Hi

I am new to the forum so let me start off by saying that I have very little electronics knowledge so apologies if my following question appears to be very basic!

I am wondering if it is possible to use a modern day smartphone charger (Common EPS - 5v, 1amp) charger in a way that could be used to power an LED/array of LED's when required but to also charge the phone when they are not in use? I understand that all chargers under the new EPS standard use a female usb port at which point I would like place an intermediate device that would sit between the charger and the charger cable so the phone is still connected and charging yet is able to power an LED when commanded to do so (see PDF for basic diagram). The switching function needs to come from the smartphone (a custom app) and using the USB cable. The LED brightness does not need to be much more than that of an LED found on a current generation smarthphone and from what I have seen online, many people have used phone chargers to power LED's but I wish to maintain an element of control and phone charging.

I am not asking how this can be done but whether it is possible at all or if there any obvious flaws?

Thanks in advance.

 

No problem. Only limit is the power rating of the charger. The LED array should not draw more than, say, 2/3 of the rated power of the charger. You could go higher, but that would put your charger at risk of overheating and premature death.

I think you'll also need to figure out what happens if the LEDs are in place when the phone is plugged in. Do you anticipate this scenario?

 

The switching function needs to come from the smartphone (a custom app) and using the USB cable.

This is the tricky part. What smartphone do you have in mind? Smartphone programming interfaces typically put tight controls over what apps can do with the USB connection. It's not wide open like on PCs. It seems to me like most smartphone apps that control external devices use wifi instead. Would that be an option?

 

Perhaps it would be possible to use the Bluetooth capability of the phone to control the LED.

 

No problem. Only limit is the power rating of the charger. The LED array should not draw more than, say, 2/3 of the rated power of the charger. You could go higher, but that would put your charger at risk of overheating and premature death.

I think you'll also need to figure out what happens if the LEDs are in place when the phone is plugged in. Do you anticipate this scenario?

Well the LED would only be on for about 20 minutes at a time, possibly a bit longer but certainly no way near as long as it would take to charge a phone. So does a charger not run at full capacity for the time it is charging?

The idea is that the phone (any smartphone that uses the new charger standard) will be plugged in charging and then the when required the power will switch over to the LED whilst it is connected without any interference from the user. So the power will be diverted away from the phone automatically.

This is the tricky part. What smartphone do you have in mind? Smartphone programming interfaces typically put tight controls over what apps can do with the USB connection. It's not wide open like on PCs. It seems to me like most smartphone apps that control external devices use wifi instead. Would that be an option?

Perhaps it would be possible to use the Bluetooth capability of the phone to control the LED.

If that is the case then wi-fi or indeed bluetooth would absolutely be an option, I was just thinking of it in terms of keeping cost down but it is possible.

Any idea what sort of luminance I could achieve from an LED setup like this? How would this compare to the luminance of an LED on a smartphone (for example)? The colour needs to be somehwere between yellow/white/blue.

 

So does a charger not run at full capacity for the time it is charging?

Perhaps, but whoever designed it had the advantage of ...designing it, and knew all the risks, specifications and limitations. When you come along later without intimate knowledge of the design, it's wise to be conservative.

The idea is that the phone (any smartphone that uses the new charger standard) will be plugged in charging and then the when required the power will switch over to the LED whilst it is connected without any interference from the user. So the power will be diverted away from the phone automatically.

I missed this initially, and now I better understand. But what do you mean by "when required"? Do you want the lights to come on when the current drawn by the phone drops below some level (as it will when full charge is reached)? It would be easy to to use a comparator to watch the voltage drop across low-ohms (like 0.1Ω) shunt resistor, compare it to a preset reference that you could set, and turn on the LEDs when there's little current flowing. With another comparator (could be on the same IC) you could detect a connection, so that the LEDs would not come on unless something is connected AND drawing low current.

 

Perhaps, but whoever designed it had the advantage of ...designing it, and knew all the risks, specifications and limitations. When you come along later without intimate knowledge of the design, it's wise to be conservative.

OK well that makes sense, I haven't ever known of a phone charger to go bust but if I go along with the intention that it doesn't exceed the demands the phone would place on it then it hopefully won't be a problem.

missed this initially, and now I better understand. But what do you mean by "when required"? Do you want the lights to come on when the current drawn by the phone drops below some level (as it will when full charge is reached)? It would be easy to to use a comparator to watch the voltage drop across low-ohms (like 0.1Ω) shunt resistor, compare it to a preset reference that you could set, and turn on the LEDs when there's little current flowing. With another comparator (could be on the same IC) you could detect a connection, so that the LEDs would not come on unless something is connected AND drawing low current.

I should have been more clear, the lights will come on at a pre-determined time (controlled by the phone app), most likely to be when the phone has long finished drawing a full charge although in reality it will be regardless of the state of charge of the phone, the LED's need to take priority.

I found an example (or at least a written example) of somebody using a 6v 500ma phone charger to power up to 30 White LED's but I leave their integrity up to your scrutiny . I would need much less LED's that amount but then I don't know what sort of luminance they are achieving or at if the charger is at 100% capacity.

http://homemadecircuitsandschematic...ng-cell-phone-charger-for-making-led.html?m=1

Please let me know if this is worthy reference material or not!

 

... the lights will come on at a pre-determined time (controlled by the phone app)...the LED's need to take priority.

Ah, I see. That means you'll need some way to receive and interpret an instruction received from the phone. Above my pay grade.

That LED article is valid, but he had the benefit of a charger that put out enough voltage for 2 LEDs in series plus the current-limiting resistor. Most chargers I see these days are USB 5V chargers, and this will not light more than 1 LED, again with its current-limiting resistor. Most of the LEDs you might want to use need at least about 3V each.

You can still put many LEDs in parallel, up to the safe limit for current supplied by the charger.

 

Most phone chargers are "dumb" and are just a regulated DC power supply. the smart bit that turns the charging on/off is usually inside the phone.

So for many chargers you can just treat the charger like a DC plugpack and plug whatever you like into it (as long as you don't overload it of course).

 

Ah, I see. That means you'll need some way to receive and interpret an instruction received from the phone. Above my pay grade.

That LED article is valid, but he had the benefit of a charger that put out enough voltage for 2 LEDs in series plus the current-limiting resistor. Most chargers I see these days are USB 5V chargers, and this will not light more than 1 LED, again with its current-limiting resistor. Most of the LEDs you might want to use need at least about 3V each.

You can still put many LEDs in parallel, up to the safe limit for current supplied by the charger.

That's right all of the modern chargers are 5v because fo the Common EPS standard agreed by all major phone manufacturers so that is my absolute limit. I would need more than one LED so it I guess its worth just experimenting with some in series to see if i get the desired output.

Most phone chargers are "dumb" and are just a regulated DC power supply. the smart bit that turns the charging on/off is usually inside the phone.

So for many chargers you can just treat the charger like a DC plugpack and plug whatever you like into it (as long as you don't overload it of course).

That makes it easier I guess, I think I would need to stay well within the limits of the charger.


Thanks all for your help! I need to start looking at if it I can achieve the desired light output with just 5v before I do anything else.

 

I would need more than one LED so it I guess its worth just experimenting with some in series to see if i get the desired output.

Oh you can still have plenty of LEDs, they just need to be in parallel. A 5V supply might light 2 reds in series, but probably not any others.