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 that 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 degrees 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 degrees F, glowing all 10 indicates 80 degrees F. The rest of the 8 LEDs indicate 2.5-degree temperature increments.

## LED Thermometer – Temperature Measurement Circuit

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 degrees Celsius by replacing sensor LM 34 with LM 35. Both are interchangeable. The circuit operates off 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.

Parts list

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

### LM 34/35 pin assignment

This is a cool project involving sensors, driver pics. We live in a world where sensors have more importance like never before. Nowadays we depend a lot on data and the data points are collected by sensors. It is very important for an electronics engineer to learn & understand sensors.

#### 11 Comments

1. tipek136

Hi, I’ve got problem. Voltmeter reads on pins 6 and 7 max. 2.00 V. What I made wrong? Please help

2. nikhiltakke

heyy dude…if ma all leds are glowing when circuit is powered ….den wat is d problm in ma circuit…plz plz plz tell me…..!!!!!

3. Seetharaman

Hi Mike refer to National application data use circuit shown in Figure 4. Ensure connecting leads are insulated from heat, else it will sense that temperature. use dab of cement to insulate. Kindly go through National application guide.
http://www.national.com/ds/LM/LM34.pdf

4. My son and I built this circuit. It works fine with the LM34 located on the circuit board. But we want the sensor to be 15 ft remote. We looked at the data sheet where it talks about putting resistors and a capacitor from output to ground etc. but the led’s don’t light up unless you tweek the trimmer pots and then the graph is off and not true to ambient temperature. We are very frustrated. Can anyone help?
Mike

5. To interchange between mode (dot/bar) with the LM3914, dis/connect the Pin 9 to the voltage supply

6. Micah Vanderstappen

Hey man, everytime I try to open up posts I just wind up viewing a small part of the page. I do not understand if it is my browser(Safari) or not, is someone else experiencing this thing?

• @ Micah Vanderstappen
We do not find any problem while loading the page in Safari. Anyway try the site in Firefox.

7. thax for help a bout bar/dot graph display of the led thermometer, but now i request for all work that has been done on led thermometer

8. please, am designing LED thermometer that measures temperature a round room temperature using simple materials. but i have failed to know how i can change from DOT to BAR display, i need help, thanks