About the PNP Circuit
If you give voltage to the transistor it turns off making the buzzer turn off as well.
EEME Dad's comments: Give voltage to the base prong of the PNP transistor
About the Pushbutton LED Circuit
When you push the button it closes the circuit, allowing electricity to turn on the LED.
How does the circuit work?
It works by flowing the electricity from the positive side to the negative side. without interrupting.
How does the IR LED and phototransistor circuit work?
We have two circuits here; the first circuit, to the left, simply turns on the infrared LED; the one to the right lights up the LED. But the phototransistor must receive infrared light to close the circuit to light the LED.
Sharing Current Explained
The reason why all the segments are dimmed is because 3 of the LED's share a negative and positive with the others while the last has only a shared positive so this makes it so that all the segments have too share the power.
How does the reed switch work
it closes near magnet