HackSpace_-_April_2020

FORGE

Above

Perfect for

high-visibility

text readouts

Below

E-ink displays are

slow to refresh, but

retain their display

without power

ADAFRUIT £9 thepihut.com

PI SUPPLY £40.99 uk.pi-supply.com

ADAFRUIT £18.50 thepihut.com

7-segment

LED display

PaPiRus

eInk display

Adafruit 16×2 LED

display with keypad

Let’s start with one of the most basic types of display:
LEDs. Light-emitting diodes have been a stalwart of the
maker community since their invention, and they will
feature heavily in this article. The classic seven-
segment LED display became popular in 1970s alarm
clocks, thanks to its bright display and the futuristic
wizardry of a fully electronic system. It’s probably
because of the millions of alarms clocks built since
then that this classic four-digit display is so common.

As is often the case with LED displays, they require
a lot of pins to work. Unless, that is, you have a
dedicated driver chip such as the MAX7219. Using a
technique called multiplexing, the entire display can
now be driven using Raspberry Pi’s I^2 C interface. This
build from Adafruit contains the display and the driver
chip on a single board, although a small bit of soldering
is required. As ever, Adafruit backs this up with a
Python library to make controlling the display in code
as easy as possible. If your project is time-based, or
requires a numerical output,
this is a cheap and effective
solution. Got a long number
to display? Using I^2 C, you
can always chain multiple
displays together.

A seven-segment LED display is all very well for

numbers, but it’s not so great at text. You can make

approximations and even add scrolling effects, but

there are better alternatives. The next step up is

this LCD that can produce 32 characters on two

lines of 16. It’s a common display type,

originally designed for providing simple interfaces

for machinery. There is a nice retro feel to this

display, but it can have real purpose if you need

something bright.

Again, this is an Adafruit kit and provides everything you need

in a single package. The 16×2 panel itself is widely available and

very cheap but, just like its smaller cousin, it requires many GPIO

pins to work. This kit features a GPIO input expander and five

tactile switches to create a small control unit that only requires a

few pins. Available in a range of colours, it’s perfect for bright

text readout and small projects, such as media players.

One thing that displays of all types tend to have in

common is their relatively high power requirements. If

you have a project that is battery-powered and only

requires occasional updates, then an e-ink screen is

worth consideration. This technology comprises

thousands of pigmented balls ‘floating’ in a panel. By

applying positive or negative charge, the balls are either

pulled up or down. When up, their pigment can be seen;

when down, they’re covered. Their unique feature is that

they remain in place permanently. Power is

only consumed when the screen is refreshed.

This makes e-ink perfect for low-power

projects. The cons are a very slow refresh rate

(you can forget about anything animated), and

you are typically restricted to monochrome,

although two-colour screens are available.

The PaPiRus range from Pi Supply offers a

very simple and effective way to add an e-ink

screen to your project. These come in a range

of sizes and all connect to Raspberry Pi GPIO

as HATs. Installation is just connecting the

display and installing some software.

Using I^2 C, you can

always chain multiple

displays together

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Below

Great for retro

projects, and simple

and easy-to-read

information displays