1. Goal of the project : By utilizing a sequence of button presses, make multiple leds to radiate. Like a concept of lock, each led represents whether it is locked / unlocked.
2. Core components : 4 buttons, 4 x 10K ohms resistors, yellow and blue leds, 2 x 220 ohms resistors, breadboard, arduino board, jumper wires
3. How it works : I made a circuit that enables a certain rule of button press to make each led shine. When button 1 and button 3 are pressed at the same time, yellow led emits, and if button 2 and button 4 are pressed at the same time, blue led emits.
4. Problems encountered : A little confusion with making a circuit for multiple buttons – where to the power and output connections go – but I figured out with reminding the basic mechanism of buttons.

Source : https://github.com/minhyekwak/picot-combination-lock