If you want to know how much current your electronic devices are drawing when they are turned off this DIY Standby Current Monitor will do just that. Back in the day when a power switch physically turned off the power to our electronics this was not a problem but these days the power button is normally just a soft input that simply turns the unit into standby mode. This is a trade-off to have things like the power button on your remote to turn on your TV.

“The current required by the electronic devices under test is monitored through a current transformer.  I made it out of a regular wall adapter: I removed the low voltage secondary winding and replaced it with 4 turn of paired 1.5 square mm insulated wire. Current flowing through the new winding produces a voltage across the untouched original 110Vac side of the transformer. The higher the current, the higher the voltage.  The untouched 110Vac primary is then connected to a filter stage, a variable gain amplifier  a full-wave rectifier, a peak detector and a threshold detector feeding a transistor and a low power relay.”