Hi Dear members of "All About Circuits",

This is my first thread and I am certain that it is a simple problem to most of you, but truth being said I have searched everywhere for an answer and there seems to be no discussion anywhere with this particular problem.

I am making a Backlight for my TV using a 12v 5050 Single Colour LED Strip (White) with 60/m, since the 5v provided by the TV usb port are not even close to provide enough power for the approximate 2.5m of LED Strip I will be using I am getting an external 12v Power Source (around 5A just to be safe). This is fairly easy to do, my problem starts here:

- I want to make (if possible) the LEDs to only light up when the TV is ON, and to light down when the TV is OFF. Basically I want to make a circuit that is Closed when tv is on and that is Open when tv is off (or similar). My brain went immediately to the most obvious way (USB port) but now I am stuck and cannot think of a solution.

Question 1: Is there any way to connect the USB from the TV to the LED circuit in such way that the circuit is only closed/powered when both power supplies are ON (12v 5A DC Adapter and USB port) ? Maybe a switch or something that I cannot think of?
Note: If a 5v to 12v Step-Up is required, it is not a problem.

Question 2: If the above question is not easily achievable, is there an easier solution (or only solution) not including the USB port of the TV ?

My Background: I finished High-School about 10 years ago where I was extremely good at physics and circuits and was 1 year attending a Computers Engineering Degree (didn't finish it, am now graduated in Banking) but since then, aside from some minor projects, I never studied anything related to circuits and electricity. I can do simple soldering work with cables and basic circuits but apart from that you can treat me like a school kid that I won't take it the wrong way, started researching this "field" that I always loved a couple of days ago and am planning to make this a serious hobby so needed to start somewhere..."LEDs!!!" I thought to myself, "what can be simpler than that?" lol

Thanks in advance for your feedback, any help is appreciated, this is part of a bigger project that I am planning to make in the future, as soon as I start with it I will be glad to keep this Thread updated with photos and further developments (will require your guidance and expert opinions for sure).

 

Welcome to the forum!
Messing with mains voltage is not something a novice should be doing.
Perhaps you could use an off-the-shelf system and power both the TV and a suitable power adapter for the LEDs from a wireless-controlled mains outlet?
(Personally, I would find a TV backlight distracting/annoying .)

 

Welcome to the forum!
Messing with mains voltage is not something a novice should be doing.
Perhaps you could use an off-the-shelf system and power both the TV and a suitable power adapter for the LEDs from a wireless-controlled mains outlet?
(Personally, I would find a TV backlight distracting/annoying .)

Thank you for the swift reply Alec,

Most people I know share the same opinion as you in regards to not enjoying the TV backlight, personally I don't mind them but I also don't mind not having them However, in a few months I want to dig myself into an Ambilight TV project (ok, I always liked ambilight on Philips TV so this project is a total must for me), but before moving to higher grounds (where a mistake might cost me much more than a 10$ led strip) I want to start with the basics of LEDs, TV USB's, etc.

I didn't mention that the 12v DC adapter I have is not homemade, it is a branded one of a different electronics product that already "died" so I guess that so long as the + / - are well stripped and connected, even a novice like myself is (sort of) safe from any massive danger ? Never crossed my head about messing around directly with AC, probably would end up stiff as a rock laying down on the floor...

Also, I already have an IR LED Remote on my "Shopping Basket" if by any chance I cannot find an easy solution to my problem (and by easy I mean, automatic ON/OFF that doesn't require myself to be in "high-shocking" situations) I will just buy it, plug it to the LED strip and use the remote to shut it down...But that's not really my goal.

I will keep doing some research and if I come across any idea that seems suitable I will share them here to get your thoughts, any other comments/solutions are welcome.

Thanks

 

Question 1: Is there any way to connect the USB from the TV to the LED circuit in such way that the circuit is only closed/powered when both power supplies are ON (12v 5A DC Adapter and USB port) ? Maybe a switch or something that I cannot think of?

Yes, a simple 5V relay could be used to switch on power to your 12V adapter. This has an advantage of complete isolation between the two power supplies.

