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From: Paul <nospam@needed.invalid>
Newsgroups: comp.os.linux.misc,alt.os.linux.mint
Subject: Re: smart plugs???
Date: Fri, 17 Jan 2025 21:46:24 -0500
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On Fri, 1/17/2025 11:48 AM, Mike Scott wrote:
> On 16/01/2025 13:25, Alan K. wrote:
>>> If you're up to a bit of wiring, try a USB relay. E.g. Annadue Relay Module (try Amazon, 2 channel £10.62)
>>> Interface: apt install usbrelay
>>> usbrelay --debug for information
>>>
>>> 1, 2, 3, 8 channels available
>>>
>>>
>> https://www.amazon.com/DSD-TECH-SH-UR01A-Controller-Computer/dp/B09PYML6Q7
>> That's the coolest.  Damn! I wish I had that back when!   Of course Linux Mint and good computers etc weren't available when I was a huge DIYer and tinkered, but it really would have been fun.
> 
> 
> Nice idea... but I'm looking at probably 3kW at 240V to run a heater.
> 
> The power level is just a bit outside my comfort zone for home construction, especially for unattended operation.
> 
> It's why I'm trying to find a commercial solution.

You can use a relay, to switch a relay.

It's like amplifiers in a sense.

You need to know the "pull-in current", to make
the "power relay" work. The relay is like a solenoid,
in that there is a coil of wire, a metal slug, and when
you put enough current into the coil of wire, the metal
slug moves and causes the relay contact to close.

If the relay is DPST (Double Pole Single Throw), then
you could disconnect hot and neutral on a 240V 13A load.
That sort of thing. Safety ground remains connected.

The relay on the USB module, you power the output contacts
from a power source providing the volts and amps the
coil on the power relay needs.

And you can test the power relay, without mains connected
to the secondary side. You can listen for the solenoid metal
slug moving and the relay contacts making the "ker-chung" noise.
If you use DC current for the relay coil, then the relay
coil will not hum.
                         DPST  Double Pole Single Throw

                           \
Hot         Line-side ---x  x--- Load side    Hot        (Fusing and fixtures
                                                          as appropriate)
                           \
Neutral     Line-side ---x  x--- Load side    Neutral
                   a----- ||
                        \ ||  Power
                        / ||  Relay (mains-rated contacts
                   b----  ||

With only signal connected to a & b, you listed for the
right sound effect. Later, you plug in the two ends on the
mains part of the project (the upper part). to switch the load.
The left hand side is a plug that goes into the wall. The electric
fire on the right, plugs into an outlet box on your wood strip with
the relay screwed to it.

Need to figure out amount of current into relay coil
at a and b, to achieve "pull-in" of the solenoid. The
more hefty the relay and its "return spring", the more
current needed on the coil to move the slug.

   Paul