Wiring

Every connection in the box, pin by pin. GPIO numbers are the Pi's Broadcom names; pin numbers are positions on the Pi's 40-pin header. Pin 1 is the one with the square solder pad, at the memory card end of the header. Odd pins are on the inner row, even pins on the row along the edge.

Power

The reference build feeds the supply's 5 V to the Pi and to the amplifier separately, so the amplifier's current doesn't pass through the Pi. On the Pi it uses two test pads: PP1 (5 V) and PP6 (ground). Find them by the labels printed on your own board, not from a picture of another board, and check them with a meter (below) before you solder.

Leave the Pi's header pins 2 and 4 (5 V) unwired. If your barrel socket has a third, switched contact, insulate it and leave it unconnected.

Before you power up, with everything unplugged:

• Check PP1 and PP6 with a meter. PP1 must show continuity to header pin 2 or 4 (5 V), and PP6 to header pin 6 (ground). If they don't, you have the wrong pads: stop.
• Check the barrel socket with the plug in. Plug the supply's plug in (supply unplugged from the wall) and check which socket contact meets the centre pin and which the sleeve. The centre is +5 V.
• Check for shorts between 5 V and ground, and between 3.3 V and ground.

When you use it:

• Use a regulated, protected 5 V supply, centre positive. Our prototype uses a 2.5 A adapter, but no current rating for the whole box has been worked out yet. For the first power-up, use a bench supply with a current limit if you have one.
• Never power the box from the PC's USB port and the barrel socket at once. That is why the USB cable's +5 V wire is cut (see below). Unplug the barrel power before you connect a USB cable that still carries +5 V, and unplug that cable before you plug the barrel power back in.
• Never plug or unplug bare wiring while the box is powered.
• An inline fuse in the 5 V lead from the barrel socket is a good optional extra.

The screen and the lamp board (I2C)

The OLED and the MCP23017 share the Pi's I2C bus.

• The firmware looks for the OLED at address 0x3c, and the setup page has no setting for another address, so use a 0x3c module. Check the address on your module before you power up.
• For the Waveshare MCP23017 board, leave the A0/A1/A2 address straps open: that gives 0x27 (on that board, closing all three gives 0x20). With any other MCP23017 board or a bare chip, set the address to 0x27 (on the chip itself that means A0, A1 and A2 high) and check it. With a wrong address the lamps stay dark. If a bare chip's RESET pin isn't pulled up, tie it to 3.3 V.
• The Waveshare expander board already has pull-up resistors on SDA and SCL. Check what your other modules have before you add any more.

The eight lamps

Each lamp is wired from one expander output, through its own 470 ohm resistor, to the LED's anode (the longer leg). The LED's cathode goes to ground. An output that goes high lights its lamp.

Fit them left to right in that order on the front of the case. Leave the GPB pins unwired. Never connect anything to GPB7: the firmware drives it as an output.

The speaker (I2S amplifier)

• The speaker goes between OUT+ and OUT-. Neither of them is ground: never connect either one to ground.
• Leave SD and GAIN unconnected only if your board's own notes say that gives 9 dB gain and the (L+R)/2 mix; ours were taken from the seller's photos, not a schematic. Don't connect SD to a Pi pin: its pull-up may go to 5 V.
• No MCLK wire is needed.

The PC connector (DA15)

The box has one connector for the PC: a female DA15 socket, 15 pins in two rows (8 on top, 7 below; not a three-row VGA socket). USB and the serial line share it. This is our own pinout, not a standard one, so never plug an ordinary 15-pin cable (a joystick, network or VGA cable) into it. The full drawing is page 7 of the wiring plan (PDF, on the Downloads page).

• No pin carries power: the box runs from its own barrel supply and must never be powered from the PC.
• Find PP22 and PP23 by their printed labels on the Pi and check them with a meter against the data pins of the Pi's USB socket (D- is its pin 2, D+ its pin 3) before you solder.
• Look at the socket's moulded numbers: seen from the front and from the solder side the order is mirrored. The metal shell is not pin 5.

The PC cable starts at a male DA15 plug and splits in two:

Keep the two USB data wires twisted together and short. Check the cable with a meter before you first plug it in: no USB pin may touch a serial pin, and the USB +5 V wire must reach nothing.

The serial port (being tested)

Three MAX3232 boards, called S1, S2 and S3 here, each with VCC to 3.3 V and GND to ground. Their RS-232 side goes to DA15 pins 1-9; from the PC cable's DE9 branch, the serial cable to the PC must be straight-through with all nine wires.

• Fit a 10 kilo-ohm pull-up from 3.3 V to each transmitter input in use: S1 T1IN, S1 T2IN, S2 T1IN, S2 T2IN and S3 T1IN.
• On S3, tie the unused T2IN to 3.3 V. Leave every other unused pin unconnected, and never join two outputs together.
• The DA15 socket's metal shell stays unconnected. Pin 9 is the ring indicator, not power.
• The port runs at 115200 bps, 8N1, with RTS/CTS flow control. The box keeps RI off for now: there are no incoming calls yet.
• On Windows, add the modem yourself (don't let Windows detect it): pick a standard modem that allows a maximum port speed of 115200, on that COM port. Set the maximum port speed to 115200 and the flow control to hardware (RTS/CTS).

Next: the case.