Here is an easy-to-construct temperature indicator-cum-controller that can be interfaced with a heater’s coil to maintain the ambient room temperature. The temperature control system is based on Atmega8535 microcontroller, which makes it dynamic and faster, and uses an LCD module to display and two keys to increase or decrease the set values.
Temperature control system circuit
Fig. 1 shows the circuit of the temperature control system. It comprises microcontroller Atmega8535, temperature sensor LM35, regulator 7806, an LCD module and a few discrete components.
The 230V, 50Hz AC mains is stepped down by transformer X1 to deliver a secondary output of 9V, 500 mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and regulated by IC 7806 (IC1). LED1 acts as the DC power indicator. Resistor R1 acts as the current limiter. A 4.8V rechargeable battery provides battery backup.
The ATmega8535 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture. ATmega8535 has such features as 8 kB of in-system programmable flash memory (i.e., read-while-write capabilities), 512-byte EEPROM, 512-byte SRAM, 32 general-purpose input/output (I/O) lines, 32 general-purpose working registers, three flexible timers/counters with compare modes, internal and external interrupts, a serially programmable USART, a byte-oriented two-wire serial interface, an 8-channel, 10-bit analogue-to-digital converter (ADC) with optional differential input stage with programmable gain, a programmable watchdog timer with internal oscillator, an SPI serial port, and six software-selectable power-saving modes.