General guidance and project review request, mainly with power supply questions

HI all, I would appreciate feedback on an idea I am working on. I have some specific power supply questions as well. I'm a software engineer by trade so I know code, but electrical is new to me.

My project is a wearable rocket pack for my kids.
It will have pot joysticks that will play sounds and flash leds when used. There will be multiple switches to cause logic to run turning lights on or off, or a buzzer to buzz, a 10 segment bar led to countdown. There will also be some 5v flashlights that a switch can turn on.

I'm looking at about 2 10 segment bar leds, 10 switches, 8 pots, 4 flashlights, and some speakers.
Leds are in a single group of 16, and 2 groups of 4.

I will be using a mega as the base, with a wav trigger from sparkfun for the sounds. I know about max mA per pin and max for the board. There will be at most 12 leds on at a time from the board, excluding the bar leds which I plan to power via 2222 transistors and either MAX7219s or shift registers. The 4 flashlights I plan to power via 2 RFP30N06LE mosfets (overkill I know), turned on via switches triggering some code.

Everything will be mounted in a chestbox and backpack, made from abs and extruded Styrofoam, covered in tyvek.

Any comments/advice on the overall setup would be appreciated.

My main questions are regarding the power supply.

My current plan is to use a single USB power bank, approx 22000mAh, with a usb plug going to a circuit board, with multiple connections going out. One to the barrel jack on the mega, and one line to each transistor and the wav trigger.

Can I split a single USB power bank this way into multiple destinations without trouble? Do I need to use 2 USB power banks, one for the mega and one for everything else? Do I ground them all together (power banks and mega sharing the same ground)? Is my approach the correct one for powering this monstrosity?

Thank you in advance for your advice

In addition: will the USB 5v power bank work to run everything? I'm assuming that since everything is 5v that it will work.

Hi redblurr,
I am very dubious of the current capacity claims of some packs but what you need to look at is not the total capacity but the maximum current it can supply at one time.

This is usualy around one to two amps that is the limiting factor. As an example, you have a water tank that holds 22,000 litres but your tap will only allow you to drain 1000 litres at a time . Not good if you need 5000 litres.

Assuming your current is under the one to two amps you can hook everything to the one battery by cutting a usb cable or using a usb hub to distribute the power.

These power packs usualy run by using a single lithum cell and an inverter booster to bring it from 3.6 volts up to 5 volts and this is the cholk point.

Daz1712:
Hi redblurr,
I am very dubious of the current capacity claims of some packs but what you need to look at is not the total capacity but the maximum current it can supply at one time.

This is usualy around one to two amps that is the limiting factor. As an example, you have a water tank that holds 22,000 litres but your tap will only allow you to drain 1000 litres at a time . Not good if you need 5000 litres.

Assuming your current is under the one to two amps you can hook everything to the one battery by cutting a usb cable or using a usb hub to distribute the power.

These power packs usualy run by using a single lithum cell and an inverter booster to bring it from 3.6 volts up to 5 volts and this is the cholk point.

Hi Daz, thanks for the reply!

I am hoping that total draw will be under 2A, but I will not know until the flashlights get here. I think they will be the most current hungry parts. If need be I can run a separate power pack just for them. I guess there is nothing to do but prototype it out and see what happens. Just have to wait until the flashlights get delivered..

Just in case the USB packs don't work, what other options do I have to use for portable power? I do need to keep it as lightweight as possible.

I have done tons of online research on this, and from what I have gathered i should be doing everything correct, or so I hope.. but i feel better moving forward having the advice of those who are more experienced with this :slight_smile:

Another thing about USB power banks, they usually switch off when not enough power is drawn. So if your project is idle (with maybe 1-2 LEDs and the Arduino active), after a few seconds it’ll switch off.

Consider running the whole thing at lower voltage (get a 3.3V Arduino - or build your own, it’s not that hard - the main thing is you have to run the processor at <12 MHz so 12 MHz crystal or 8 MHz internal oscillator) and power them with a regular lithium ion battery pack. You’ll have to get a separate charge for this of course. Adafruit sells such battery packs (the largest being 6,600 mAh) with protection circuits built in.