You could also use the 5V signal to switch a MOSFET to complete the current loop to the LEDs, so that the 12V supply could be left on but will not power the LEDs unless the USB power is present. To retain isolation, I'd use an optoisolator with this solution so that, as with the relay, there is no connection between the power supplies.

 

Yes, a simple 5V relay could be used to switch on power to your 12V adapter. This has an advantage of complete isolation between the two power supplies.

You could also use the 5V signal to switch a MOSFET to complete the current loop to the LEDs, so that the 12V supply could be left on but will not power the LEDs unless the USB power is present. To retain isolation, I'd use an optoisolator with this solution so that, as with the relay, there is no connection between the power supplies.

Hi Wayneh,

Thanks for both advises, never worked with relays before but by giving a quick look at some "relay basics" it seems exactly what I am looking for, since it will be the first time working with a relay I don't want to be writing without knowing what I am talking about so I'll do some homework first and then edit this reply.

By the way, my guess is "yes" but can you give me your opinion if this cheap one 5V relay would work ? Still need to figure out what the middle pin is for but I'm sure I'll find it fast later today (I'm at work so can't waste too much time on this at the moment).

Many Thanks, will post something later tonight.

 

FYI..
Here is the difference in relay contacts (form A vs form C)
http://www.ni.com/white-paper/4782/en/

That relay should work just fine as its rated to switch 10A @ up to 28VDC. (assuming when the TV is off the 5V on the USB is also off)
Personally for ease of wiring I would find a socketed one or one with quick connect terminals so you don't need a PCB or delicate soldering skills.

 

There is a few ways to do this , the easiest way depends how much you know about the design of your sets power supply, plus if your leds etc are connected directly to your set they have to be assrmbled in such a way to ensure that they dont present a shock hazard if handled incorrectly. ....give me the make and model of your set and ill look the service manual up to see what can be done.

 

...can you give me your opinion if this cheap one 5V relay would work ?

Looks fine to me. That relay will draw 0.38W/5V=0.076A = 76mA. That should be fine for even a 100mA USB port.

 

FYI..
Here is the difference in relay contacts (form A vs form C)
http://www.ni.com/white-paper/4782/en/

That relay should work just fine as its rated to switch 10A @ up to 28VDC. (assuming when the TV is off the 5V on the USB is also off)
Personally for ease of wiring I would find a socketed one or one with quick connect terminals so you don't need a PCB or delicate soldering skills.

Hi McGyver,

Thanks for the link, will be studying that for a while.

So, although I like soldering and so far I have been doing a decent job, I'll take your advise save myself some time and buy one with the terminals already connected, THIS one should do the trick since the relay is the same I was going to buy, still need to study the connections since it's the first time I'm working with relays but that shouldn't be an issue.

Once again, thanks for the comment and hint, helped a lot.

PS: Tested the USB port from the TV yesterday, completely off when the Tv is also off.

 

There is a few ways to do this , the easiest way depends how much you know about the design of your sets power supply, plus if your leds etc are connected directly to your set they have to be assrmbled in such a way to ensure that they dont present a shock hazard if handled incorrectly. ....give me the make and model of your set and ill look the service manual up to see what can be done.

Hi Sheldons,

The Power supply I'll be using is in the garage at my other apartment, will only go there this weekend so I can only update you this Saturday, but I will do it for sure, thanks for making yourself available to help.
The Led strip is not fancy and I'm not planning on getting anywhere near it with my hands while it's on, you can check it HERE (exact same model as the one I bought). But since I'm going to put some sort of "white glass" protecting it to make the light more uniform (sorry, I'm Portuguese and don't know the correct term for this) I don't think I should worry too much...at least while I don't have kids...should I ?

By the way, this "white glass" will be with enough distance from the leds and not covering it totally (leaving a gap between Tv back and the "white glass"), this way the air will flow and it won't overheat. Just in case this was crossing your mind when I pulled the subject on.

Many Thanks, will post you the Power Supply Specs/Model on Saturday.

