Electronic Thermometer Project by LM35 and LM3914

Description: 

As shown in the schematic, temperature sensor of our electronic thermometer is LM35DZ. There are some kinds of LM35 IC, since it is cheap and easy to find we used LM35DZ in our project. It measures from 0°C to 100°C with a very linear output graph.For one degree change, it increases its output 10mV.On the other hand, this circuit measures temperature values only between +10°C and +39°C.

2 numbered (middle) pin of the sensor is connected to the 5 numbered pins of LM3914 ICs. So every IC determines how many leds of bargraph will bright due to the analog  signal received from the sensor. 2.2 microfarad tantal capacitors are connected between the 2 and 3 numbered pins of LM3914. Resistors in the circuit have %1 tolerance values.

Click here to download PCB (1:1 scaled) and layout files

For proper operation, voltage values of the nodes shown in the circuit must be adjusted to 90mV, 190mV, 290mV and 390mV by using multiturn trimpots. This adjustment determines the range of the shown values for each led bar display. For instance first display shows from 10°C to 19°C, second display shows from 20°C to 29°C and so on.

9V battery can be used for voltage supply. In this situation, circuit operates properly until voltage of the battery decreases to 4V. Current consumption depends on the number of lit LEDs. Every led draws about 5mA, at about 18° 16 LEDs will be lit and total LED current draw will reach to 80mA. If the circuit left operating,  battery will discharge in a short time. To prevent this situation, circuit should be opened only when needed. So putting a on-off switch will be sufficient. If a 9V AC/DC adaptor is used, circuit can be left operating continuously with no drawback.

Last step is placing the board into a proper box.If it is put in a box, temperature sensor should be placed outside. You can draw a scale divided in to 2.5mm parts. If you put a piece of glass or plexy on the led bargraphs, they will be more visible. 

To test the circuit, you can use a mercury thermometer as shown in the image. We can see our thermometer gives right results.

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Comments (17)

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GOD HELP ME
written by MEHDI , December 29, 2007

I decide to make this thermometer for the Girl i love and she wanted it for her project in university and God help me if will not work!
Pray For me! LOL!

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written by math, December 29, 2007

hello I want to do this thermometer too please write the result for me man.. It will be nice if you send some photos of the circuit also. good luck :)

change the range from 0 to 100 instead of 0 to 39c
written by Rens, January 15, 2008

What resistor values should I change If i want to make the range from 0 to 100c? I'm sorry, I only know how to solder and I do not know how to recompute the circuit.

Take a look at a look at schematic
written by Alan, February 17, 2008

Take a look at the schematic. The LM35 output is scaled 10mV/degree C. Look at the reference voltages on the three stages U!-3 and see how they are scaled (90,190,290,390 mV). If you want the highest reading to be 1000 mV, then you could add more stages (a really long bar graph!) or you could adjust for different reference voltages (250, 500,750,1000 mV) on pots 2,1,3,4 respectively.

ETH 256 AMP
written by aziz, February 19, 2008

hi . . I am using the lm35dz sensor to measure the body temperature by connected to ETH 256C AMP . Am not getting the result output from the AMP .
hope you e mail me
Thanks

my comment
written by ilham afandi, February 20, 2008

thanks for all, now i wiil try it in my college,s laboratory!

Nice
written by Bill, February 20, 2008

I wonder if you could substitute an LM34 (Fahrenheit) directly? It appears to have the same scale division as the LM35 (10mV/F).

Max Temp
written by Alex, February 29, 2008

Hi. The LM35DZ measures up to 150°C. What voltages do I need for a thermometer from 50°C to 150°C? Can the LM35DZ in some way measure 200°C?

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written by cremer, March 01, 2008

You should change the ref voltage values shown in the schematic (90mV, 190mV, 290mV and 390mV). Starting from 490mV to 1500mV. I think it is not possible to read 200 °C by using LM35DZ because it doesnt give a linear output at the values greater than 150 °C

What sensor?
written by Alex, March 01, 2008

Do you know what sensor is capable of measures 250°C? Thanks

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written by cremer, March 02, 2008

I searched for parts but couldn't find out any sensor in this range. National has a sensor which measures maximum 200°C. You can check out National Semiconductor Temperature Sensor Handbook http://www.national.com/appinfo/tempsensors/files/temphb.pdf

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written by Alex, March 03, 2008

I cant get a range of 500 mv to 1500mv with the 1k trimpot. What should i change?

Thanks

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written by cremer, March 04, 2008

try decreasing the R2 R3 and R4 values. Lets see what will happen

Question
written by Caleb, March 08, 2008

Hi! I just want to ask what the 330R and 75R are...

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written by marok, March 08, 2008

Hi they arae 330 ohms and 75 ohms

Still
written by Alex, March 10, 2008

Hi, i´ts me again. I´m trying to get these values: 600mv, 900mv, 1200mv and 1500mv so i can measure up to 150°C. I reduced R3 and R4 to 1k2 and R2 to 1k, and i can get 1500mv, 1200mv and 900mv. But i cant get 600mv in the wiper of POT 2. What should i do?
Thanks

where'e the code?
written by buddy, April 07, 2008

can i have the codes?