INDEX

Temperature sensor:
Commonly available temperature sensors are LM35,DS1621,thermistor. Thermistor gives resistance proportional to the temperature. But accuracy is not good in thermistor. DS1621 gives digital output in I2C format, so you require a microcontroller to interface to see the temperature. Thermistor
require accurate resistance in series to get good reading with accuracy The resistance of thermistors are 100ohm,1Kohm(the one i have heard). But thermistor creates some headaches although it costs Rs.5. LM35 have 3 terminal Vcc, ground, Vout. So it is easy and gives analog output and it costs Rs.50
LM35:

The most commonly available LM35 is LM35DZ measures temperature from 0 to 100 degree. Normally sensors become inaccurate with age. But LM35 will not have this problem. We get LM35 in TO-92 package, just like small transistors like BC547. The main problem you will be creating is
interchanging leads. Vs ranges from 4V to 30V. Output is 10mV* degree in Celsius. That is if the temperature is 29 degree then Vout=.29V. Let's try for a circuit which will glow an LED if the output voltage is greater than some voltage. Use comparator with a reference voltage at one end and LM35 output at other end, so that comparator become high when the temperature is above reference.