LM35‑Based Temperature Control System Using AT89S52 Microcontroller

Temperature‑Based Device Control System using the AT89S52 microcontroller and LM35 temperature sensor is an embedded solution that automates household appliances—heaters, fans, coolers, and more—by continuously monitoring ambient temperature. This project is an excellent starter for students in electrical and computer engineering, providing hands‑on experience with hardware interfaces, ADC processing, and real‑time control logic. Before diving into the design, ensure you understand how to interfere LM35 with AT89S52 by reviewing the dedicated guide. The system reads the temperature from the LM35, then compares it against two preset thresholds representing summer and winter limits. If the sensed temperature exceeds the summer threshold, the microcontroller activates a fan. If it falls below the winter threshold, a heater is turned on. Temperatures within the intermediate range keep both devices off, maintaining a comfortable environment. This logic can be extended to any number of external devices as needed. In the schematic, pins 1^6 and 1^7 connect to resistors that drive transistors Q2 and Q3. Because the AT89S52’s port outputs cannot directly energize AC appliances, relays are employed (as shown in the diagram) to switch high‑power loads safely. Relays protect the microcontroller from voltage spikes and isolate the low‑voltage control circuitry from the high‑voltage AC side, ensuring reliable operation and user safety.