Explorer HAT Pro - backpowering the Pi?


#1

Hi, is it possible to reasonably reliably and reasonably safely, backpower the Raspberry Pi from the Explorer HAT Pro? If so, how?

I picked up an EHP from the Birthday Bash as it looked ideal for a number of projects, notably my Pololu Zumo Chassis + metal gearmotors + Raspberry Pi Model A+ . (Great to meet you guys, by the way - I’m Andrew from Cotswold Jam, the chap with the daughter who loves the Lego bases).

The Zumo has a 4xAA battery compartment giving 4x1.5v=6v . I know I can channel this through the EHP’s H-bridge motor controllers to give power to the motors, my question is whether the EHP could also be used somehow to use the same batteries to power the Raspberry Pi.

My alternative option is to use a stepdown transformer to take the 6v and turn it into 5v, and then split this across both the Pi and the motors. My DC:DC stepdown transformer takes anything from 3-12v and converts it to USB power (5v) but ideally I’d like to do away with it and use the EHP for everything.

Third option is to use a cheap USB emergency phone charger, which I already have, to power the Pi and leave the 4xAAs solely to power the batteries. But that’s inelegant, and adds bulk.


#2

I think 6v is theoretically possibly on the Pi, but since it’s technically out of spec I wouldn’t attempt it! Basically the 3.3v and 1.3v ( I think ) are regulated down from the input voltage anyway, with the raw 5v input voltage supplying the 5v pins on the GPIO and the 5v supply to the USB ports.

So powering the Pi at 6v, if it doesn’t immediately explode, will yield out-of-spec USB and GPIO voltage. If you’re putting this 6v into the GPIO then that’s not really an issue.

If you want to regulate down to 5v, the safe option, you’ll need a super low dropout 5v regulator to get it working at 6v- ours need 7v I believe.

Best option, and safest, is a mobile phone charger- I love them and use them all the time- but like you say it’s bulky!


#3

Cheers. The DC:DC transformer I’m thinking of using is this:

This takes anything from 3-6v input (up to 12v if modded) and converts to 5v, with a max of 1 amp. My plan is to solder a couple of jumper cables onto the output pads (and possibly desolder the USB port for neatness). (Why not just use a USB cable to power the Pi? Because the restricted space on the Zumo plate, means you can’t have both a WiFi USB adaptor and a USB power cable when using a Model A+; it clashes with the tracks, or you have to mount the A+ up on stilts).

Without the Explorer Hat Pro, I’d place these jumpers directly onto GPIO pins 2 (5v) and 6 (GND). But since there is no GPIO pass-through on the EHP, could I plug them into the 5v and GND connectors above touchpads 2+3? Will that backpower the Pi?


#4

Having done a bit more research, the best option might be to use the DC:DC transformer and then solder 5v direct to PP1 on the underside of the Pi (5v input before the polyfuse) and GND direct to PP3.

List of PP test points on RPI A+/B+/2 models:

http://www.raspberrypi.org/forums/viewtopic.php?f=28&t=89522&p=628505&hilit=pp1#p628505


#5

Heya! Yes, you could do that. The 5V and GND pins behind the cap touch are connected directly to the Pi GPIO too (as you intimated!).

I would definitely make sure you have a solid regulated 5V supply though or you may be in for bad times!

1A total isn’t a huge amount to power both the Pi and some motors, but you may be OK! :-)


#6

Yeah. With a model A+ being only seventeen quid, this could fit in to the “acceptable losses” category of sacrificial electronics experiments.

Not something I’d try with a £35 Model 2 until the prices come way down!

I’ll let you know how I get on. And how “solid” the three quid eBay DC:DC transformer turns out to be!

As you say, current draw may prove to be the limiting factor; although the A+ only sips at batteries, the motors will draw more, and WiFi can eat quite a bit if in a poor signal area. Adding a camera will almost certainly be the straw that breaks the camel’s back, but worst case scenario I lose an A+ and have to resort to a mobile emergency charger after all.


#7

Oh with an A+ that does change the game a little :-) Good luck and report back!