LED Temperature Thermometer ProjectIntroduction To LED Temperature Thermometer This LED temperature thermometer project will display the room temperature where it is placed using 10 LEDs, with eachLED representing 2 degree Fahrenheit step, forming a bargraph display. Electronics hobbyists will learn how to use thetemperature sensor and LM3914 LED driver. Another method of doing this project is using a low cost microcontroller if one is familiar with it. Otherwise, this is a good project for beginners to electronics. The LM3914 is a monolithic integrated circuit that senses analog voltage levels and drives 10 LEDs, providing a linear analog display. Current drive to the LEDs is regulated and programmable, eliminating the need for resistors. Much of the display flexibility derives from the fact that all outputs are individual, DC regulated currents. Various effects can be achieved by modulating these currents. The individual outputs can drive a transistor as well as a LED at the same time, so controller functions including "staging" control can be performed. The LM3914 can also act as a programmer, or sequencer. Temperature Thermometer Circuit Description 
The schematic diagram shows how temperature sensor LM34DZ is connected to LM3914 IC. It produces a voltage between VCC and Ground that is linearly proportional to the temperature that it senses, usually 10mV/degree F. In this circuit,it is connected to R1, R2 and R3 to form a gain output of 40mV/degree F. The output of LM34 is fed to LM3914 pin 5. LM3914 has 10 internal comparators with outputs connected to LED1-LED10. Theinput is compared to the voltages at pins 4 and 6 which will determine which LEDs are lighted up. Pin 6 must be calibratedto a voltage of 3.345V and pin 4 2.545V in order to display the range of temperature mentioned by adjusting variableresistors R5 and R7. Start by adjusting R7 to obtain pin 6 calibration voltage and then R5 to obtain pin 4 calibration voltage. Use a standard thermometer and measure the air temperature at the vicinity of LM34. Adjust variable resistor R1 so thatthe proper LED for that temperature are lighted up. Next, measure the voltage between pin 5 and GND. Its value should beclose to the calculated equation below: V=0.225 + (0.04 X T) where T is the temperature in degrees Fahrenheit.
Make sure that LM34 is position away from any heat source of the circuitry as this will affect its reading. Adequate holesshould be drilled on the enclosure to ensure ventilation to the sensor. Temperature Thermometer Parts List 
The specifications of the Temperature Thermometer LM34 and LM3914 Specs can be obtained from National Semiconductor website.
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