Download Kit Instructions

LED Thermometer
PART NO. 2260261
We receive information about temperature or use thermometers almost every day. But have you ever wondered how temperature is
actually measured?
After completing this great starter kit user will get nice interactive LED thermometer. With this kit 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:
Temperature measurement range: 0-40 degree Celsius
Indication: 20 LEDs (2 degree Celsius per LED)
Power Supply: 5V - 9V (Recommended 9V battery or 5V from PC USB port)
Supply Connectors: 9V Battery snap and female USB port (male to male USB cable included in kit)
Switches: Tactile switch for momentary temperature indication and Slide Switch for permanent indication
Dimensions: 2.7cm x 17cm (1.0630 inch x 6.6929 inch)
This thermometer indicates temperature with 20 blue 5mm LEDs and measures temperature in ranges from 0 to 40 degrees Celsius,
each LED corresponds to 2 degree Celsius. LEDs will turn ON in the same way as Mercury rises in standard thermometer.
Power Supply for LED thermometer can be standard 9V battery, but there is also USB connector on the PCB so this device can be
supplied from PC USB port (do not connect 9V battery and USB power supply at the same time!)
Kit includes 9V battery snap and 6 feet USB male to male cable.
Each LED with large viewing angle is driven with 12.5 mA current which ensures their brightness. Tactile switch is pressed to turn ON
device to indicate temperature and when tactile switch is released device will turn OFF. Also there is sliding switch which will keep
thermometer permanently turned ON or OFF.
Check out this link if you want to see 3D Model of assembled LED Thermometer:
https://grabcad.com/library/led-thermometer-1/files#
Click on 3D view marked with red.
WARNING: If you are unsure of the dangers involved with your particular project, consult with someone who is
experienced. Always wear eye protection and gloves. FAILURE TO INITIATE AND FOLLOW YOUR OWN SAFETY
PROCEDURES MAY RESULT IN BODILY INJURY OR DEATH!
Time Required: 1 to 2 hours depending on experience
Experience Level: Beginner
Required tools and parts:
Soldering Iron
Soldering Iron Stand
Solder
Wire Cutters
Multimeter
Bill of Materials:
Qty
Jameco SKU
Component Name
2
300003
U1, U2 - 10 Segment LED Driver
1
155740
U3 - Temperature Sensor
1
23966
U4 - General Purpose Dual OP Amp
1
254028
RV1 - Potentiometer 50K Ohm
1
691083
R1 - Resistor Carbon Film 8.2k Ohm
1
690865
R2 - Resistor Carbon Film 1k Ohm
1
690945
R3 - Resistor Carbon Film 2.2k Ohm
1
691171
R4 - Resistor Carbon Film 20k Ohm
20
2211080
Blue 5mm LEDs
1
94001
C1 - 2.2 uF 16 Volt Tantalum Capacitor
1
109171
Slide Switch Single Pole Double Throw
1
153252
Tactile Pushbutton Switch
1
109154
9V Battery Snap
1
2096181
USB-A Right Angle Connector
1
2150985
6 Foot Blue USB 2.0 Male to Male Cable
Step 1 - Schematic
Kit 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
The PCB is single sided with components on top and copper on the bottom layer. All holes are non-plated and PCB dimensions are
2.7cm x 17cm (1.0630 inch x 6.6929 inch).
There are small mounting holes at the edges of the PCB.
Step 3 - How does this circuit work?
Temperature sensor used to calculate temperature is the 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 two LM3914. This small voltage is
amplified by a factor of 6.25.
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,
but 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.
Let's give an example. Ambient temperature is 20 degree Celsius. Question is how many LEDs are turned ON?
At 20 degree Celsius, output voltage of LM35 temperature sensor is 200 mV. This voltage is then amplified by a factor of 6.25 and now
at the output of the amplifier there is 1250 mV which is 1.25 V.
Total reference voltage of two LM3914 is 2.5V and that means that each LED is at 0.125V voltage level. So 1.25V/0.125V equals 10,
which means that 10 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 6.25
amplification factor, potentiometer resistance should be set to nominal 43k Ohms.
This circuit works with power supply voltages between 5V - 9V and its measurement quality is not dependent on voltage, so its power
supply can be a 9V battery.
Temperature is indicated with 5mm blue LEDs with a large viewing angle and each LED corresponds to 2 degree Celsius temperature
increase.
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 t h e orientation of some components. Two LM3914 (U1 and U2), temperature sensor LM35 (U3)
operational amplifier LM358N (U4), capacitor C1, blue LEDs (D1 - D20) and battery holder are all polar.
When mounting in the PCB, note that the notch in 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 temperature designation numbers.
The red wire from the battery pack is positive.
Tactile switch and USB 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 four resistors and one potentiometer, which need to be soldered on the PCB.
R1 is 8.2k designated with: GRAY RED RED GOLD
R2 is 1k designated with: BROWN BLACK RED GOLD
R3 is 2.2k designated with: RED RED RED GOLD
R4 is 20k designated with: RED BLACK ORANGE GOLD
Potentiometer RV1 is 50k Ohm and it can be placed only in one direction in PCB, this potentiometer should be set to 43k 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 43k. It should be set after you solder potentiometer on the PCB.
To correctly set 43k, you can use multimeter to measure resistance at nodes shown on the image above.
If you don't have multimeter, you can set potentiometer slider to be at 86% 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 8.2k 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 1.25V at amplifier output to turn ON 10 LEDs so:
Vout = 0.2 * (1 + 43k/8.2k) = 1.25V
A = (1 + RV1/R1) = should be as close as possible to 6.25
Sometimes calculated values can deviate from real world ones. That is because each component have its tolerances. In that case,
there is one simple practical way how you can calibrate your thermometer.
Simple 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.