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Not technically entirely passive
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.. so fails the requirement. They also turn on mains things so another PSU required.
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The posh answer is a hall-effect current sensor plus associated electronics.
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But again not passive.
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just put a series resistor in the mains supply
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Not keen on a hot part, also I want the relay elsewhere so needs to be an isolated feed.
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Does the telly have any USB sockets which are supplied with 5V when it's switched on?
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Brilliant. I should have thought of that.
But I am seeing snags: I mainly have a supply of 24V relays (maybe should have mentioned that). The TV outlets have the wretched "double insulated" aspect - i.e. an electrostatic voltmeter sees them as mains.
I already have issues with this and the audio which is about to be improved to remove the last traces of hum.
So still looking for the ideal solution - making a safe current transformer.