Electrical guide, manual structure.

docs/electrical.md

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+# TeraHz Electrical Guide
+This section briefly explains the neccessary electrical connections between the
+Raspberry Pi and the sensors you'll need to make to ensure correct and safe
+operation.
+
+As mentioned before, TeraHz requires 3 sensors to operate. The simpler UVA/UVB
+sensor and the ambient light analyzer connect to the Raspberry's SMBus (I2C)
+bus, while the spectrometer connects via high-speed UART.
+
+![pinout](imgs/raspi-pinout.png)
+
+## PCBs vs breakout boards & jumpers
+The Raspberry Pi GPIO port includes enough power pins to require only jumper
+cables to connect the sensors to the Raspberry Pi. However, this is not a great
+idea. During development, jumper cables have repeatedly been proven to be an
+unreliable nuisance, and their absolute lack of rigidity helped me fry one of my
+development Raspberry Pis. For this reason, I wholeheartedly recommend using a
+simple PCB to route the connections from the Pi to the sensors. At this time,
+there is no official TeraHz PCB, but it shall be announced and included in the
+project when basic testing will be done.
+
+GPIO can be routed to the PCB with a standard old IDE disk cable, and terminated
+with another 40-pin connector at the PCB. Sensor breakouts should be mounted
+through standard 0.1" connectors, male on the sensor breakout and female on the
+PCB. A shitty add-on header and a shitty add-on header v1.69bis can't hurt, either.
+
+## SMBus sensors
+SMBus is a well-defined version of the well-known I2C bus, widely used
+in computer motherboards for low-band bandwidth communication with various ICs,
+especially sensors and power-supply related devices. This bus is broken out on
+the Raspberry Pi GPIO port as the "I2C1" bus (see picture).
+
+Pins are familiarly marked as SDA and SCL, the same as with classic I2C. They
+connect to the SDA and SCL pins on the VEML6075 and APDS-9301 sensor.
+
+## UART sensor
+Spectral sensor attaches through the UART port on the Raspberry pi (see picture).
+
+The Tx and Rx lines must cross over, connecting the sensor's Tx line to the
+computer's Rx line and vice versa.
+
+## Power supply
+As the sensors require only a small amount of power, they can be powered directly from the Raspberry Pi itself, leeching power from the 3.3V lines.
+
+## Ground
+There's not a lot to say here, connect sensor GND to Pi's GND.