The LED thermometer project uses a normal precision LM 34 temperature sensor which has a linear voltage yield based on the room temperature that is measured in Fahrenheit. To display the temperatures in Celsius, you can apply the LM 35 temperature sensor.
There are three packages that are available which are; TO-92 plastic package, SO-8 molded package or TO-46 metal can package. The use of TO-92 plastic package is more appropriate since it is readily available in the market.
The advantage of LM 34 compared to a thermistor is that the output energy is linear whereas thermistor is designed with a resistance characteristic which is non-linear. An LM 34 is also more accurate when compared to a thermistor. However, it is more expensive compared to a thermistor.
The LED driver for displaying the temperature is offered by LM 3914. A temperature ranging between 62 to 80F will be exhibited using the bar mode-display setting. The jumper JP with its header will need to be installed in this mode.
The room temperature can be displayed using different LED colors. For example; L1, 2, 3 and 4 can be displayed in green, L8 to L10 red and L5 to L7 can be orange. This project uses 9V DC as the supply voltage. This is easily available since you can get it from an ordinary 9V dry cell battery.
An ordinary LM34 can provide a yield of 10Mv/deg Fahrenheit. By the use of a few resistors, energy yield can be amplified to 40mV/deg Fahrenheit hence providing a wider range of voltage.
Power up the circuit by linking to 9V battery then switch on using switch S. calibrate the LM 3914 using a digital multimeter. Adjust VR3 till pin 6 of LM3914 displays a value of 3.385V. Repeat the same procedure by adjusting VR2 for pin 4 till it shows a value of 2.505V.
After this, place a thermometer close LM34 in a room with temperature range of 62-80 F but not touching it.