Isolating the bus


This is possible. The circuit is rather complex due to the bi-directional nature of the I2C bus.
The following figure shows a possible solution:
 
Component Values:
5 and 5' : PNP like 2n2219 or BC557
6 and 6' : NPN like 2n2222 or BC 547
1 and 1' : 270 Ohm
2 and 2' : 3300 Ohm
3 and 3' : 1800 Ohm
4 and 4' : 1000 Ohm
Optocouplers : 6n139 , 4n27 or Til 111

Note: Since the speed of the I2C bus can be rather high, it is recommended to use a fast optocoupler. However, this circuit will not work on speeds higher then 10KHz. A 6N139 will do the job in all cases. The two PNP and the two NPN transistors can be any standard type, e.g. 2N2219 and 2N2222 (USA) or BC547 and BC557 (EUROPE).

How does it work ?

The problem with bi-directional lines is that a buffer tends to get stuck on a certain level. This case has been taken into account in the above schematic. In the following explanation we assume that the left side is transmitting and the right side is receiving (the circuit is symmetrical)

Let's assume we send a logic 1 into the left side. The LED of the top optocoupler will stay dark. Since its transistor does not receive any light, it is not turned on. The next transistor does not get driven and the line at the end is being pulled high via resistors 1' and 3'. The PNP transistor 5' will not get driven. Therefore the LED connected to it will not light up and there is no feedback signal.

Now let's see what will happen if we send a logic 0. The first transistor 5 will be turned on, therefore the led connected to it will start emitting light. This results in the fact that its matching transistor will turn on. The transistor connected to the emitter will be turned on also. The output line is now being pulled low via resistor 3'. This low level would turn on the PNP transistor, which would result in the other optocoupler to light, its transistor to turn on etc. In other words, the circuit would go into a lock-up. However, since the NPN transistor is pulling the anode of the LED to ground, this will not happen. This way we have eliminated the deadlock

© Vincent Himpe 2016