PiClaw Conversion Part 2.2 - Failure analysis & PCB V0.1 Design

In this blog post I will be detailing the analysis of the reason for failure and designing the first revision PCB.

Analysis

While initially I believed the Power supply was at fault it seems it was not. I opened it up and powered it up with no load and everything was fine. No magic smoke or anything.

I then plugged it back into the electronics connecting the ground and one voltage rail at a time. The 5 & 12V Rail connected fine and powered up (however there was no voltage LED on the Pi). I then plugged in the 24 & 48V Rails and once again the power supply cut off.

I then disconnected each rail and found it was something to do with the 24V Rail which was the motor controllers. I then removed one chip at a time from them and powered them up and found that one of the motor controller ICs had an issue.

That explained the 24V rail not working but not why the Pi did not power up. However I believe that a surge went through the motor driver IC and into the pi killing the BCM chip.

Notes to take for V0.1 PCB

  • Better protection: The Pi needs better protection from the motors, in this case I'm going to be using Diodes & Opto Isolators where possible.
  • Sockets & Plugs: For ease of maintenance

Designing the PCB

To begin I had to pick which PCB software I wanted to use. I usually use Eagle however knew this board would have been over my paid edition license of 10x8CM.

However apart from Kicad the other two main software I could find (Designspark & Circuit Studio) both are windows only and require wine which when I tried was just not working easily.

However with the Autodesk takeover of Eagle while the price may now be monthly if you're a student you can get it for free! Activating it against my open university credentials got me a full copy to use :D

I first planned everything I needed in a schematic, I used the same connector as originally used for the claw assembly but have decided to replace the rest with my own wiring system which will be using VGA connectors (I have a bunch of spare sockets) and network cable for wiring the front panel.

Screenshot-from-2017-10-17-15-07-40

I then put everything on a 10x10CM PCB on the board editor and ran autoroute. While people don't like autorouting I use it at this stage for a simple reason. If the auto router can route it, It's possible to route and I can then do it better.

I then ripped up all of the traces and started to move items arrive and finally routed the board. Here's what the V1 Board looks like.
Screenshot-from-2017-10-17-15-09-23

The diodes on the motor driver ICs are to add extra protection, 4 per motor are for flyback diodes and two are from the GPIO to the driver IC. So if the chip does fry itself all I have to do is replace the chip which isn't an issue due to the low cost and socketability. As I only have 3 motors you can see one motor driver has only one side being used.

The front panel has the two buttons which in turn uses 2 Lightbulbs to light the lights up. I'm using the TIP120s for these as while overkill for two lightbulbs at 12V I have quite a few. It also meant I could copy and paste the circuit from the claw driver.

One of the VGA / DE-15 connectors will then go to the front panel connecting the front buttons & lights, the I2C bus is also connected through as there will be a driver chip to drive the 2 7 Segment Displays in the front panel along with a chip to interface to the coin mech.

The other DE-15 connector is free to lead to the top of the machine and also has a I2C connection with some power rails. If I then want to either connect or create my own PCB to have a mini game in the top section I can.

And that's about it! It'll be here in about 2 weeks from China. I paid £20 for 5 PCBs with DHL which I was very impressed with! (A UK Manufacturer wanted ~£40 for 1!)

Next time The next post will either be assembling and testing this board or redecorating the machine. Stay tuned!


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