Voltage drop circuit by means of diodes
In this project, you are going to learn that the diode develops voltage when forward current flows into it.
When you've finished wiring, turn power ON. You can see LED 1 lights up but it is dark. This is because of two diodes which are connected to LED 1 in series. You could guess the action of the diodes. Yes, the voltage applied to LED 1 is reduced since the voltage drops by the diodes.
Turn S1 ON and watch what happens to LED 1. It becomes brighter than before as one diode is short-circuited by S1.
Then turn S2 ON. You can see LED 1 becomes its brightest light as both diodes are short-circuited by S2.
With this experiment, you'll see that the diodes are used to make the voltage drop at point A in the schematic.