This circuit measures temperature in Celsius
scale and displays it on an alphanumeric LCD screen
When temperature rise to 40 C an alarm is activated
and at the same time a relay is also activated which
drives a fan to keep the temperature at a level.
Another feature of this circuit is that you can use
the keys "1,2,3,4" of a Philips TV IR remote to turn
on or off three relays, The key '4' is used to turn
on or off the over temperature alarm.
The brain of this circuit is AT89C51 microcontroller. LM35 is a
3 pin chip which is easily available in TO-92
package. LM35 can sense temperature from 0 C to 100 C but it
gives analogue output the microcontroller does not understand
analogue data, so ADC0804 (analogue to digital converter) is
used to convert it to digital form.
This digital data is given to port 1 of microcontroller. (See
the circuit diagram) this data is processed by microcontroller
and temperature is displayed on lcd connected to port 2.The
control pins of lcd are connected to port 0. port 0 also
controls the relays and alarm.
The ULN2003 chip is used to drive the relays because the
microcontroller pins don't have enough current to drive them. so
relays cant be connected to microcontroller pins directly
further more the relays are inductive load and reverse current
is generated in them. Pin 1 to 7 are the inputs and 10to 16 are
respective outputs. Pin 8 is ground and pin 9 is connected to
the output of 7808 voltage regulator.
The 7805 voltage regulator drives rest of the circuit. I used a
standard buzzer driven by LM555 timer/Oscillator chip. The chip
is wired as a monostable multivibrator and at its output (i.e.
pin no 3) a buzzer is connected.
Use any IR module and connect its Data out to pin 10 of
microcontroller. The Relay connected to pin 13 of ULN2003 turns
on when temp rises above 40 C so connect the fan to this relay.