This module is rather simple. Just a simple method of connecting a bank of buttons to an AVR and checking the results using polling. You have already seen the code for polling, that was in module #1.
The theory is rather simple. We drive the input lines at 5V when off and send it to zero when on. This is known as ACTIVE LOW, because the line will be active, when the voltage is low.
The diagram is as follows.
When the button is not pressed, the 5V is fed into the AVR. But when the button is pressed, we are driving the line down to ground. Hence sending the AVR line to 0V.
Next module. The power supply.