You can find every information about Feather MO Adalogger in: https://learn.adafruit.com/adafruit-feather-m0-adalogger/overview

I have used all pins of Feather and I would like to have more pins available. These are the connections of the set:

Pin00- RX (Tx GPS) 

Pin01- TX (Rx GPS) 

Pin02- n/a (no external pin)

Pin03- n/a (no external pin)

Pin04- SD card CS (no external pin)

Pin05- Touch Interrupt ( irq – CAP1188)

Pin06- (TFT_CS)

Pin07- SD card CD (no external pin)

Pin08- SD Red LED (no external pin)

Pin09-  A7 - Battery sensor (This analog input is connected to a voltage divider for    

            the lipoly battery so be aware that this pin naturally 'sits' at around 2VDC  

            due to the resistor divider)

Pin10- Interrupt (Lightning sensor)

Pin11- Interrupt (Spectrograph)

Pin12- Noise (RadiationWatch)

Pin13- Signal (RadiationWatch) - Feather red LED 

Pin14- A0 to (TFT_DC)

Pin15- A1 to (Eletrostatic sensor circuit)

Pin16- A2 to (EMF Low Frequency Antenna)

Pin17- A3 to (RF sensor)

Pin18- A4 to (Speaker) 

Pin19- A5 to (RGB sensor LED control)

Pin20- SDA

Pin21- SCL

Pin22- MISO (TFT, SD)

Pin23- MOSI (TFT, SD)

Pin24- SCK (TFT, SD)

You can find every information about TCA9548A in:

https://learn.adafruit.com/adafruit-tca9548a-1-to-8-i2c-multiplexer-breakout/overview

I have use all case select of this multiplexer to bypass all I2c address conflict issues. These are the case selects connections:

tcaselect(0); i2c multiplexer set to GRIDEYE

tcaselect(1); i2c multiplexer set to RTC

tcaselect(2); i2c multiplexer set to BME280

tcaselect(3); i2c multiplexer set to TSL2591

Tcaselect(4); i2c multiplexer set to VL53L1X

Tcaselect(5); i2c multiplexer set to TCS34725

Tcaselect(6); i2c multiplexer set to CCS811

Tcaselect(7); i2c multiplexer set to MAX30105

This is very simple circuit that you can see detailed in; http://talkingelectronics.com/projects/200TrCcts/200TrCcts.html#8

A1 from Feather board is connected at the junction between the 220 R and the third transistor collector. In fact, this circuit is extremely sensitive and can detect everything from the displacement of electrostatic charges to variations in electrical fields such as electrical discharges from tasers or electrified fences.