To identify the light switch pins I suppose I could have a standard where they are all from D1-Dx and Dx+1 has some kind of simple identifier on it that would get picked up by a scan. Any I2C devices could start from the last pin and work backwards.
For example if I have 20 light switches they would be on D1-D20 and D21 would be the "end of light switches" device.
What kind of cheap and simple thing could I plug in to D21 for this purpose?
Thanks,
Phil