Dont forget the good old hardware store with all those battery packs and chargers for power tools that you can buy so cheep and seperatly from the tools.
These with a couple of Boost-Buck units frrom ebay and you have what ever voltage you need.
Units come with voltage inputs of 4 to 30 volts input and 1.2 to 36 volts output. Current limiting and some have an input to turn the units on and off (really have to hunt for those)

redblurr:
Just in case the USB packs don't work, what other options do I have to use for portable power? I do need to keep it as lightweight as possible.

There will be suggestions to use Lithium batteries in various forms, but be aware that very few have so called 'protection circuits' that will cut off the battery at a reasonable level, 3.3v or perhaps 3.0v.

Most 'protection circuits' will cut off at 2.4v or lower, and whether that is safe to carry on using the batteries after such a low discharge is debatable. Depends on the application of course.

Remember that if the Lithium batteries are abused significantly they may become dangerous.

So unless you build in decent low voltage cutoff protection, it would be safer to stick to Alkalines, NiMh or LiFePo4 rechargeables etc.

Excellent, a hardware question where I can actually be of use!

Ok, so as said, your choke point is the current output from the power banks.

I have some power banks that will deliver 2.4A, they are fairly large but they are fantastic (in the UK, I got them from RS components, they have a very large capacity).

(The first thing I did was ditch the USB board from them, and made my own, but my applicaiton was very different to yours. Don't do what I did, as said they have protection circuits in them to stop you from doing damage. Believe me, when they go up in smoke, they REALLY go, you don't want that strapped to your kids when they do!!).

A power bank is safe to use, for a given value of "safe" (the inherant problems with lithium batteries when they are damaged, shorted out or over-discharged). If the kids are likely to start throwing water around with hteir super cool sounding backpacks (please post pics!!!) then lithium packs are NOT the way to go.

If the flashlights are LED, I would almost certainly say a power bank will be just fine to power the whole lot from an electrical point of view.

However, have you looked at other batteries? NiCD or NiMH for example?

for a kids toy, I would be tempted to just rig up some C or D cell rechargables rather than use a power bank fo the sake of ruggedness, ease of replacement and repair and because kids + batteries often = doing things they shouldn't with them, but that's just me.

But back to specifically, the questions you asked... No you won't run into trouble splitting off a USB and dividing the supply between different components. The optential between all the components in a paralell circuit will be the same, i.e. they will all be at 5 volts. yes you can use a common ground between them all.

Use one large battery pack rather than two smaller ones for convenience, there's no reason you can't use two to power different parts of the circuit but it adds unneccesary complexity.

The one thing you must NEVER do is try to double up the lithium cells in power banks (that is, splitting them open and sticking the two cells in parallel). Likewise, it is unwise to put outputs from two USB boards together in parallel unless you really know what you are doing. The reason? Complicated things about charge, surge currents, and other fancy words that when put together basically mean "Fire". But since you aren't going to open up a power bank, you will be just fine.

Hope that helps!

Dave

Thank you all for the responses, Karma given!! (Especially you Dave :wink: )

Lithium is a non started from the start for safety reasons. This is being built for a 6 and 9 year old, so I have to make sure it is built as idiot proof and over redundant as I can... that ruled out Lithium.. at least for kids builds.

I also discarded regular batteries due to weight considerations. The power will be plug in via USB, so I can try out later some D cells to see how well they work if the power banks prove problematic.

Going down to 3.3v or other non-off the shelve tricks are unfortunately not going to work, for this project. (filed in the back of my mind for later ones though). Same applies to batteries, hence why the USB power bank appears to be the best option. Again, filing all these ideas for later projects :wink:

I have some simple 4.5v e10 LED flashlight bulbs ordered, and after thinking about it, they do not really need any logic, just a simple on/off/on switch. So, i modified the design to have them wired directly and bypassing the arduino.

