HPDL-1414 display operation

There are a great many different kinds of displays on the market – LCD, LED, OLED, etc. We encounter most of them on a daily basis, but there are also some more classic designs, such as the HPDL-1414.

All in one

HPDL-1414 Displays

HPDL-1414 is a small-sized LED display module capable of displaying up to four alphanumeric characters. Each of them was built with 14 luminous segments, activated depending on the displayed character. Looking through the catalog note of the display, the term “smart alphanumeric display” is immediately striking. However, this is not just a marketing term designed to attract consumer attention. The device does indeed have built-in RAM, an ASCII character decoder (ASCII 32-95) and a circuit to power the individual display segments, making it much more similar to popular LCD displays than ordinary LEDs in terms of design.

Microscope view of a single digit

As mentioned, the device can display up to four alphanumeric characters in red thanks to the use of gallium arsenophosphide as a semiconductor material. The dimensions of the component are really small, it can be easily plugged into a contact board, while the displayed character itself is 2.85mm (0.112 inches) in size.

Pinout description (https://www.farnell.com/datasheets/76528.pdf)

The display is powered by a voltage from 0.5V to 7V, so it can work with popular platforms operating in 5V and 3.3V logic standards. In addition to the power pins, the device also has address inputs – allowing you to select one of four digits, data inputs – a binary code assigned to a specific alphanumeric character is fed to them, and a /WR (write) input – the appearance of a low state on it causes the module to store in memory and display the character of your choice.

HPDL-1414 support

Supported characters (https://www.farnell.com/datasheets/76528.pdf)

Operation of the HPDL-1414 is actually very simple. We use the address inputs to select one of the four available digits, then to the data input we give the code for a particular character according to the table above, and finally all we need is a short pulse given to the /WR input and then the display will show the character we selected. If we want to run more than one display module, we connect the data and address inputs together, only the /WR input is dedicated to a specific device.

Writing code that would be able to handle this type of display in an Arduino, for example, would not be anything difficult, but in fact we don’t have to do it, because the appropriate library already exists. After installing it, you can handle the display in an even simpler way.
#include <HPDL1414.h>

const byte dataPins[7] = {2, 3, 4, 5, 6, 7, 8}; // Segment data pins: D0 - D6
const byte addrPins[2] = {A1, A2}; // Segment address pins: A0, A1
const byte wrenPins[] = {A0, A3, A4}; // Write Enable pins (left to right)

HPDL1414 hpdl(dataPins, addrPins, wrenPins, sizeof(wrenPins));

At the beginning, of course, we need to include the library and create a three-array in which we will put the designations of the pins used. The first two structures are of fixed size, that is, we need to specify seven outputs for data and two address pins. The last array stores the designations of the /WR outputs. In my case, I was testing three displays at the same time, so I put three outputs A0, A3 and A4.

void setup() {

In the setup loop I placed only two commands. The first one initializes the display modules while the second one makes the text to be wrapped. This means that if we want to display a string longer than 12 characters (three displays) the text that exceeds the range will appear at the beginning. If the command were removed, the excess characters will be truncated.

void loop() {
  hpdl.print("HPDL TEST");

The two basic functions that support the HPDL-1414 are hpdl.clear();, which clears the display, and hpdl.print(“”);, inside which we put the text or variable we want to display. Note, however, that a single display can only handle four characters, any excess text will be ignored.

Display during operation

After running the code, the display will show the text “HPDL TEST” every second. You have to admit that HPDL-1414 is quite an interesting design. Easy to control and, above all, small. If the device you are designing needs to display only simple messages or error codes, this type of display can be a good alternative to classic 7-segment designs or LCD displays.


  • https://www.farnell.com/datasheets/76528.pdf
  • https://github.com/marecl/HPDL1414

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