Raspberry Pi v1 Model B Synth Board – Part 2

This is a bespoke PCB design for my Raspberry Pi v1 Model B Synth Board which I was using with my “Bare Metal” Raspberry Pi Synth.

Update:

• Build Guide here: Raspberry Pi v1 Model B Synth Board – Part 3

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, single board computers and electronics, then see the Getting Started pages.

The Circuit

This is essentially the same circuit as used with my protoboard version: Raspberry Pi v1 Model B Synth Board.  I’ve not used a MIDI THRU socket though as I wanted to have DIN sockets, and there won’t be room for three on the PCB.  But I have left it optional if the four potentiometers are fitted or not.  There are breakout headers to connect to up to eight external potentiometers if required all connected to the Pi via an MCP3008 multiplexer.

One addition I did want though was to add a DAC, so I’ve added a GY-PCM5102 module.  But as this is a V1 Raspberry Pi, I need to provide a way to hook up to the additional P5 GPIO header to get access to I2S as described here Raspberry Pi I2S Audio Interface.

PCB Design

Key features to note with this design:

• It will not work with a V1 B+ or later board – it is designed for the 26-way GPIO boards with the additional P5 header only.
• In fact, it probably won’t fit any of the other boards either!
• The board will have to be used with extended headers to ensure it can clear the Pi’s Ethernet port, unless you use it with a V1 model A (with no Ethernet socket).
• However, if things are a little tight then the fourth potentiometer could be omitted allowing the PCB to sit more closely to the Pi.
• I’ve used two GND zone fills, one on each side, to make routing easier and hopefully to reduce interference when in use.
• Whilst there are two mounting holes on a V1 Pi B, one of them will end up underneath the PCM5102 so it hasn’t been included in the PCB.  The other one is present though and can be seen just behind the MIDI OUT socket.
• This probably has a sub-optimal approach to using AGND for the MCP3008 and the potentiometers.  Basically they are all just connected to the common GND which isn’t a very good way of doing things as I understand things.  But for this prototype idea its kind of ok (if you squint).

Closing Thoughts

I hope the measurements for the P5 header are ok – I’ve not been able to find the mechanical drawings of a V1 non-B+ Pi anywhere… but I’ve measured it against my existing boards, so fingers crossed!

When I get this back I might get back to some of my “bare metal” experiments with those older Pis.  I’ve been somewhat distracted of late with Zynthian and MiniDexed!

This is the second of a batch of boards that is being manufactured by Seeed Fusion thanks to some discount vouchers I was sent.

Kevin