is built using two 2N4401 transistors. Any similar NPN transistors will also work here for example 2N3904, BC547 etc.
The second stage is built around a 555 timer IC. This stage is used to activate the UM66 melody for a certain time period. Once an output will be received from the transistor Q2 on the trigger pin 2 of the IC. The activate time of the melody can be increased by increasing the value of the 47uF capacitor that is marked as Cx in the schematic.
The third stage is a UM66 melody circuit. UM66 is a CMOS IC available in TO-92 transistor package. It is available in many different melodies. Every melody has a different code, which is written on the IC. Before purchasing one it is better to check the datasheet to purchase your desired melody. A BC548 transistor is used to increase the output of UM66 IC to drive an 8 ohms speaker.
After building the circuit fix it in any doorbell type box and make a hole in the box from where the LDR will receive light. Attach the box on your door or near the door.
The circuit can be operated with 4.5 volt DC. You can also use a 4.5 volt power supply with the circuit to operate it with the mains voltage. Any scrap charger of cellphone can also be used as power supply.
Here is a very interesting project of a light sensitive visitor welcome doorbell circuit. The circuit will activate the UM66 melody for a set time period when any visitor comes near to your door or pass near from the door.