[Solved / Archived] White blinking light in standby?

Posted by: @subzero1248

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Is it drawing any power from the power source when in standby to power the white light?

Yes. That is the only power source it can draw current from.

I don't know anything about the witty Pi mini, but I observed an odd blinking phenomenon of which I also wondered where is the power coming from ...

A few weeks ago I observed something odd on a Pi4 that was connected to its official power supply. I think I just had a battery powered RTC module (DS3231 with rechargeable LR2032 and a modified charging circuit [a series diode]) on it that I was testing.

When the Pi was halted (shut down), the 5V power supply remained connected to it although the AC power to it was cut. Yet, every so many seconds the red LED on the Pi would blink very shortly.

I looked quite some time for the cause of this odd blinking. First one notices in the corner of one's eye, but then you start waiting for the next blink. First I suspected something weird going on with the RTC and its battery, but that really couldn't be the case. Its connection to the Pi's 5V header pin had been improved with a series Si-diode to drop some voltage, because the charging circuit on the Chinese RTC board is rather dodgy for an LR2032, so no back powering was even possible.

Ultimately I found the culprit: the smart plug that the power supply was connected to. You may know the type that is connected to the internet through wifi and you can switch on and off using a smart phone app. The smart plug was a Blitzwolf BW-SHP6 Pro.

Anyway I found out that the smart plug has a one pole relay only. Depending on the way is is plugged in, it cuts the neutral wire, and so 'phase' remains connected to the Pi's power supply. There is some AC leak current from phase to ground from the smart plug to the power supply, which as s switching type has a high voltage rectifier circuit followed by a capacitor at the input. This leak current will slowly charge up said capacitor until the voltage is high enough to start the switching buck converter for a short while. It then produces a pulse of 5 V DC to the Pi and the Pi's red LED will flash. After that the cycle starts over again, so it will draw your attention for sure, although it may not actually be harmful.

I had to check which side of my numerous extension power strips is connected to the electric phase and mark the switched pin on the side of all smart plugs to prevent this leaking current phenomenon. I also have some smart plugs of another brand, too, and sure enough they also have just a single pole relay as well. I didn't open the cases, but used a 'phase detector' (which an electrician apparently had dropped under my cottage where I found it and cleaned and dried it until it worked again :-).

Even if this does not apply, it's something that you might see sooner or later, as smart plugs are in quite wide use these days, I think. You can use it to remotely start a Pi which has halted/shut down.