DUAL-TONE SINGLE BELL For Two Doors

In modern homes, apartments, and offices, multiple entrance doors are common. Traditionally, each door requires a separate doorbell, which increases cost and can confuse residents. Hearing a bell without knowing whether it is the main entrance or the back door is a frequent inconvenience. To address this, a single bell device has been designed for two doors, using a common bell that produces two distinct sound patterns depending on which door's switch is pressed. Fig. 1 shows the device prototype without its transformer.

- By SURESH CHANDRA DWIVEDI

Circuit and working

Fig. 2 shows the circuit diagram of the dual-tone single bell device for two doors. It uses a popular LM556 dual-timer IC, which has two 555 timers built into one package. The entire circuit is divided into three sections: the power supply section, the tone generation section, and the output section.

The power supply section is built around transformer X1 and LM7805 (IC1) and provides a regulated 5V DC supply. A step-down transformer (X1) reduces the mains 230V AC voltage to 15V AC, which is then rectified by a bridge rectifier (BR1). An electrolytic capacitor (C1) smooths the DC output, while another capacitor (C2) removes high-frequency noise. Finally, the LM7805 voltage regulator ensures a steady and ripple-free 5V DC, which powers the LM556 timer IC and associated components.