This LED thermometer is designed for in home use, to read temperatures between about 60 and 78 degrees Fahrenheit. It is based around a precision temperature sensor IC, the LM34DZ.
http://aaroncake.net/circuits/ledtemp.asp
This circuit uses a Dallas DS1621 temperature sensor which indicates the temparature of the device. The temperature sensor has an termal alarm output, which becomes high when the temperature of the device exceeds a user defined value.
http://www.avrprojects.net/index.php?option=com_content&view=article&id=77:temperature-indicator&catid=37:avr-projects&Itemid=57
A simple op-amp circuit that will trigger a relay when a preset temperature is reached. Please note that there is no hysteresis in this circuit, so that if the temperature changes rapidly, then the relay may switch rapidly.
http://www.zen22142.zen.co.uk/Circuits/Misc/tempalarm.htm
This is extremely simple to build PIC Temperature meter that allows to measure temperature in two different locations at the same time. Never before such a useful and powerful circuit could be built with so little components and yet provide endless possibilities. This is all possible thanks to the use of PIC16F628 microcontroller and 2into16 character LCD display that act like a small computer which can be customizable thanks to upgradeable hex firmware.
http://electronics-diy.com/pic_temperature_meter.php
This circuit warns the user against fire accidents. It relies on the smoke that is produced in the event of a fire. When this smoke passes between a bulb and an LDR, the amount of light falling on the LDR decreases.
http://www.electronic-circuits-diagrams.com/alarmsimages/alarmsckt13.shtml
Just follow the picture exactly in less than 1 minute and you have to download a source code and make this thing work.
http://zedomax.com/blog/zedomax-diy-109-build-a-digital-thermometer-in-one-minute/
This one is using a 16F876 PIC, MCP1047A temperature sensor ( X2 ), MCP1541 voltage reference and MCP6022A opamp.
http://www.geocities.com/constantinescuradu/content/thermo.htm
http://www.uoguelph.ca/~antoon/circ/auto-fan.html
Sometimes we have a need to measure the temperature of a room, of an object or just maybe a terrarium;
http://www.e-dsp.com/how-to-measure-temperature-with-the-dallas-maxim-ds1820-sensor/
Using a thermistor in the position shown makes a heat activated sensor. A change in temperature will alter the output of the opamp and energize the relay and light the LED. Swapping the position of the thermistor and 47k resistor makes a cold or frost alarm.
http://www.mitedu.freeserve.co.uk/Circuits/Misc/tempalarm.htm