cell phone + charger project

[pearlteeth]

I'm thinking of starting a project with a number of cell phones, all hooked up to their adapters, constantly on the 'low battery' threshold.

I am wondering what the easiest way to control this would be. I have experience with picaxe and am thinking that I can use that and a TIP to turn the chargers on and off. Is there a way to monitor the power left in the battery so that it's not based on time assumptions but on actual battery power?

Also, how many adapters could I plug into a normal 15A circuit before causing problems? could I run more than one phone off of each charger?

The phones would be of various makes and ages, and as such each one would have a different 'low battery' threshold.

Thanks for your help.

 

[Resqueline]

Umm.. I'm not going to ask, & I don't even know how to get you started on the first part..

You'll need a video camera and some nifty software to recognize the lo-bat symbols in each display, unless you want to run thin wires out from all batteries and into a voltage-measurement system. All phones have an under-voltage shutoff.

Second part.. You're in the US, right? 15A x 115V = 1725W.
Each charger draws 2-3W, which means you can charge 862-575 phones at the same time.

No, each charger can only deal with one phone (or vice versa) since the charging process in the phone pulls down the charger voltage. Unless you time-share one charger among several phones of course.

 

[Mitchekj]

(Curiousity is getting the better of me...)

What? Why?

 

[(*steve*)]

What? Why?

Yeah, me too.

If the phones are all fairly modern (i.e. they all have LiPo or similar batteries) then the low battery indication is likely to be a function of voltage alone.

It may be easiest to monitor this voltage externally (after determining what the threshold is)

It may be even easier, assuming many phones have the same or very similar threshold to wire in an external source to their battery contacts and run them from an external regulated voltage source (without chargers)

 

[pearlteeth]

It may be even easier, assuming many phones have the same or very similar threshold to wire in an external source to their battery contacts and run them from an external regulated voltage source (without chargers)

Yeah, somebody I asked today said something similar. This seems like the way to go. I wasn't aware that the low battery detector was based on battery output voltage. I'm going to plug in my old cell and see what the battery voltage is when it hits that threshold. It is a Li-ion battery. Any foreseeable problems with that? Why are there three contacts for the battery?

A video camera? recognition software? Thanks for the wattage calculations though

I think I'm doing a sort of chandelier thing as an art project. I'm mainly looking for the beep-boop sound en masse.

 

[Resqueline]

Well, you only stated you wanted to keep a large number of them at the low-bat treshold, without any further explanation than that it should be automated, and a camera recognition system looking at their displays is the "only" thing that can do just what you said. Knowing what you said now it's obviously easiest to use one adjustable supply for each group of similar phones (w/o batteries).
The turn-off point is based on voltage alone, but the level of the bars and the point where they start to beeep has more complex calculations behind it.
The batteries contain electronics (temp sensor etc.) that the phone needs to communicate with, hence 3 contacts.

 

[(*steve*)]

I think I'm doing a sort of chandelier thing as an art project. I'm mainly looking for the beep-boop sound en masse.

Well, ya see, this is why it's really handy to know why you want to do something, rather than how you envisage it might be done...

Another issue you might have to deal with is that the phones will probably only do the beep-boop infrequently when they detect the low charge. You may have to do something else to trick the phones into doing it more often.

One thing that comes to mind is ramping the voltage up and down. It is likely that the phones will detect slightly different voltages to do the alarm, and this may "encourage" them to issue it more often.

I'm sure you will need to experiment.

 

[pearlteeth]

Another issue you might have to deal with is that the phones will probably only do the beep-boop infrequently when they detect the low charge.

I'm not really concerned about that, as the number of phones used (hopefully 40 or so) will be chirping out of sync and will therefore not stay silent for long. One thing I did notice though is that my old cell gives a low battery signal twice then shuts off (not for lack of power, but to protect the batt from over draining). I don't think most other phones work like this as my roommates phone can be heard chirping away all night. If I used this phone, I would have to lower the voltage only when I wanted the sound to go off, then raise it again so the phone wouldn't shut off.

Lastly, I found that the battery was outputting about 3.7V at the first signal, and was still powering the phone ok at 3.35V (if not for the auto shutoff). what could I use to feed the phone a voltage in that window? I did a bit of looking around and it seems like an LM317 is what I need. Any thoughts?

 

[(*steve*)]

LM317 sounds fine for that. You can adjust the voltage with that.

If some phones need slightly different voltages you can use multiple 317s each set to a slightly different voltage.

You can purchase already amde up regulator boards based on LM317s. check out these: http://cgi.ebay.com/LM317-DC-In-5-2...emQQptZLH_DefaultDomain_0?hash=item51906661d8

Watch out -- they are not 1A devices, and they don't have heatsinks, but I imagine the quiescent current of mobile phones is actually quite low.