LED Thermometer

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This LED thermometer is designed for in home use, to read temperatures between about 60 and 78 degrees Fahrenheit. It is based around a precision temperature sensor IC, the LM34DZ. This sensor require no calibration and can measure temperatures of between -50F and +300F. While the circuit shown here does not use the full range of that sensor, it can be modified to do so by simply changing the voltage reference to U2 at the sacrifice of precision.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
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 U1 and U2, Case

Notes

The pinout of U1 depends on the version of the IC you purchase. These options are shown below:

Pinout of LM34 IC variants

You will want to build the circuit with U1 and U2 in sockets in order to be able to complete calibration (which requires removal of these ICs).

You can use any LED you want for D1 - D10, however blue LEDs have a higher voltage requirement so if you want to go blue for a modern look, they may appear more dim then red, yellow or green.

By leaving pin 9 of U2 disconnected, the graph will operate in dot mode and R8 should be 100 Ohm. If you build the circuit with pin 9 tied to 9V, the circuit will be in graph mode and R8 should be 15 Ohms.

To calibrate the circuit, you will need a voltmeter. Power the circuit up and let it sit for a few minutes for temperature to stabilize. Ground the negative lead of the meter and connect the positive lead to pins 6 and 7 of U2. Set R7 so the meter reads as close to 3.345V as possible. Now connect the positive lead of the meter to pin 4 of U2 and adjust R5 until the meter reads 2.545V. Finally, disconnect power to the circuit and remove U1 and U2 from their sockets. Measure the value of R3 with an ohmmeter and remember that value. Connect the ohmmeter across R1 and adjust R1 to a value of exactly 3 times the value of R3. Reinstall U1 and U2 and the circuit is ready for use.

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Comments

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anonymous LED Thermometer Friday, May 01, 2009 6:49:49 AM

it is not a trust worthy experiment i have tried this but the result is the temperature is incorrectly shown

anonymous LED Thermometer Tuesday, November 18, 2008 2:01:36 PM

I was able to get this to operate by removing the capacitor C2. You should only be using this capacitor if your LED leads are more than six inches in length.

$ethi LED Thermometer Thursday, November 13, 2008 6:28:51 AM

I designed the circuit bt the problem is that all the LED glow immediately when u supply the voltage..So it doesn't;t show normal temperature like a barometer... Waste of time in designing this circuit...

Vashisth LED Thermometer Sunday, September 07, 2008 2:04:41 AM

Thats a good project but I'm not able to know that how we can desplay o/p temperature and second question is "Can we use LM35DZ instead of LM34DZ

anonymous LED Thermometer Wednesday, August 27, 2008 9:58:06 AM

Anyone make works this circuits?

anonymous LED Thermometer Monday, August 25, 2008 10:51:17 AM

did anyone get work for this project???anyone see this comment,please answer it, coz i really need to make this project works but i can't

Peter LED Thermometer Tuesday, August 12, 2008 2:27:23 AM

simple and nice idea

anonymous LED Thermometer Monday, July 21, 2008 5:05:28 AM

Extremely good. But tell that how o/p is displayed?? How we will come to know the temperature?? I want to design it as college miniproject.

dinesh circuits Friday, March 28, 2008 6:50:13 AM

really very good. excellent ideas. keep it up

jaydip LED Thermometer Wednesday, March 19, 2008 3:50:42 AM

how i can make led thermometer? plz give me it's process

anonymous LED Thermometer Sunday, March 09, 2008 6:48:36 AM

how did I know if the LED thermometer we did is correct.how did we check for it??????????????? thank you

misho LED Thermometer Sunday, January 27, 2008 12:22:47 PM

what is the device that displays the temprature? is it the leds or what???

elamaran LED Thermometer Sunday, December 30, 2007 6:36:57 AM

this page is veryused in my project work thankyou

anonymous LED Thermometer Monday, November 12, 2007 1:34:53 AM

it is very useful to me

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anonymous	LED Thermometer	Friday, May 01, 2009 6:49:49 AM
it is not a trust worthy experiment i have tried this but the result is the temperature is incorrectly shown
anonymous	LED Thermometer	Tuesday, November 18, 2008 2:01:36 PM
I was able to get this to operate by removing the capacitor C2. You should only be using this capacitor if your LED leads are more than six inches in length.
$ethi	LED Thermometer	Thursday, November 13, 2008 6:28:51 AM
I designed the circuit bt the problem is that all the LED glow immediately when u supply the voltage..So it doesn't;t show normal temperature like a barometer... Waste of time in designing this circuit...
Vashisth	LED Thermometer	Sunday, September 07, 2008 2:04:41 AM
Thats a good project but I'm not able to know that how we can desplay o/p temperature and second question is "Can we use LM35DZ instead of LM34DZ
anonymous	LED Thermometer	Wednesday, August 27, 2008 9:58:06 AM
Anyone make works this circuits?
anonymous	LED Thermometer	Monday, August 25, 2008 10:51:17 AM
did anyone get work for this project???anyone see this comment,please answer it, coz i really need to make this project works but i can't
Peter	LED Thermometer	Tuesday, August 12, 2008 2:27:23 AM
simple and nice idea
anonymous	LED Thermometer	Monday, July 21, 2008 5:05:28 AM
Extremely good. But tell that how o/p is displayed?? How we will come to know the temperature?? I want to design it as college miniproject.
dinesh	circuits	Friday, March 28, 2008 6:50:13 AM
really very good. excellent ideas. keep it up
jaydip	LED Thermometer	Wednesday, March 19, 2008 3:50:42 AM
how i can make led thermometer? plz give me it's process
anonymous	LED Thermometer	Sunday, March 09, 2008 6:48:36 AM
how did I know if the LED thermometer we did is correct.how did we check for it??????????????? thank you
misho	LED Thermometer	Sunday, January 27, 2008 12:22:47 PM
what is the device that displays the temprature? is it the leds or what???
elamaran	LED Thermometer	Sunday, December 30, 2007 6:36:57 AM
this page is veryused in my project work thankyou
anonymous	LED Thermometer	Monday, November 12, 2007 1:34:53 AM
it is very useful to me