Frequently Asked Questions
Yes! We'll supply common types including North America and the EU.Last updated:
Yes! We actually thought about the power button already -- often projects will want to mount this away from the Pi itself -- so we're doing solder points on the PCB to support that. We hadn't thought about the LEDs -- thanks for the tip! (Though maybe not for all of them -- the RGB has a lot of connections...)Last updated:
Weve been looking at putting the pass-through on prototype 3.
The downside of always supplying the pass-through is cases: it is hard to fit it inside all cases when there are another set of pins on the top.
We're definitely aware of the benefits though... perhaps we'll offer two versions, depending on demand.Last updated:
MoPi uses a switched-mode regulator (a TPS5430), so almost no power is wasted.
The voltage range is 6.2V to 25V.
In contrast to the Pi's LDO, which is "the hottest component on the board" according to Eben, our regulator runs cool because it is switched mode (and also has plenty of headroom). The downside is that we can't accept voltages lower than 6.2V.
(There's no step-up circuitry so the battery has to supply 6.2V at minimum.)Last updated:
Just click on the reward that you want, then it offers you a "how much to pledge" option, and you add the £2.25 there.Last updated:
Yes and no!
The issue: MoPi supplies power to the Pi over the GPIO pins, instead of via the micro USB connector (which it leaves free so that mains power can be plugged in at the same time as batteries if required). The 5V feed from GPIO involves bypassing the Pi's built-in fuse (which is in line with the USB connector).
Is this safe?
Given a correctly functioning Pi and peripherals, then the answer is a clear "yes". But what about if the Pi is faulty, or if a pin is shorted out, or if someone connects a 3 bar electric heater to it? :-)
First, MoPi is currently capable of supplying up to 2A, after which there is an internal current limiting circuit. It uses standard voltage and power regulation circuitry which is robust and mature, and will not allow over-volt or over-current on the output.
Second, we're supplying from batteries, not mains -- so as long as we don't over-consume from the battery (which in principle we can't short of a major internal fault) there's a limit to the amount of power we can supply to a faulty Pi.
Third, not having an internal fuse is pretty standard behaviour -- poke around in the electronics of all those power supplies sitting around in your desk drawer and you won't generally find fuses.
Having said that, we'll look into adding an autorecovery fuse to version 3 just to be ticking all the boxes :-)
Thanks for the question!Last updated:
There are two sets of connectors that come off the top of the board -- labelled "supplies" in this pic: http://pi.gate.ac.uk/static/images/mopi/mopi-with-key.png
In prototype 2 they're soldered on and then attached to PP3 or car charger or etc. connectors. In prototype 3 and the final version they will be screw terminals, and we'll supply PP3 connectors by default because these are standard on lots of battery holders, e.g. the ones shown with prototype 1 here: http://pi.gate.ac.uk/static/images/mopi/prototype-regulator-05.jpg
To use a different type of supply (e.g. an old laptop charger, or car cigarette lighter plug) just bare the wires, push them in the terminal holes and screw down.Last updated:
There's no soldering -- the board is fully assembled. We thought of offering a kit version as well, but we use surface mount components which are really tiny. My eyes hurt even thinking about soldering them...Last updated:
Sure -- but we would have to use lower quality components and that would inevitably increase the failure rate. The Pi itself is miraculously reliable -- we want to meet the same standard!Last updated:
We'd love to -- but it raises a whole load of tough issues to solve. Dealing
with multiple battery chemistries is already hard -- each type of rechargeable
battery has a minimum discharge level, for example, and if you go below it
then the battery will likely not recharge any more. Then we have any multiple
of each type that will generate more than 6 and less than 25 volts. If we try
to charge all these different configurations then we'll a) have to reply on
really good and accurate data from the user, and b) damage batteries when the
data isn't correct. Perhaps we can add charging for a small set of
configurations -- but probably this will be as a stretch goal!
Also, part of the point of allowing users to connect any old battery is that
lots of rechargeables that people already own have chargers with them. In
which case, the benefits of adding another charging system are relatively
Finally, we also let you swap out one battery pack, connect a separate
charger, then swap it back in without shutting the Pi down. So another of the
use cases for on-board charging (to continue working without shutdown) is at
least partially catered for.
Let's keep talking!Last updated:
The software on the Pi that talks to MoPi is open source and available here: https://github.com/hamishcunningham/pi-tronics/tree/master/simbamon
(See also http://pi.gate.ac.uk/pages/download.html .)
It listens to MoPi and logs the data in a standard manner, which other applications can then consume as required.
The Raspbian maintainers have added this software to their repository, so you can install it in the same way as any other Pi sofware.Last updated:
This should work so long as you connect the mains-derived power to one of MoPi's inputs (it will have to be a supply >= 6.2 volts). MoPi will then use mains whenever the solar output dips below the mains supply. The ideal would be a solar panel rig that outputs e.g. 15v coupled with a 12v mains supply, or some similar config.Last updated:
Rechargeable batteries don't like being completely discharged. We always need
to stop drawing from the battery at some distance above the minimum voltage
level. This distance varies for different battery chemistries (NiMH, LiPo,
lead/acid, etc.) and for different multiples (eight AAs vs. two PP3s vs. one
car battery, etc.).
MoPi listens continuously to the voltage level on the battery inputs. How does
it know when that level is getting low? Let's identify two separate low-level
points: first, the absolute low voltage level is the point where our
micro-controller can no longer reliably run the switching regulator to produce
a constant 5V (around 6.2 volts). The second low voltage point depends on the
particular battery configuration (for eight NiMH 1.2 volt cells it is around 8
volts). For non-rechargeables, or solar panels or a water turbine, it is the
same as the absolute low level (6.2V).
MoPi is responsible for deciding the input voltage is low (for either or both
of its two supplies), and then warning the user and/or shutting the Pi down
cleanly (from software first, using "sudo halt", and then by cutting power).
How do we decide when to do this?
First we have a default configuration, which is the one which we will be
supplying in the "battery heaven", UPS and outdoor kit rewards: eight
high-capacity NiMH AA cells. In the absence of any other data, we assume that
this is what is connected, and when the output voltage descends to around 8.5
volts we turn the main indicator LED red and send warnings to any user logged
into the Pi. (There are various other levels defined that choose blue -- the
highest level indicator -- vs. green, and so on, and separate LED indicators
for low level on the two supply inputs.)
Of course this default configuration won't work for all cases, so then we
allow choice of another set of options from the Pi itself. (Currently we're
looking at a set of 16 different options, but that may change in the next few
weeks as we finalise the third prototype.)Last updated:
Please give me a shout on hamish at gate dot ac dot uk, or any other method listed here: http://hamish.gate.ac.uk/pages/contact.html
(Oh, Timbuktu should be fine, btw, but no harm in checking!)
This would be for cases where the Pi is assumed to be thrashing or otherwise dead to the world...
We can't get the Pi itself to reboot in that case, but we can do a hard reset by cycling the power.
We'll add this to the wishlist for the firmware.Last updated:
Don't see the answer to your question? Ask the project creator directly.Ask a question