Temperature Controlled LEDs using LM35

This temperature-controlled LED circuit operates entirely through analog electronics without needing any microcontroller or programming. Power is supplied by a 9-volt battery, and an IC 7805 voltage regulator steps this down to a steady 5-volt rail to ensure stable operation for the sensor and comparator. The core sensing is handled by the LM35 temperature sensor, which outputs a precise linear voltage proportional to the ambient temperature in degrees Celsius. This voltage signal is sent to the LM358 operational amplifier, which is configured as a comparator. A 10k ohm variable resistor acts as a voltage divider to set a specific reference threshold voltage. When the temperature read by the LM35 rises above this threshold, the LM358 changes its output state. This output drives the BC547 NPN transistors, which act as electronic switches. Depending on the op-amp's output, current is directed through the 1k ohm current-limiting resistors to illuminate either the yellow LED or the red LED. For instance, the yellow LED can stay lit during normal temperatures, while the red LED switches on when the temperature crosses the unsafe limit set by the variable resistor.