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> But I was thinking about something like modulate a 15V line
> with the i2c signal, isn't it possible?

> Do you think it is very dificult to design something like this?

Attached is a circuit that works for a remote control using a signal
and ground. It's a unit I made a long time ago from the Ferranti
handbook. The idea is that the signal from the left side momentarily
shorts the 5V line, causing negative-going pulses to appear at the
signal pin of the receiver on the right

While this simple circuit works well for one-way remote control, I
think it would be much more difficult to implement 2-way I2C. You
would probably need a lot of signal conditioning to rebuild the data
and there's the added complication of a clock line

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