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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Reetu LED Thermometer Saturday, November 16, 2013 5:10:16 AM

plz tell me how the tempermerature is measured now ? and is LM35DZ can be used in this instead of LM35DZ??

Richard LED Thermometer Friday, January 11, 2013 1:35:06 PM

Please sir i constructed LED thermometer using LM340[instead of LM34DZ] and LM3914. I don't know where to read the temperature from in the circuit. Please how can i read the temperature of human being.Thank you Sir.

kiran LED Thermometer Saturday, November 27, 2010 9:52:22 AM

project is not succeeded

anonymousRic LED Thermometer Monday, September 13, 2010 2:58:35 PM

I take it this is an ac voltage not dc is that true

Jen LED Thermometer Sunday, January 24, 2010 5:14:05 PM

I found this circuit in my old Popular Electronics magazine from July 1996 on page 45. I decided to breadboard the circuit and test it. It worked perfectly after I calibrated it.

anonymous LED Thermometer Thursday, December 10, 2009 12:05:07 PM

Hi, please anybody till me how i know this project working or no. when i connect the voltage all leds lighting, pleaeaeaaeeaeaeaeaeaeaeaeaeeaes tiil me.

Jim LED Thermometer Monday, November 23, 2009 11:18:35 PM

I have built an extremely accurate digital thermometer, using a 4-1/2 LCD panel meter and an LM34DZ. Dont breath at it because the temp will go up 5 degrees from a couple feet! Simply purchase the PM328 meter from All Electronics and order the temperature device, download the PDF data sheets,and you will be very pleased. One word of caution. Because National Semiconductor cant seem to get their act together and test these devices, most are being distributed in the Centigrade format! Nine out of ten that I ordered from All Electronics were FORMATTED IN CELSCIUS and the tenth one was FAHRENHEIT.

Jim LED Thermometer Monday, November 23, 2009 10:27:37 PM

The reason most of you folks are having trouble with these projects is because the manufacture of the LM34DZ has failed to test them! they are mostly CENTERGRADE that's right, CELSIUS! I found out the hard way by ordering a dozen or so from ALL ELECTRONICS for 2.50 each. 8 out of 12 where CENTERGRADE. Therefore I have thrown 30 dollars in the sewer because of the incompetance of NATIONAL SEMICONDUCTOR? When I asked for my money back, they, in so many words told me to piss-off.

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.

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Reetu	LED Thermometer	Saturday, November 16, 2013 5:10:16 AM
plz tell me how the tempermerature is measured now ? and is LM35DZ can be used in this instead of LM35DZ??
Richard	LED Thermometer	Friday, January 11, 2013 1:35:06 PM
Please sir i constructed LED thermometer using LM340[instead of LM34DZ] and LM3914. I don't know where to read the temperature from in the circuit. Please how can i read the temperature of human being.Thank you Sir.
kiran	LED Thermometer	Saturday, November 27, 2010 9:52:22 AM
project is not succeeded
anonymousRic	LED Thermometer	Monday, September 13, 2010 2:58:35 PM
I take it this is an ac voltage not dc is that true
Jen	LED Thermometer	Sunday, January 24, 2010 5:14:05 PM
I found this circuit in my old Popular Electronics magazine from July 1996 on page 45. I decided to breadboard the circuit and test it. It worked perfectly after I calibrated it.
anonymous	LED Thermometer	Thursday, December 10, 2009 12:05:07 PM
Hi, please anybody till me how i know this project working or no. when i connect the voltage all leds lighting, pleaeaeaaeeaeaeaeaeaeaeaeaeeaes tiil me.
Jim	LED Thermometer	Monday, November 23, 2009 11:18:35 PM
I have built an extremely accurate digital thermometer, using a 4-1/2 LCD panel meter and an LM34DZ. Dont breath at it because the temp will go up 5 degrees from a couple feet! Simply purchase the PM328 meter from All Electronics and order the temperature device, download the PDF data sheets,and you will be very pleased. One word of caution. Because National Semiconductor cant seem to get their act together and test these devices, most are being distributed in the Centigrade format! Nine out of ten that I ordered from All Electronics were FORMATTED IN CELSCIUS and the tenth one was FAHRENHEIT.
Jim	LED Thermometer	Monday, November 23, 2009 10:27:37 PM
The reason most of you folks are having trouble with these projects is because the manufacture of the LM34DZ has failed to test them! they are mostly CENTERGRADE that's right, CELSIUS! I found out the hard way by ordering a dozen or so from ALL ELECTRONICS for 2.50 each. 8 out of 12 where CENTERGRADE. Therefore I have thrown 30 dollars in the sewer because of the incompetance of NATIONAL SEMICONDUCTOR? When I asked for my money back, they, in so many words told me to piss-off.
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.