Regarding power and the supply shutting off.. I have a few ideas. At minimum the 2 bar LEDS will be running at all times, as well as a voltage led readout. If this proves to be not enough to prevent shutoff, then I can add a few LED "navigation" lights, or maybe some resistors to bump up the current draw.
I think the best way to prevent shutoff though is to change the communications setup (old Motorola talkabouts) Instead of being completely removable and independent, I can create a "battery" pack for that is wired up with a plug, and hook that up to the power bank as well. I'm pretty sure that will do the trick.

And again, thank you all for your input!!

When considering power, how long does it need to run between charges?

Not for a Marathon? How long for the kid to get tired of running? Five whole minutes? Two?
Suppose it could be one minute and it's time to recharge the rockets?

Do you see how a few rechargeable AAA cells might just do?
It's just a shame that a bank of supercaps probably wouldn't do all of that for even a minute, it'd be quick to charge!

Digikey lists 90F, 7V supercaps for just over USD 17 a piece. Those should do the job for a few minutes of fun, and should indeed be really quick to recharge.

redblurr:
Lithium is a non started from the start for safety reasons. This is being built for a 6 and 9 year old, so I have to make sure it is built as idiot proof and over redundant as I can... that ruled out Lithium.. at least for kids builds.

Most mobile phone powerbanks contain exactly those batteries.

Going down to 3.3v or other non-off the shelve tricks are unfortunately not going to work, for this project.

That's not a "trick" or so. AVR processors are rated to work just fine on 3.3V, but you have to go down to 12 MHz or 8 MHz. There are 3.3V Arduinos out there, including versions of the Pro Mini. Or use the ESP8266 which needs 3.3V, it can't even run on 5V.

It's a design choice rather than a trick. 3.3V is the future - almost everything works at that voltage, and more and more ICs and sensors can't even handle 5V.

wvmarle:
Most mobile phone powerbanks contain exactly those batteries

Indeed so.

And these are the batteries are prone to catching fire and\or exploding when physically damadged or possibly dropped.

On a couple of ocaisions I have crashed RC planes, which used Lipos, and had the batteries start smoking and catch fire sometime later ................

LithiumFePo4 are supposed to be a lot more durable and resistant to abuse.

Runtime i'm hoping to get an hour or two out of it. I might be over optimistic but 30 minutes would work as well, but thats the minimum.

Probably used the wrong wording for trick, i mean more of anything that is not out of the box (I had to google 12mhz just to know what you were talking about). I have a mega on hand, so that's what I am stuck with to use for the project. I want to keep it simple and basic so in case someone wants to replicate it later, it'll be easier.. of course easier is completely relative. It took me 2 months just to get to where I am now :wink:

well poop on the power banks.. time to research some more, maybe look for a heavy duty & safe power bank or the LiFePo4 or try a different approach. The bank I have i'm pretty sure is standard, so for prototyping only it is. Maybe I can make a custom padded enclosure for it... regardless, this needs more thought.

Google is my friend 0.o

Thanks again all :slight_smile:

I have found a few heavy duty, drop proof power banks.. Looking at a 5v 2A one right now. Going to research more but I think these will calm my lithium fears.

Apologies, I hadn't realised that you didn't know USB banks are almost exclusively lithium.

Here's an idea though, following on from the Supercaps idea posted by wvmarle...

yes supercaps would not give you huge amounts of time, but why not make that part of the fun? Make a "launchpad" or a "space station" or similar that is a charger, and the kids have to run back to "base" to refuel every nowand then..? The capacitors would charge in seconds rather than minutes, certainly quick enough for sugar-charged hyperactive kids to not lose interest and for it to be part of the game, especially if it makes noises and flashes while charging.

All USB power banks are 5v. I have a LiPo 20800mAh (77Wh) bank sat in front of me which has two outputs, a 2.5A and a 2A. Made by Ansmann (model number 1700-0068). Now, what I can tell you about this particular model is it is large (about 150mmx80mmx20mm), for a power bank it is relatively heavy fora powerbank. This is exactly the type I opened up and used in tandem for the solar powered Raspberry Pi project I was comissioned to make work. They are pretty rugged (this one goes travelling with me), but as I said before, when they go up in smoke, they go up big. However, it will charge a mobile phone several times before depleting, and runs a raspberry pi for well over 24 hours depending on the settings. Perhaps overkill for your needs!

