This standalone digital thermometer controls the temperature of a device according to its requirement. It also displays the temperature on four 7-segment displays in the range of –55 to +125°C. At the heart of the circuit is the microcontroller AT89S52, which controls all its functions. IC DS1821 is used as temperature sensor. IC DS1821 Dallas Semiconductor’s IC DS1821 is one-degree precision temperature sensor in a 3-pin pack like a transistor with single-wire communication protocol. It can operate as a standalone thermostat with user-programmable trip-points (set-point) or as an 8-bit temperature sensor with a single-wire digital interface. The open-drain DQ pin functions as the output for thermostat operation and as the data input/output (I/O) pin for single-wire communications. The single-wire interface lets user access the nonvolatile memory (EEPROM) thermostat trip-point registers (TH and TL), status/configuration register and temperature register. When configured as standalone thermostat, temperature conversions start immediately at power-on. In this mode, the DQ pin becomes active when the temperature of IC DS1821 exceeds the limit programmed into the TH register, and remains active until the temperature drops below the limit programmed into the TL register. The DS1821 uses Dallas’ exclusive single-wire (one wire) bus protocol that implements bus communication with one control signal.