LED Thermometer – Temperature Measurement Circuit
This is a simple electronic project suitable for everyone. This project is about making a LED bar graph thermometer using two ICs, one for the sensor.
What is a bar graph thermometer?
A bar graph thermometer is one which shows temperature reading using an array of LEDs instead of a digital numerical output. Here this project has 10 LED lights which indicate temperatures ranging from 60 to 80 degree F. They are arranged in a single line, and by counting the number of LEDs lit you can determine the temperature. Here in this circuit, the glowing of a total of 1 LED indicates 60 degree F, glowing all 10 indicates 80 degrees F. The rest of the 8 LEDs indicate 2.5-degree temperature increments.
The circuit given here is a precision LED bar graph Fahrenheit thermometer, which can be used as a temperature measurement circuit, build around IC LM 34 (sensor) and IC LM 3914 (bar graph driver). The circuit can be modified to read in degree Celsius by replacing sensor LM 34 with LM 35. Both are interchangeable. The circuit operates of a volt power supply.
To calibrate the circuit, you will need a voltmeter. Power the circuit. Ground the negative lead of the voltmeter and connect the positive lead to pins 6 and 7 of IC LM 3914. Adjust R7 so the meter reads 3.345V as possible. Now connect the positive lead of the meter to pin 4 of IC LM 3914 and change R5 until the meter reads 2.545V. Disconnect power to the circuit and remove both IC’s from their holders. Check the value of R3 with an ohmmeter and remember that value. Connect the ohmmeter across R1 and set R1 to a value of exactly 3 times the value of R3. Reconnect both IC’s and the circuit is ready. Here the circuit is designed for home use and reads between 60 to 80 degree F.
If pin 9 of IC LM 3914 is left disconnected the display will be in dot mode and R8 must be 100 Ohm. If pin 9 is connected to 9 V display will be in bar mode.
Part. Qty. Description .
C1 1 1uF 25V Electrolytic Capacitor
C2 1 10uF 25V Electrolytic Capacitor
R1 1 2.2K 1/4W Resistor
R2,R5,R7 3 1K Trim Pot
R3 1 1K 1/4W Resistor
R4 1 1.5K 1/4W Resistor
R6 1 470 Ohm 1/4W Resistor
R8 1 100 Ohm Or 15 Ohm 1/4W Resistor (See Notes)
D1-D10 10 LED
U1 1 LM34DZ Precision Fahrenheit Temperature Sensor
U2 1 LM3914 Bar/Dot Graph Driver IC
MISC 1 Board, Wire, Socket For IC’s, Case
Bar Graph Thermometer Circuit diagram
LM 34/35 pin assignment
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