You'll remember from the previous chapter that most things to do with the GPIO operate on a +3.3V level, rather than the +5V level that is often associated with digital circuits. This is the same with our I2C-based shift registers—they need to operate on +3.3V levels as well, in order to work with the Raspberry Pi.
You'll also recall, however, that there's not much +3.3V juice available directly from the Raspberry Pi—in fact, just 50mA. This is really not enough for our interface. So, before we go any further, we're going to build our own +3.3V power supply, which is sufficient for our system.
For our power supply, we're going to use a basic 3.3V voltage regulator (type LD1117V33) that will take our slightly more plentiful +5V supply from the Raspberry Pi and regulate it to a nice smooth +3.3V supply. We should be able to draw a few hundred milliamps from this supply—enough for the I/O circuitry on our security system.
The parts required for our power supply are as follows:
A LD1117V33...
