HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks
Adrian