Using an HD44780 character LCD with the Raspberry Pi

[Tech2077] is one of the lucky ones who already got his hands on a Raspberry Pi. He’s been looking into different interface options with the GPIO header and just posted a guide to using an HD44780 character display with the RPi. We like this approach because instead of doing some hard-core LCD work he’s using prototyping equipment you probably already have on hand.

Getting a character LCD running should be really simple. The gotcha is the logic level gap between the devices. If you’ve been working with Arduino, your add-ons are probably meant for a 5V power rail ¬†and logic levels. The RPi outputs 3.3V logic. You could use a level converter (you’d need at least 7 pins to be converted in this case) or you can be a bit more clever. [Tech2077] grabbed an I2C port expander that uses just 2 of the RPi lines to address even lines of the display (four data bits plus three control bits). This is a bit of a hack, as the 3.3V logic is 0.2V below the recommended minimum for a digital 1 on the port expander. But it seems to work just fine! If it didn’t, a couple of NPN transistors would do the trick as well.

Addressing the new peripheral is just a matter of loading the i2c module and writing some Python.

Parts: 8bit IO Expander (PCF8574)

pcf8574

Sometimes a project has more sensors, buttons, or LEDs than your microcontroller has pins. The PCF8574 is an easy way to add 8 low-speed input or output pins to a microcontroller. A configurable address lets multiple PCF8574s exist on the same bus, so two microcontroller pins can control dozens of IO pins. We’ll show you how to use this chip below.

Continue reading “Parts: 8bit IO Expander (PCF8574)”