Automatic hand dryers and toilet flushers typically have light-sensitive switches installed in them. This is a straightforward light switch that can also function in regular lighting conditions. You can install it on the primary entrance to your home so that it serves as both an automatic doorbell and a burglar alarm. The moment that someone's shadow is detected by the sensor of this apparatus, the bell starts to ring. Circuit with a light-operated switch.

Fig. 1: Light operated doorbell The light-dependent resistor (LDR) sensor is constructed out of a CdS semiconductor and has a resistance that varies depending on the amount of light present. The presence of light causes its resistance to decrease, while the absence of light causes it to increase to a higher level. Timing circuit IC 555 and melody generator IC UM66 are used in this circuit. The UM66 may operate at a maximum working voltage of 3.3V. In order to supply the maximum voltage that is necessary, a Zener diode is utilized. The input for the melody IC UM66 comes from the timer NE555, and the output of the melody IC is sent, through resistor R2, to the base of the transistor T1. In order to alter the operation of the circuit's threshold, a 470-kilo-ohm preset, denoted by VR1, is utilized. Circuit operation When light hits LDR1, its resistance drops, and the potential gap between pin 2 of IC1 and the ground is smaller as a result. Under these circumstances, the output of the timer continues to be high. Due to the high voltage that has been applied to the ground

terminal of the UM66, its operation has been halted, and you will not hear any melodies. When someone casts a shadow over the LDR sensor, the resistance of the sensor goes up, which in turn causes an increase in the potential difference between pin 2 of IC1 and ground. The value of the output from the timer drops. The operation of the UM66 is kicked off by this low voltage at the ground terminal. The signal that is amplified by transistor T1 and sent to speaker LS1 is produced by UM66, which generates the pulse. You will be able to hear a pleasant tone produced by the LDR once light no longer falls on it.

The circuit requires 3-6V DC to function, which can be generated by pencil batteries rated at 1.5V. Install it in an appropriate cabinet once you have assembled it on a PCB that may be used for a variety of purposes. If you want the circuit to perform more effectively, you should avoid dry-soldering on the PCB. You can mount the device on the door in front of a regular lamp so that light falls directly on the LDR. When someone walks in front of the door, they block the light from reaching the LDR, which triggers the bell to ring. To make the circuit work in the opposite direction, that is, to make the bell ring when light falls on the LDR and remain silent when someone's shadow falls on it, disconnect resistor R1 from the power supply rail and connect it to pin 3 of IC1. This will make the bell ring when light falls on the LDR and remain silent when someone's shadow falls on it. After that, detach the ground pin 3 of UM66 from the pin 3 of IC1, and then connect it to the ground rail.