer kit to show same temperature by changing resistance of potentiometer.