ON the block one the switch turns on the IR led and then it starts emiting the infrared and then it crosses through the photodiode and across the diode,this will turn on the transistor and the other one toobecause it is connected to the ground and it turns on the NPN.Transistor wil pass or conduct the power y the 8 pin of the 555 and turns it on ,then the led and the speaker whiich areconected to the 3 pin of the 555 (output) turn the led on and make the speaker emit the sound.