For the transmitter part. Im going to use +5v , infrared led , and 330ohm.
When the infrared led is released the beam toward the phototransistor. The base of phototransistor will received the energy and now the Q1 is open and the emitter also release voltage to the NPN transistor.
Now the NPN transistor's base have the voltage source + Vcc and now the NPN is open and the voltage will directly flows toward to gnd. At this time, pin 2 is now in "LOW" state and output is "HIGH".
Is my theory correct? But my prof. tell me the output will be "HIGH" when something between it. Or can someone briefly explain it to me.
NOTES:
I'm going to change the speaker to buzzer. So for the transmitter part i'm not going to create fix frequency by using 555 timer.