0x2A: 16 bit port expander and LCD driver Pi Top Module




The Pi Top is becoming a popular platform for software development using the Raspberry Pi singleboard computer. However, interfacing hardware to the Pi Top can be challenging, since the hub does not conform with the RPi GPIO header. This module provides an additional 16 digital inputs/outputs as well as a convenient driver for Hitachi HD44780U-based LCD displays.


This design features the popular PCF8574 8 bit port expander. Using, for instance, the wiringPi library, a digital input/output module with an old-fashioned LCD display can easily be realised. Graphical user interfaces can be created using Qt and wiringPi. See the download section on this page for some example source code and the Qt tutorial for getting started.

License terms
The 0x2A design is published under the Creative Commons Unported 3.0 License.  I do not profit from this design, neither should you. You are welcome to adapt, modify, and redistribute this design as long as the license terms are respected.

Physical size of PCB

84 mm (width) by 56 mm (height). Double-sided.

Rendering and photos

DipTrace 3D rendering, frontside with components


DipTrace 3D rendering, backside with components 


0x2A module mounted in a Pi Top 2


0x2A module with LCD display on top mounted in a Pi Top 2


Screen dump of Qt interface running on the Pi Top 2 (Polaris)


Bill of materials

5 pcs. decoupling capacitor, ceramic, 100 nF, 1206

3 pcs. capacitor, tantalum, 10 uF, 16 V, SMD (optional)

3 pcs. pullup resistor, 10k, 1206

2 pcs. I2C pullup resistor, 1k8, 1206

1 pcs. AMS1117-3.3 voltage regulator, SOT-223

16 pcs. pull up resistor, 100k, 1206

16 pcs. LED current limiting resistor, 390R, 1206

2 pcs. resistor, 1k, 1206

1 pcs. resistor, 100R, 1206

1 pcs. trimpotentiometer, 1k, RM065

1 pcs. 2N2222, NPN transistor, SOT-23

1 pcs. 3×16 male header, 2.54 mm pitch

1 pcs. 1×16 female header, 2.54 mm pitch

1 pcs. LED, green, 1206

16 pcs. LED, amber, 1206

1 pcs. 2×17 90 degree 2.00 mm pitch male header

1 pcs. Hitachi HD44780U compatible LCD display (e.g. 2×16 or 4×20)


Estimated cost

Components, display, and circuit board should not cost more than EUR 15,-.



Schematic PDF: 0x2A_schematic

DipTrace v. 3 schematic file: 0x2A_schematic

DipTrace v. 3 PCB design file: 0x2A_ratlined_public

Gerber files: 0x2A_Gerber

Qt 5 GUI source code: QtGUI_0x2A



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