HI, new to thr forum, I have a basic set of skills and understanding of BASIC Electronic circuits etc etc, I am looking to build a very basic Monitor device that will allow me to see if an AV Amplifier is ON or OFF, the reason being the Amplifier is hidden away in a cupboard (the cupboard itself is visible to the Television viewer) and is controlled by an IR Extender, the only downside of this is that the Amplifier is quite high rated in terms of power consumption and dissipates heat, and with it being hidden from view is very prone to being left on.
So Id like to create a very basic sensor circuit that will "sense the unit is on" and perhaps illuminate a small LED that can be place discretely on the outside of the cupboard and serve as a reminder to turn the amp OFF or into standby. I have considered using one of those energy saving sockets, ie when the TV goes into standby of the AMP will be turned off at the socket outlet, However Id rather the AMP be instandby so it can retain the settings etc etc. and it would obviousley be enjoyable to create something. Can anyone suggest a starting point especially the ON/OFF sensor part. my first thought would be the Current flowing into the unit, obviousley in standby it would be minimal, while in use it would be much higher, but I have no idea what to use to do that with reference to include in a circuit, perhaps powered by a battery.

 

Make a current transformer like this ----> https://images-na.ssl-images-amazon.com/images/I/31Y+-8BHU7L.jpg
Insert one AC wire in it.

 

A little low technical, but if there is a power-on indicator with enough light, then a Plastic Fiber Cable might do the job.

 

Another option, if there is a LED, you can put a photo-transistor to detect the LED being on and turn on an external one.

Since, it is hidden, let's look at initially at some light you can detect.

Back in the "dark ages" when there were turntables, I had a timer that had two ranges, 0-10 min and 0 to 2 hours.It was normally at 10 min/1 hr.
If there was no sound for 10 minutes, the system turned off. It could monitor the speakers directly or the Tape Out and had a sensitivity control.
No such thing as retaining settings. For the purpose of this discussion, there wasn't.

Here's http://www.tauntek.com/tinyir2-learning-ir-remote-control-receiver.htm something that could be the basis of a timer based system. e.g. lack of optical out for 20 minutes, sends the IR command to go to standby. Caveats are is there an idle command or only a toggle command.

These are just ideas.