HEAT SENSOR HOW? :(

[louise john cacot]

hello guys, im a beginner in electronic things
i just want to know HOW can i build a heat sensor with this available components except resistors etc. that are easy to find
NTC thermistor (200 ohms)
NTC thermistor(300 ohms) when extended by a wire
9v power supply 750mA
buzzer 6-12 volts

im in rush with this project heat sensor
so guys ,please help me.

 

[Mike odom]

Here's a basic temperature control circuit using an NTC thermistor. Note your 200 ohms is at 25°C, and has a negative curve as the temperature rises.

The output of the comparator is open collector to ground, and when the temp voltage (set by R1/Rtherm) goes below the temperature set point (set by R_HI/R_LO), the output pulls to ground (to turn on a relay or other device).
Note that R_HI and/or R_LO can be a pot or pot/resistor combination to adjust the temperature set point.

The 392 ohm resistor is not set in stone. I calculated that for a 5K (@25°C) NTC thermistor the best value was 3.92K (actually 3.75K but I had the 3.92K in stock) gave the highest change in voltage for a corresponding change in temperature (do you know calculus?).

Also note that the '339 is a quad voltage comparator, whereas a '393 is a dual comparator.