Temperature Controlled SWITCH

This simple temperature controlled circuit triggers automatically when the ambient temperature goes beyond a set limit of, say, 50 degrees centigrade. This temperature setting can be changed as per requirement through the potentiometer in the circuit.

- S.C. DWIVEDI

The circuit has three parts: the sensor, main control unit, and relay driver. It works on 230V AC mains. A bulb used in the circuit acts as a load for demonstration purpose. When the room temperature goes beyond the set limit the bulb switches on automatically. The author's prototype is shown in Fig. 1.

Circuit and working

Circuit diagram of the temperature controlled switch is shown in Fig. 2. The circuit comprises step-down transformer X1, bridge rectifier BR1, 5V voltage regulator 7805 (IC1), temperature sensor LM35 (IC2), op-amp LM35 (IC3), 5V single changeover relay RL1, and a few other components. Capacitor C1 connected across the supply terminals minimises any ripples or noise signals in the voltage.

The circuit needs 5V DC to operate, which is derived from the 9V, 500mA secondary side of transformer X1. The 230V AC mains is connected to the primary of X1 via CON1 in the circuit whose 9V AC secondary is connected to bridge rectifier BR1 for rectification. The rectified and filtered voltage (by C1) is given to IC LM7805 to get regulated 5V for the circuit.