Simple and Efficient Temperature Controller Without MCU

This analog temperature controller functions entirely without a microcontroller by translating thermal variations into switching actions using discrete semiconductor devices and an integrated sensor. At the heart of the power and reference system, the LM7805 voltage regulator (IC1) steps down an input voltage to provide a stable, regulated 5V DC rail for the control circuitry. The TMP36 temperature sensor (IC2) monitors the thermal environment, producing an output voltage that varies linearly with temperature changes. This voltage signal is routed into a discrete transistor arrangement formed by the BC547 NPN transistor (T1) and SK100 PNP transistor (T2). As the temperature rises or falls past a threshold determined by the surrounding resistor network, the base-emitter junctions of these transistors bias accordingly, transitioning them between saturation and cutoff regions. When the transistor stage changes state, it modulates the current flowing through the coil of the 5V one-changeover relay (RL1). The 1N60 diode (D1) is placed in reverse bias across the relay coil to act as a flyback diode, safely shunting away the high-voltage inductive kickback spikes generated when the relay coil de-energizes, thereby protecting the delicate semiconductor switches from breakdown. When activated, the relay's mechanical switch changes over, allowing the user to open or close an external circuit to manage heating or cooling loads.