LED Temperature Thermometer

LED temperature thermometer project will display the room temperature where it is

placed using 10 LEDs, with each LED representing 2 degree Fahrenheit step, forming a

bargraph display. We use the temperature sensor and LM3914 LED driver interegated

circuit.

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. The input is compared to the voltages at pins 4 and 6

which will determine which LEDs are lighted up. Pin 6 must be calibrated to a voltage of

3.345V and pin 4 2.545V in order to display the range of temperature mentioned by

adjusting variable resistors 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 that the proper LED for that temperature are lighted up.

Next, measure the voltage between pin 5 and GND. Its value should be close to the

calculated equation below:

V=0.225 + (0.04 X T) where T is the temperature in degrees Fahrenheit.

Advantage:

-We can determine the room temperature just by watching the number of glowing LED.

Submitted by:

Smriti Pandey

Sonal Khandelwal

4th year E.C.E.