soldered_pcb
Wordclock

The LED Matrix

The LEDs I have chosen are PLCC6 packaged RGB LEDs. Each package contains 3 separate LED’s bundled together with there anode and cathode exposed across 6 small metal pads. They are inexpensive at about £30 for 400 (Plenty spare ;)). I plan to use a scanning technique to drive them, where each column’s cathodes are connected together and each row’s anodes are tied. The 48 cathodes columns (16 LEDS*3 Colours), will be connected to 4 TLC5940N constant current sink PWM drivers. Which will allow variation of each LED’s brightness and colour mixing.

Each TLC5940N has 16 PWM channels with a resolution of 12bits per channel. The data is driven over SPI with a couple of control wires for latching the data, The drivers can be chained together to allow sharing of the SPI lines

Each LED Row will be driven by a Power MOSFET that should be capable of switching at at least 2.8A 48LED * 60mA when an entire row is at full brightness. The scanning works by enabling each row at a time and having the correct PWM data in the cathode drivers. By switching the 16 rows very quickly, the human eye wont be able to see the difference giving the illusion that all the lights are on at the same time.

To further reduce the number of IO required, I plan too chain two eight bit shift registers onto the end of the TLC5940N, where there outputs will be used to enable the row MOSFET drivers. This allows the entire matrix to be driven via SPI, which is a very simple interface available on almost every microcontroller.

From previous experience,  Placing the LEDs in a display can be a nightmare. This is especially true of the wordclock as I’m using 256 surface mount LEDs.  However soldering 256 surface mount LEDs together in a cohesive manor is not necessary an enjoyable / sensible task. The simplest route is to get  a PCB/PCBs made to help align the LEDs and to remove the monotony of wiring.

At this size of project, PCB fabrication can cost a lot depending upon size and quantity. Especially considering most of the PCB will not be utilised. For this reason I decided against buying a huge PCB ( I did look, the quote was £400!). Splitting the PCB down into smaller modules is a cheaper solution but depending upon how many LEDs per board, would require wiring fun…

My compromise came in the form of 10cm x 10cm boards fabricated by Seeed Studio. £35 for 20 double layer boards including delivery.

The PCBs are double sided with one side designed to take 4 x 4 LEDs and the other for 4 MOSFET row drivers. The row drivers will not be required on all boards however the single PCB design can be reused.  For PCB interconnection I decided to use ribbon cables so that I could reduce the amount of tiny wire soldering. It also solves the problem of crimping hundreds of wires. This was a lesson learnt from building an LED cube.

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When the PCBs arrived I was not disappointed. The quality of them is excellent considering the price! This is also my first attempt at getting PCBs professionally manufactured, fortunately there were not mistakes!

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