# LED Nightlight

Discussion in 'The Projects Forum' started by twinpapa, Feb 4, 2013.

1. ### twinpapa Thread Starter New Member

Feb 4, 2013
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I'd like to make an LED nightlight for my twins that runs along the top of their wall and shines light up onto their ceiling (sortof like ) I've read Bill's tutorial on LEDs and think that I have a basic understanding of the project but am concerned about safety and also am not sure how to figure out what the heat output will be for the LED. I don't want a fire risk!

My preliminary plan is to wire 24 blue LED (datasheet) in 6 parallel strands of one current limiting resistor (100 Ohm) + LED in series. Each LED has a Vf of 3.4v @ 20 mA. I used the LED.Lineara1.Org to design the LED array.

I plan on buying a 12VDC voltage regulator to plug into the wall like this one rated at 1000mA.

I thought that I would mount each LED in wooden molding (maybe like this) into which I had drilled an appropriately sized hole. I would mount the LED so that 80% of the bulb was exposed to the air. The wire, resistor, and soldered connections would be stapled to the back of the molding so that it was sandwiched between the molding and the wall (or ceiling).

I could use some help with a few questions. First, would something like this work? Second, would it be safe i.e. Am an idiot for thinking of putting something homemade in my childrens' room? (I am anticipating a future conversation with my wife). Third, how do I calculate the appropriate gauge wire to handle the current (the wizard calculates it at 160 mA for the total array). I guess a related question is whether 160mA at 12V is dangerous if one of my children decided to pull out an LED or something similar.

We've built simple robots together using our arduino and breadboard but nothing that would permanently plug into the wall. I really like the idea of building this with them to model a DIY spirit but don't want to burn the house down either.

Any guidance would be appreciated.

Dan

Last edited: Feb 4, 2013
2. ### tracecom AAC Fanatic!

Apr 16, 2010
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I don't think there is any danger with LED night lights running on a 12V wall wart. I am not sure exactly how you intend to wire them, but the most efficient way would be 3 LED's in series with one resistor. I haven't yet calculated the value of the resistor, but that won't be a problem. The thing you want to remember is to drop as much of the voltage as possible in the LED's and as little as possible in the resistors.

Also, 24 LEDs will produce quite a lot of light. You might want to consider a variable power supply so that you can dim the LEDs.

3. ### tracecom AAC Fanatic!

Apr 16, 2010
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91 ohms would be a good resistor choice for each 3 LED string. You would be getting 85% efficiency. Eight strings would then be wired in parallel, and would draw 8 x .02 A = 160 mA, which is not much current, so 26 AWG wire would be more than plenty big. A 300 mA 12 V wall wart would be sufficient with no overload problems.

You could also make a little adjustable regulator using an LM317.

A 24 LED 12 V schematic is attached.

• ###### 24 LEDs 12 V.png
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Last edited: Feb 4, 2013
4. ### twinpapa Thread Starter New Member

Feb 4, 2013
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Thank you Tracecom! I appreciate the guidance. I am a bit confused now and admit that I am a total Noob. But how would a adjustable regulator dim the LED? Would the regulator be adjustable in terms of V or I? I thought that the LED was either on or off and when on always took its Vf of 3.4 and If of 20 mA? Would the current limiting resistors also need to be variable? Again, I apologize for the noob question and appreciate your help.

5. ### tracecom AAC Fanatic!

Apr 16, 2010
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I don't mean to confuse you, and you may not need an adjustable regulator. It would just allow you to dim the LEDs from full brightness to off by varying the voltage from 0 to 12 with a potentiometer. It would be a small circuit that would go between the wall wart and the LED strings and would have a knob on it to adjust the brightness. If you plan to use the LEDs as indirect lighting, that lessens the concern.

You probably will need a regulated power supply (wall wart) to ensure that the voltage does not exceed the 12 V that the LED strings are designed for. Some non-regulated wall warts can exceed their rated voltage quite a bit with a light load such as LEDs.

You can look for a regulated wall wart, or you can build a simple outboard regulator, but if you decided to build an outboard regulator, I would suggest that you do make it adjustable.

Have you bought the LEDs? Can you post a link to them?

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6. ### twinpapa Thread Starter New Member

Feb 4, 2013
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Thank you! I really appreciate the help. Its definitely not too much information. I am eager to learn how this all works. The data sheet is located here. Would the regulator control current or voltage? I really like the idea of a dimmer. I hadn't realized that such was possible. But given that its a night light a dimmer would be really cool.

7. ### tracecom AAC Fanatic!

Apr 16, 2010
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It would contol voltage, varying the voltage from 0 to 12V. You would simply take the output from the wall wart and run it through the regulator circuit and then into the LEDs. I have attached a photo of a regulator that I built for some LED lights that I use for table top photography. I didn't bother to put the circuit board in an enclosure, as there is no shock danger, but for appearance sake, it would be better to put it in a little plastic box. The reason that there are two sets of wires going to the lights is that I have two lights. In your case, you would need only one pair of wires to the entire LED string.

The LEDs in the datasheet have a 10 degree viewing angle, which means that if you look at them directly toward the end, they are very bright, but off to the side they are not very bright. Once again, if you are using them for indirect lighting, a narrow viewing angle isn't bad, but if you intend for them to be seen directly, you may not be satisfied.

Are you planning to put them high on the walls pointed toward the ceiling, or what?

ETA: I have to turn in now. Good night.

• ###### LM317T Wall Wart Regulator 2 lo res.jpg
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8. ### GRNDPNDR Member

Mar 1, 2012
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7
LEDs are current controlled devices, you can do damage by varying voltages, a small change in voltage results in a LARGE change in current, pump too much current through the LED and it blows up.

Also to diffuse the light or mount them, try something like this
http://www.superbrightleds.com/more...es/led-lens-mount-low-profile-clear/746/2020/

The LED clips right in, then you can lock the lens into a small hole. this way it'll avoid having bright spots on the ceiling where the LEDs are pointed at.

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9. ### tracecom AAC Fanatic!

Apr 16, 2010
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If the current limiting resistors are sized for the maximum voltage, there is absolutely no risk in using a voltage regulator to dim LEDs. Else, how would you keep LEDs that are battery powered from "blowing up" as the battery voltage falls?

10. ### GRNDPNDR Member

Mar 1, 2012
449
7
as the voltage falls so does current....in significant increments.

besides, you could just use an appropriately sized pot to do the adjustment. it's only 12V.

11. ### Audioguru New Member

Dec 20, 2007
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The expensive DC power supply you found DOES NOT say that its voltages are regulated. It is expensive because it has a switch and many output voltages. Pick a less expensive power supply that is just 12VDC and regulation is not needed.