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 for remote telephone ringer


Total Qty.
C111uF 25V Electrolytic Capacitor
C2110uF 25V Electrolytic Capacitor
R112.2K 1/4W Resistor
R2, R5, R731K Trim Pot
R311K 1/4W Resistor
R411.5K 1/4W Resistor
R61470 Ohm 1/4W Resistor
R81100 Ohm Or 15 Ohm 1/4W Resistor (See Notes)
D1 - D1010LED
U11LM34DZ Precision Fahrenheit Temperature Sensor
U21LM3914 Bar/Dot Graph Driver IC
MISC1Board, Wire, Socket For U1 and U2, Case


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

    Pinout of LM34 IC variants

  2. 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).
  3. 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.
  4. 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.
  5. 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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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??
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.
LED Thermometer
Saturday, November 27, 2010 9:52:22 AM
project is not succeeded
LED Thermometer
Monday, September 13, 2010 2:58:35 PM
I take it this is an ac voltage not dc is that true
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.
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.
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.
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.
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
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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