Let's summarize what we have learned in this chapter. We connected a 7-segment module to a Raspberry Pi board through eight GPIO pins. Then, we show a number on the module. To minimize the GPIO pin usage, we implemented a shift register using a 74HC595 IC, so we only need three GPIO pins. A shift register is also used to drive a 4-digit 7-segment module by cascading two 74HC595 shift registers. At the end of the section, we tried to build a simple program for a countdown timer.

In the next chapter, we will build a digital clock using several LED modules. The chapter will introduce LED modules related to the digital clock stack.