Easiest way to control a pis power supply via usb

Question

This may be an easy question for electronically inclined, but I am not too savy in that area.

I have a little bramble in a rack, where I don't necessarily have access. The bramble has an x86 master board that I can always access (KVM switch). The pis all are powered from the same psu as the server, so I can reboot them all by cycling the server. However this is cumbersome and I'd prefer to reboot them individually even in situations where the OS isn't running or responsive.

I would love for someone to share schematics for a cheap, easy to build usb controlled circuit breaker, that could reboot them individually (or in smaller groups though I doubt that buys us anything).

Answer 1

You can buy relay boards that are controlled via USB. E.g. this:

You would wire the power supply of each Pi to an individual relay, the power supply for each Pi can then be controlled by a computer connected to the relay board's USB interface.

Another possibility would be to use the Pi's built-in watchdog. See this answer to How do I hard reset a Raspberry Pi?

Answer 2

While a USB Relay Module will work, relays are bigger, louder, and some require an additional power source. The more channels you need, the more it takes. They are useful for situations when you have to physically cut power though.

The other option is a USB GPIO Module. Essentially the same thing, but without the relays and the passive parts needed for them.

Model B Rev 2 Raspberry Pis have a header labeled P6. It is tied to the Run (Enable) Pin of the BCM2835 SOC that runs the RPI. Shorting Pin 1 to Pin 2 (Ground), will cause the BCM2835 to hard reset. From the RPi Schematic:

Using a USB GPIO Module with a 1kΩ Resistor and any common NPN transistor, you can electronically control this.