This is my first instructable and it is a LM35 and LM3914 based room temperature thermometer. I built this circuit on a bread board but you could use a veroboard or a pcb.

Thank you for taking interest. Please comment and rate.

As per request i would like to add somethings.
1.The LM 35 is the actual sensor which senses the temperature and sends an analogue signal according to the temperature. It has a range of 0 to 100 degrees Celsius but we are only using 10 to 39. It's output increases 10mV for every degree.

2. The LM3914 are ic's which receive the analogue signal and convert it into a visible indication by lighting a specific number of led's according to the input signal.

3. The two capacitors i used are for filtering the signal from the LM35 so that the circuit functions properly,

Step 1: Materials

You would need:
Resistors;
1k preset...4
2.2k...2
2k...1
330ohm...1
75 ohm...1
Semiconductors;
Lm3914...3
Lm35...1
Capacitors;
10 uF Electrolytic
10nF Ceramic
LED's;
Any Color...30

Step 2: Schematic

This is the schematic we are going to follow. I omitted the 2.2 uF capacitors.

Step 3: Adding the LM3914 IC's

First step is to add the three LM3914 ic's. Pay attention to the notch. Insert them with five hole gap.

Step 4: Adding the LED's

I added the leds with each ic. Make sure to insert them correctly. The positive + side goes to the common rail and the negative - side goes to the ic. I also made a + power connection.

Step 5: Adding the Resistors

Nest i added the resistors. First i added 1k variable and 2.2k fixed to two of the ic's. After that i added the remaining 2 1k variable and the 2k and 330 ohm resistors with the third ic.

Step 6: Making Connections

My nest step was to link the three ic's together. This is done my connecting 4 and 8 pins of one ic to sixth pin of the next. This is better understood by the schematic.

Step 7: Adding the LM35

Next step is to add the LM35 and make its connections. Pay proper attention to the pins. The signal pin connects to pin 5 of all three ic's. Next insert the 75 ohm resistor and the two caps to complete the assembly.

Step 8: Finalizing

To finalize the project i added a diode and made some final connections which i missed the first time. I also connected the pin9 of all ic's to positive rail. I then connected it to a 6 v battery and voila is worked. The power supply must be between 5 and 9 volts or else it wont work properly.

Now you must calibrate the circuit. Take a good quality multimeter preferably Fluke, Unit-t or Mastech and set it to the volts setting. Connect the negative terminal of the digital multimeter to the ground and power on the circuit. Then connect the positive terminal of the digital multimeter to the pin 4 of the first ic which is on the left side in the picture and bottom one on the schematic. Then turn the preset connecting to pin 4 until you achieve 90 mV. Then connect on the pin six of the same ic and turn the second preset until you achieve 190 mV. Then connect the multimeter to the pin 6 on the middle ic and turn the preset to achieve 290mV. And then connect it to the last ic's pin6 and turn the fourth preset to achieve 390 mV.

In This way the thermometer would start its reading from 10 degree Celsius for the first led for every degree rise in the temperature there would be an addition of one led thus making the maximum to be 39 degree celcius.