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Digital Thermostat with High Accuracy
-6,00 to 46,00 °C
Digital Thermostat
We build a digital thermostat for home applications, which controls the room temperature. I
used the AT90S2313 microprocessor of ATMEL. The digital thermostat opens the main heating valve of
the apartment and the hot water flows in the radiators.
Digital Thermostat Board
Bitoulis Spyridon (Digital Thermostat) Page 2 of 4
Power and Oscillator
The system is supplied a 6 Volts DC current, we use a 7805 to stabile the voltage to 5 Volts
DC. The microprocessor is running on an 8MHz crystal so the pulse measurement is in microseconds
(us).
Watchdog Timer
The AT90S2313 enables the watchdog timer in a period of 2 seconds the microprocessor runs
the program and finally goes to power down sleep mode. It can wake up only from the RESET of
watchdog. The CPU is mainly in sleep mode and the actual current is very low (0,04mA).
NE555 – Pulse measurement
The temperature is measured using the NE555. In order to reduce the power consumption the
PB1 pin gives the ground to the NE555, in normal operation it consume up to 3mA. The resistor R0
reduces the current of the NTC Thermistor in order not to influent the real resistor of the thermistor. In
the AT90S2313 we store in the eeprom memory the NTC thermistor curve. Using the ICP pin in the
AT90S2313 we measure the pulse duration of the NE55 output. The AT90S2313 calculates the actual
temperature using the NTC thermistor curve and also an average temperature of four measurements is
also performed. The temperature measurement is performed every a specified period of time, by default
every 1 minute.

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