Your weight concerns... I think if you use the slimline "pencil" type USB banks (the kind that are in aluminium housings), they will be light enough and rugged enough. One of those would run for a good while, and I wouldn't worry too much about the "idle" current, all you need is an LED with enough draw as an "on" light to keep it ticking over, that is easy peasy. Like this one:

There is lots of talk of lithium batteries exploding and catching on fire.
Yet still every product comming on the market that runs on batteries is fitted with a lithium battery.
From your phone that you drop (admit it you do it at least once a week) to laptops, tablets and nearly every power tool being sold.
Did i mention kids toys?

Yes caution is warrented and this does happen but the ones i have heard of resulted from catastrophic damage to the battery or citcuits resulting in dead shorts on said battery or faulty charging circuits that Over Charged the battery resulting in gassing and overheating (these are the ones i see on youtube).
Over discharge of a battery (eg below specified voltage) usualy results in the life of the battery being shortened.

Comes down to picking the right battery for the right job and the reason lithiums are being used more and more is because they give the most output for the weight.

BTW. I work in communications and for a long time now portable radios that get all sorts of abuse have been fitted with lithium batteries of all sorts and the biggest problems have been the life of the cells and customers fitting the new batteries into the old nicad chargers and the "YES they do blow up" usualy with a fizzle and smoke.

Oh yes and i do class lithiums in model planes that crash as prone to catastrophic failure!
Have you any idea of the gee forces exerted on the batteries and their internals when that plane hits the ground?

Daz - agree with you to a point.

However, there are already restrictions on some airlines about lithium batteriers, and fully discharged ones are no less safe. Over discharge can result in the battery shorting it's self out because the copper can dissolve in the electrlite, stick to the annode when you next charge and cause shorts (the same things happens with overcharging, but it is lithium not copper that causes the issue. All you need to do with a lithium battery is remove the safety circtuit governing under/ over charge and short it out... I suspect there will be a swift move away from lithium in the coming years when the latest battery technologies mature.

Sadly, I have seen a couple of lithium disasters under "normal use", it does happen.

Most batteries do, thankfully, come with all sorts of safety features that revent catastrophic falure in normal use, which is why dropping your phone is not an issue. However, I'd still advise caution against lithium in kids toys because kids do like to find inventive ways of self destruction, and have a nasty habit of "wet testing" everything.

But, we're moving away from the OP's questions.

Daz1712:
Yet still every product comming on the market that runs on batteries is fitted with a lithium battery.

Yes, and just about every one of those products will have a proper charge and discharge control circuit, designed for long life and safety of the battery.

As an additional protection a lot, but not all, batteries have built in 'protection circuits' to limit damage if the products charge and discharge control circuit develops a fault, or the battery is otherwise abused.

Your phone, for instance, does not rely on the batteries seperate 'protection circuit' that most hobbyists seem to think is all that is needed for charge and discharge control. If it did, you would have no warning that the battery is getting low or the phone gracefully shutting down.

Lots of food for thought here. I'm going to have to pick up some of those super caps and experiment with them.

My wife mentioned last night, when i brought up regular batteries, asking me if those were the kind that leaked inside my sons nerf gun... so each power options has its advantages and disadvantages.

I think the option I am going to go with is a "shockproof" lithium power bank, there are quite a few highly rated ones on Amazon starting about $40 or so, and then I will handle recharges at the Daddy Supply and Maintenance Depot. Wiring in the talkabout will solve the issue of power draw as well. I think that is the best solution.

Thank you all for the discussion :slight_smile: If I run into any more questions i'll resurrect this thread again... as I will post a pic of the completed project when it is done, probably the end of summer.

Again thank you :slight_smile: