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Now the relays have been selected, I thought I'd better check some power figures. Relay coil resistance is 178ohm, which means driven at their rated 5V, they should draw ~28mA. The shift registers I was looking at are rated at ~20mA. The relays can be driven using a lower voltage, and using 4V would only drop the current to ~22mA, which although would likely work, it is likely to increase failure. So it's going to mean adding transistors to the drive the relays. Not a big deal, but even with quad transistors (4 transistors on single chip), is still another 9 chips to fit on a board.
The current plan, is the relay board will be separate from the controller board. I could combine them, but by keeping them separate, it means I can change the controller later while keeping the relay board.
I've also been looking at sockets. I'm currently thinking of using PCB mounted jacks, which although means the relay board will have to be a good bit bigger, it removes the added complexity of connecting jacks to the board via wire, and the jacks should support the board in the final housing.
The plan is to get all that combined into a schematic, and get a 3D model to see just how things will look. Then hopefully get some PCBs ordered within the next couple weeks.
And if I've managed to upload this correctly, this link should provide an online 3D model that you view - http://autode.sk/2D2PRLt
Now just to get the controller board designed..
It turns out there are Shift registers available with the current driving capability I need for the relays, so it greatly simplifies that part of the design. They also have inbuilt flyback diodes, so it eliminates the need for diodes on the relay board, but I'm just going to leave the relay board design as it is.
As I'd like to add Midi, I had been thinking of designing the controller board with the Midi sockets mounted directly on the board, but I can't help but feel that's going to give me a bit of a headache in terms of mounting everything easily in a housing, so I'm now thinking just having headers on the board.
I'm still debating LEDs. I think I'm going to add in an extra shift register for driving them, which will give me a total of 15 extra outputs, which I'll route to a set of headers.
Switch inputs I think I've decided on a 3x3 input matrix, which will allow me to have up to 9 buttons. I may expand it to a 3x4 matrix allowing for 12 inputs.
Power supply wise, everything is going to be run of 5V, so I'll take the easy option and use a 5V linear regulator, along with a standard 9V supply in.
Tomorrow's plan is to hopefully get it converted to a PCB design, which may take a little big of juggling, then try and get an order in before China shuts down for their new year :-/
So for those interested, here's how you go from schematic to PCB.
Taking the above control board schematic, you import it to the PCB designer, and get presented with this mess-
On initial import, all the components positions are taken directly from schematic, and all the blue lines represent connections.
First job is to move everything where you want it, so you end up with something more akin to the finished article-
The software does have an autoplacement function, however it will try for the most compact component placement possible, which means components end up in some very weird positions, and generally not in a good way. You can lock down key components, then autoplace the rest, but I find it just as easy to manually place everything.
Once your happy, you hit the autoroute button, at which point all going well, you end up with this -
At which point you just need to double check everything has routed correctly, by making sure there are no blue un-routed lines left, and carry out any slight tweaks. And having just looked at that this morning with a fresh set of eyes, I forgot to boost the size of the main 5V and Gnd tracks (not the end of the world as total current usage shouldn't be that high), and some of the components would of been better rotated around a bit to make track routing more direct. Ah well, that's why you make prototypes.
Then once you're happy with the design, you need to export all the required files for manufacture, and send/upload them to your PCB manufacturer of choice. Which in my case is Seeedstudio's Fusion service. I've used them multiple times in the past, and they're normally pretty quick, but I just need to hope they get them made and sent before shutting on Sunday for two weeks...
Now just to find the list of bits I need to mount on them..
Eqd Speaker Cranker clone
Monte Allums TR-2 Plus mod kit
Trading feedback: http://www.thefretboard.co.uk/discussion/60602/
10 of each.
Any less and you don't actually save that much money. IIRC I would of saved about £20, and probably would have to wait even longer for them. The default order quantity is 10, but the way they work, is they wait until they've filled a full size PCB panel with smaller boards, before producing them. So if you order less, you're bumped into a queue until they have enough low quantity orders to fill the panel.
Eqd Speaker Cranker clone
Monte Allums TR-2 Plus mod kit
Trading feedback: http://www.thefretboard.co.uk/discussion/60602/