i2c pins connectors

Some rules when connecting devices (modules) like sensors: with special attention to serial busses like i2c.
This is the start of a guide to show how to build circuits with multiple types of sub assemblies.
It also documents the thought processes in connecting a Raspberry Pi to external sensors. The sensors are on a standalone board and operate

from different voltages. Work is happening to interface the BBC micro:bit, Cypress boards and Arduinos.
The i2c protocol requires the SDA and SCL lines to be bidirectional (i.e. input and output)

1. Connect the ground lines
2. Check the voltage rating of the sensor e.g. 3v3 or 5v0
5. - some sensors are voltage specific and some cope with a range (compare with mains voltage power packs)
6. - some sensor breakout boards are equipped with a regulator (e.g. 3v3 sensor but 5v0 regulator onboard
7. - power up the regulator - test the output voltage and current rating. Use a DVM for this.
8. - some onboard regulators have ample capacity to power other devices
9. - voltage convertors are available to interface 3v3 to 5v0.
3. If the sensor is, say, 3v3 rated ensure that the voltage on the data lines and the VCC line does not exceed 3v3
4. template

The order of the four I2C connections on the BBC micro:bit is as follows:

1. see notes
2. Gnd
3. Vcc
4. SCL
5. SDA

The order of the four I2C connections on the Slice of Pi V1.1 is as follows: [Note power and Gnd pins swapped wrt BBC above]

1. see notes
2. Vcc
3. Gnd
4. SCL
5. SDA

This is the same sequence as for the OLED display. The addresses for the display are 0x78 and 0x7A, select one.
The Adafruit PCA9685 (16 * 12 bit PWM) has the following connections

1. Gnd
2. see notes
3. OE
4. SCL
5. SDA

Thus the BBC micro:bit and the OLED can be connected directly with a 4 pole 1 to 1 cable.
With a minor wiring change the BBC unit can be connected to the PWM unit. Vcc from the BBC should be connected