 

.. some sort of "white glass" protecting it to make the light more uniform (sorry, I'm Portuguese and don't know the correct term for this)

We would call that "translucent" or "frosted" glass.

 

We would call that "translucent" or "frosted" glass.

What you really want to use is acrylic (Plexiglas, Lucite, etc.). It is a lot lighter than glass and easier to cut and shape. There is an amazing (bewildering?) variety of colors and textures available.

 

We would call that "translucent" or "frosted" glass.

Many Thanks for teaching me the correct term, I appreciate it.

Will be drawing a sketch of the circuit today and post it here just so that the experts (i.e. You guys) can give their opinion, hopefully the circuit will be correct, then I'll move on to some calcs to make sure I won't "fry" anything and with the help of Sheldons I'll figure out if the Power Supply is safe to use with this circuit.

Have a great day all of you, will post later today.

 

What you really want to use is acrylic (Plexiglas, Lucite, etc.). It is a lot lighter than glass and easier to cut and shape. There is an amazing (bewildering?) variety of colors and textures available.

Hi,

You're right about that, it's just that I might get the frosted glass for free so if that's the case I'll probably stick with it, eitherway I'll be checking the prices for the acrylic, worked with it a few years ago and it was affordable so I guess nowadays it might be relatively cheap, don't really have the know-how or material to work with it but maybe I could purchase it already with the measures and shape I wanted.

Thanks, will let you know what I end up using.

 

Looks fine to me. That relay will draw 0.38W/5V=0.076A = 76mA. That should be fine for even a 100mA USB port.

Hi everyone,

I just draw a simple sketch of what I'm trying to do, can you have a look and see if the connections are correctly done ? Also, if I plan to add a dimmer later, would you recommend placing it between the 12v power supply and the relay or after the relay ?

Sheldons, my friend tricked me, I went into my garage today and the Power supply is 500mA and not 5A, so I'll have to look for another one, probably buy a new one since I don't think I have any good one to spare, at least not one that I could find (only 15 V and 18.5 V but I don't want to do a step-down, need to start simple with this one).

So, I guess if the circuit is OK I'll just wait for the relay to arrive and assemble everything.

Thanks for all the help.

 

Driving a normal relay coil directly from a USB port risks the back-emf spike at switch-off damaging the port. Adding a reverse- biased diode across the coil would go some way to reducing the risk, but how the port controller would respond to the diode/coil presence is unknown. IMO it would be better to use an SSR; or else an opto-isolator and transistor stage to drive a normal relay.

 

Driving a normal relay coil directly from a USB port risks the back-emf spike at switch-off damaging the port. Adding a reverse- biased diode across the coil would go some way to reducing the risk, but how the port controller would respond to the diode/coil presence is unknown. IMO it would be better to use an SSR; or else an opto-isolator and transistor stage to drive a normal relay.

Hi,

Thanks for the reply, was completely not aware of this...So, you think somethink like this SSR would reduce the risks? Is the wiring any similar to the one I made previously or this would involve a different approach? I really can't figure it out right now.

Thanks once again, sorry for the short reply but I'm without much time at the moment.

 

Hello,

A heatsink as you are showing will not do anything against the spikes.
A diode will supress the sike generated by the unpowered relays.
Here is how to add it to your circuit:

Bertus

 

That link (if you scroll down far enough) shows several SSRs. Which one do you have in mind?

 

That link (if you scroll down far enough) shows several SSRs. Which one do you have in mind?

Sorry everyone, wrong SSR link on the first post, this was the one I wanted to place, "SSR". Didn't have the time to study what exactly is the difference in terms of wiring but this one seemed ok for the kind of circuit I'm making.

Bertus, I've worked with diodes before when making a solar charger but only to prevent other panels to "charge" each-other and harm the solar panels, this seems quite simple for me to do (and cheap), I guess this would be a good way of doing it? Again, going to do some research first to totally understand the diode role on this kind of circuit but probably this is the schematics I'm going to follow.

Thanks once again.