This circuit is based on a LDR which act as a sensor to detect the shadow; NE555 is a timer IC and UM66 which is used as melody generator IC.
The circuit woks on the principle of operation of LDR (light dependence resistor). LDR is a device whose sensitivity depends upon the intensity of light falling on it. The resistance of LDR decreases when intensity of light falling on it increases and vice versa (resistance increases when intensity of light falling on it decreases). In dark or in absence of light, LDR exhibits a resistance in the range of mega ohms, which decreases to a few hundred ohms in presence of bright light.
When light is continuously falling on LDR its resistance decreases. Therefore potential difference between pin 2 of IC1 and ground is also low. In this state out pin 3 of IC1 is high. This will make the pin 3 which is the ground terminal also high and IC2 will not conduct and no musical bell sound will be heard.
But when somebody comes on door and its shadow falls on LDR. Then resistance between the pin 2 of IC1 and ground start increasing and output pin 3 of 555 timer goes low. This will provide the ground to pin 3 of IC2 for its operation and melody can be hear. In this circuit transistor T1 is used to amplify the weak signals which will then feed to speaker.
We have used a zener diode at the pin 2 of UM66 because excess voltage more than 3.3V will harm the IC. And with the help of VR1 you can change the threshold operation of the circuit.