V2_00_step_03.rst

STEP n°3: MUX board

The multiplexing of the channels is a mechanical multiplexing based on OMRON's manufacturing relays (G5LE-1-VD 12 VDC). Each relay is combined with a ZVN4206A power MOFSET. The raspberry has only 30 GPIOs, which is not enough to activate all the 64 electrodes, which represent 512 GPIOs. We used gpio expender I2C (MCP23017). We have associated these components with an I2C muliplexer of type type TCA9548A from adafruit. This combination allows to go up to 512 GPIOs and up to 128 electrodes. Each card has its own digital address between 0X70 and 0X77. In the following presentation for an Ohmpi 64 electrodes, we will use the addresses 0X70 for channel A, 0X71 for channel B, 0X72 for channel M and 0X73 for channel N. 0X73 for the N channel. 4 MUX board will be needed to multiplex an Ohmpi 64 electrodes.

PART A Assembly of MUX board

Required components

1
	Mux board pcb

2
	Installation of the 150 Kohm resistors

3
	Installation of the mosfet ZVN4206A

4
	Installation of the diode-1n4007

5
	Installation of the relay

6
	Instalation of the terminal screw

7
	Installation of generic male header

8
	Installation of Pin strip (6 pins) and pin strip (16 pins)

9
	Installation of DIP Dual In Line Socket 2*14

10
	Installation of Adafruit TCA9548A

10
	Installation of MCP23017

Note

This step must be duplicated 4 times for every Mux card.

PART B MUX board address

To build an ohmpi it is necessary to have 4 MUX boards, with 4 different addresses. It is therefore necessary to identify each board, by assigning an address, which will be allocated in the Ohmpi code. We present here the addresses selected by default.

For the A electrode board, we suggest addressing it with address 0x70:

1
	Mount the jumpers and note the value of the address and the electrode name on the mux board (A).
	Zoom on the jumper

For the B electrode board, we suggest addressing it with address 0x71:

2
	Mount the jumpers and note the value of the address and the electrode name on the mux board (B).
	Zoom on the jumper

For the N electrode board, we suggest addressing it with address 0x72:

3
	Mount the jumpers and note the value of the address and the electrode name on the mux board (B).
	Zoom on the jumper

For the M electrode board, we suggest addressing it with address 0x73:

4
	Mount the jumpers and note the value of the address and the electrode name on the mux board (B).
	Zoom on the jumper