Building and Wiring the LED Sign

Created on: 25 September 2012

Building and wiring of the LED Open sign open-source project.

Building the Enclosure

You will need some woodworking skills to build the enclosure for the LED sign. The enclosure shown in the videos has the finished dimensions of 616mm long by 229mm high by 54mm deep.

The front and back are made of 3mm thick hardboard and the sides are made from 18mm thick MDF board.

The back of the enclosure is attached with six screws so that it can easily be removed.

Drilling the LED Holes

5mm LEDs are used, so a 5mm diameter drill bit is needed for drilling the LED holes.

Open the drill template files with Inkscape, the open source vector drawing program, and print the two halves of the sign on two A4 sized sheets.

Drill template files for the "Open" sign: sign_face.zip or sign_face.tar.gz

Line up the two printed pages and tape them together. Now tape the two pages to the front panel of the enclosure. Use the marking on the template to line up the drill bit and drill the holes for the LEDs.

Wiring the LED Open Sign LEDs

Connections to the Controller Board

Each set of LEDs (i.e. the letter O, P, etc. and every third LED of the top and bottom lines) is controlled by a single channel on the controller board. These sets of LEDs are connected to the controller board as follows:

• O – channel 1
• P – channel 2
• E – channel 3
• N – channel 7
• Top line, every 3rd LED starting with LED 1 – channel 4
• Top line, every 3rd LED starting with LED 2 – channel 5
• Top line, every 3rd LED starting with LED 3 – channel 6
• Bottom line, every 3rd LED starting with LED 1 – channel 8
• Bottom line, every 3rd LED starting with LED 2 – channel 9
• Bottom line, every 3rd LED starting with LED 3 – channel 10

In the above list, the top line LED 1 is the top left LED of the top line (at the beginning of the curve). LED 1 on the bottom line is the bottom right LED (at the tip of the line, end of the curve).

It does not matter which set of LEDs is connected to which channel, but if you change them from the above you will need to modify the program that controls them to switch on the correct set of LEDs.

Wiring the Sets of LEDs

Each set of LEDs, e.g. for the letter "P", is made up of sets of 4 LEDs in series with a resistor. Where the number of LEDs in a set does not add up to a multiple of four LEDs, a different value resistor is used.

The following wiring diagram shows the wiring for the letter "P" which is made of 29 LEDs. 7 sets of four LEDs each and one set of one LED is used to make up the 29 LEDs of the letter "P".

Below this wiring diagram for the letter "P" is a diagram that shows the value of resistors needed for 4 LEDs, 3 LEDs and 1 LED.

The number of LEDs used in the sign is 232 which is made up as follows (resistors are also shown):

• O – 35 (8 × 4 in series + 3 in series) (8 × 220Ω resistors + 1 × 330Ω resistor)
• P – 29 (7 × 4 in series + 1 in series) (7 × 220Ω resistors + 2 × 1k resistors in parallel)
• E – 32 (8 × 4 in series) (8 × 220Ω resistors)
• N – 34 (8 × 4 in series + 2 in series) (8 × 220Ω resistors + 1 × 470Ω resistor)
• Top line set 1 – 17 (4 × 4 in series + 1 in series) (4 × 220Ω resistors + 2 × 1k resistors in parallel)
• Top line set 2 – 17 (4 × 4 in series + 1 in series) (4 × 220Ω resistors + 2 × 1k resistors in parallel)
• Top line set 3 – 17 (4 × 4 in series + 1 in series) (4 × 220Ω resistors + 2 × 1k resistors in parallel)
• Bottom line set 1 – 17 (4 × 4 in series + 1 in series) (4 × 220Ω resistors + 2 × 1k resistors in parallel)
• Bottom line set 2 – 17 (4 × 4 in series + 1 in series) (4 × 220Ω resistors + 2 × 1k resistors in parallel)
• Bottom line set 3 – 17 (4 × 4 in series + 1 in series) (4 × 220Ω resistors + 2 × 1k resistors in parallel)

Wiring and Soldering

The actual physical wiring is done by inserting LEDs in the sets shown above and joining them in series by bending the LED legs and soldering them together. The anode of each set of LEDs connects to every other set using a common wire which is then connected to the +12V screw terminal input on the controller board. This conveniently connects the +12V of the power supply to the anode of every set of LEDs.

The cathode of every set of LEDs is then connected to its correct resistor of value shown in the above list. The other side of each resistor is then connected together for each separate item – e.g. all the resistors that connect to the letter "O" are connected together and then wired to their correct channel on the controller board.

Gluing

Hot melt glue from an electric glue gun is used to fix the LEDs in place and as an insulator to prevent the wiring and resistors from shorting out.

Test the sign before applying too much glue as you may need to make some changes to the wiring and won't want glue in the way.

Power Supply Connector

A barrel connector used to connect the power supply to the sign is mounted in the top of the sign and wired to the power input of the controller board. This supplies 12V to the sign.

Be sure to check the polarity of the 12V power supply that you are going to use: is the centre of the connector positive or negative – check with a multimeter to make sure.

Mounting the Controller Board

Two blocks of wood can be glued into the enclosure on which to mount the controller board. The video below shows the inside of the sign and the controller board screwed to the two blocks of wood.