Soldering 1.27mm pitch RGB LEDs to wire, 96 times without going insane.

[Graebot]

Hi guys,

Started back on the flexible 12x8 RGB LED ping-pong-ball matrix I started last year.

It was abandoned primarily for the lack of brightness and high flicker when trying to PWM 8 rows, one at a time.

It was silly complicated, with about 4 uCs doing various things.

One solution, which I'm sure many of you will think is just as silly, but stick with me on this.

I've split the 12x8 matrix into 12 groups (3x4) groups of 8 LEDs

I'll drive each group with 3 (cheap) shift register / line driver modules.

The shift register has a latch, so I can feed in 24 bits into the module without changing the output to the LEDs, so can effectively drive 24 PWM outputs from a single serial line.

There are 12 of these, so that's 12 serial outs from the uC, plus clock and latch, and possibly output enable.

I'll use the SPI interface to talk to an SD card and read out raw frame data, plus some GPIO for buttons, etc.

All that will easily fit into a single 40-pin PIC, or less.

~~ THE PROBLEM ~~

I've already soldered the matrix together in a grid. Annode vertical, Cathode horizontal. I've also used heatshrink on the terminals.

The wire I chose was single-core. This was a big mistake. There are lots of breaks in the wire from striping the connections.

What I need to do, is solder the 4 x 1.27mm pitch LED terminals to wires which will end in a connector.

To clarify, we have 8 RGB LEDs, each colour will end in a 8-pin connector, so there will be 3 connectors for each group of 8 leds.

Things I've thought of:

1)  Bend the pins into place, solder them to 2.54mm pitch veroboard. Good: solid connection, Bad: potentially break or weaken terminals. A bitch to solder.

2)  Buy really rare & expensive 1.27mm veroboard. Good: no bending of pins, solid connection, Bad: A BITCH TO SOLDER.

3)  Buy some tiny crimps (do they exist?) crimp wire & led terminal, then solder. Good: No juggling hot things, solid connection, Bad: Trouble to insulate...?

4)  1.27mm pitch IC header row... Bad... Because... you're in no better situation than before.

5)  Directly solder LED terminal onto wire and heatshrink. Good: OKish connection. Bad: Biatch to solder x 10, Heatshrink is annoying.

I personally think that (#2) is my best option. 

Anyone got any 1.27mm veroboard they wish to share?

Anyone got any ideas?

To connect the wires to the driver board, I've decided to use some really, really cheap connectors.

The cheapest connectors I could find... are 40-pin IC sockets.

If you cut them up, you get 4 x 8pin fairly-low-profile, pluggable headers, plus 2 x 2pin headers (destroying 4 pins while separating them) per socket.

Each socket is like 48p so that's about 12p per 8pin header.

They're not pretty, but will stay put, you can stack them, and you can jam wires into the top of them, and solder them in place.

So that's my plan for the driver-board connectors. Solder one row into the board, solder wires into the other row (using insulating varnish over the solder joints) Bish bosh, connector.

I've attached a screenshot of my (haphazard) LED driver module.

Yes. It is veroboard. And yes, that is (pretty much) the final version. I might throw in an output enable if it's necessary.

I decided against making PCBs because I'm crap at doing it consistently, and the idea of drilling all those holes, or soldering all those SMT components, frankly gives me the willies.

The chips on the left are the shift registers, on the right are the non-inverting line drivers.

I don't have the model numbers in front of me, buf if you're interested, I'll post them later.

[Simon Howes]

Hate to say it...
But at a certain point oursourcing to a fab for an smt job may be what you want....

:)

(Of course, the roll-your-own hacker imperative may override this sage advise :p)

[Nigel Worsley]

I think the best approach would be to make up a small breakout PCB.

Nigle

[Adrian Godwin]

You could use some of these boards : http://combee.net/rgbshield/ (or
build your own)

Each board will drive 16 RGB leds, so you'd need 6 of them. The
controller chips are SPI with internal PWM, so you only have to send
the desired brightness, refreshing only when it changes rather than
every PWM cycle. They're current-controlled so you don't need the
resistors either.

-adrian

[Simon Howes]

Nice! :)

[Bob Clough]

Take regular stripboard, and use a dremel to slice down the middle of each track.  You now have 1.27mm smt stripboard!

[Graeme Job]

Unfortunately, budget is rather limited, so hot glue, stripboards and a fair amount of swearing it'll have to be.

[Graeme Job]

@Adrian that would be great if the display was small scale, but there's 80mm spacing between LEDs, and they're hotglued to flexible ribbon. If I could get 96 tiny PCBs, with an RGB LED throughhole footprint and holes for wires, and if that would be under £30, that would be ideal. But i haven't seen any such deal from manufacturers.

[Graeme Job]

I am graebot by the way, posting from my phone.

[Graeme Job]

@Parag0n That's a genius idea!! Thank you very much! =D

[cynar]

How time critical is it? What footprint is it? We have a CNC mill that can do PCBs over at RHS, I'm happy to run a board through it if you like, and post them on to LHS. It wont be anything fancy, but it should be within our capabilities.

[Graeme Job]

@Cynar Wow, I hadn't thought of that. Would it drill the holes at the same time?

I basically need something that'll allow me to solder a 1.27mm pitch RGB LED to something solid, preferably soldered right down to the base, so there's minimal stress put on the leads (The LED itself is attached to the structure)

I'll also need contacts big enough to solder wire to 400 times without having a tantrum.

As Parag0n said, This could be achieved by dremelling down the center of the copper strips on a normal stripboard, but you would have to solder on the copper side of the board for the LED and wire, which could potentially add stresses to the LED terminals, the solder joint and the copper track.

That said, I don't believe it'd be a huge problem, but it'd surely have a shorter lifespan.

So, I have 96 LEDs to solder. So, 12x8.

Maybe an extra row for faulties / replacements.

4 holes for the through-hole LED.

Either 4 holes or 4 pads, probably at something like 3-4mm pitch for me to solder wires onto easily.

So, each breakout board need only be about 15mm x 15mm.

If the mill does do holes at the same time, it'd be great to do a perforation-type thing so I can snap the pieces off.

Something like that would be ideal. Can you guys do that? What sort of file format would you need?

Cheers,

Graeme J

[cynar]

Eagle would be easiest we are still working out the procedures and settings.

Drilling shouldn't be a problem, It's currently set up for 0.8mm holes right now, anything different will require changing bits over mid run, doable but a hassle.

Design wise, let me know the spacing on the pins and I'll knock up the design in eagle. It will likely just be a small square pcb with the LED on 1 end and the wires on the other. They might not be pretty, but they should be effective.

[Graeme Job]

Fantastic!

The RGB LED pin spacing is 1.27mm 

The legs are about 0.5mm, I believe, so 0.8mm hole diameter is perfect.

For the wires, 3mm spacing would be ideal. I think just having pads without holes for these would make the soldering job a lot easier.

Thank you so much for your help!

This would save me a whole lot of grief!