# Very simple question from an electronics noobie

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#### FiletMignon67

##### New Member
Hello guys and girls, I have a simple question. But first, a little about my project:

I am doing a little project with etched plexiglass. When plexiglass is etched and illuminated by LEDs, an interesting picture is formed. Example:

I am taking on a much simpler project :

I am about to take on the electronics portion of the project now and I need your help. Here is a diagram of my plan:

Basically I am going to use an unused modem adapter to power my LEDs, which will be run in parallel.

Question 1: Is it okay to solder directly to a wall adapter like this?
Question 2: Will this system work?
Question 3: Any tips or precautions?

-Electronics N00b.

##### Banned
The resistors are going to be dissipating a lot of power, for 12 volts it would be smarter to run three LED's in series with one current limit resistor, a LOT less power will be dissipated in the resistor. Do you know the forward voltage drop of your LED's? I think green LED's a just a smidgen lower than blue LED's. You say 3.5 volts but that sounds a little high. If 3.5 is good for you then 3.5 * 3 = 10.5 volts, so you need to drop the other 2 volts, someone was just asking about a very similar project the other day and we came up with 3 LED's in series with a 100ohm resistor. Keep in mind the voltage from a 12 volt DC wallwart is going to be higher than 12 volts if it's not a regulated supply (you'll need to test it) so you may want to use 150ohm resistors to be safe. Unloaded a 12 volt wallwart can easily be over 14 volts, but it depends on what that voltage is going to be when it's loaded with the current all of your LED's use.

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##### Banned
By the way, that dragon and sword looks both white and blue, how'd they do that?

#### Torben

##### Well-Known Member
Hi there,

Short answers: Yes, Sort of, and Yes.

1) No problem soldering to the leads of the wall wart.

2) I say 'sort of' because in the diagram you indicate that you want to use the resistors to drop the 12VDC to ~3.5VDC--but LEDs are current controlled, not voltage controlled. You want to use the resistors to limit the current they will draw, since (unlike an incandescent bulb) an LED will draw as much current as it can, and if that means it draws enough to burn out or even pop, it will happily do it.

So instead of thinking in terms of the voltage you provide to the LEDs, think in terms of the current. The formula to determine the resistors you will need is R = (Vs - Vled) / I, where R is the value of the resistor in Ohms, Vs is the voltage of the power supply, Vled is the forward voltage of the LED, and I is the current you want to supply.

Do you know the forward voltage and current rating of the LEDs you intend to use? How many LEDs do you want to use? I'm guessing more than four, but that's just a guess.

There is a useful tool for calculating the resistors you need for strings of LEDs in different configurations here: LED series parallel array wizard

There is some good information for basic LED work here: Light Emitting Diodes (LEDs)

Good luck!

Torben

#### Triode

##### Active Member
I would guess with the two colors thing that its the simplest answer, they painted color into the etching. But there are more complex ways, like dual panes with an etching for each color layer, relying on the fiber optic containment of the glass to keep them separate.

#### dougy83

##### Well-Known Member
By the way, that dragon and sword looks both white and blue, how'd they do that?
Not that I know, but I'm guessing they could:
a) use multiple sheets of plexiglass (laminate) with different light sources
b) fill each engraving with some fluorescent dye (blue & white), and use either white or UV light to make them glow
c) something else...

The first suggestion sounds dumb, and there probably would be light bleeding between the sheets. So I reckon b) or c)...

#### FiletMignon67

##### New Member
Thank you all for your replies (I read all of them)--this electronics noob truly appreciates it!

I have the following information regarding my LEDs:

Green LEDs:

Forward Voltage Typ (if=20mA): 3.2
Reverse Current Max: (if=20mA): 10

I do not have any direct info on max foward current, but that "if=20mA" in parentheses makes me think the maximum forward current is 20mA?

Using the handy resistor calculator you guys recommended, I got the following:

My input:

Source: 12 Volts
Forward Voltage: 3.2
Forward Current: 20
Number of LEDs: 6

Calculator output:

2 resistors, 120 ohms each

After the calculator spit this out I tried it again with 18 for forward current instead of 20 and the calculator gave me 2 resistors of 150 ohms each. I will most likely go with resistors at this range to be safe, no?

I have some more noob questions:

Question 1: The resistor calculator website showed me a schematic where one resistor is placed on the (+) side and another on the (-) side--does it matter which side gets the resistors?

Question 2: I read on the website you guys linked to me that I should not run LEDs in parallel without having a resistor to each individual LED. So, my question is: Assuming I go with two 150 ohm resistors, can I just simply add the total, 300 ohms, and divide it by the # of LEDs and attach a resistor to each individual LED? For ex/ 300 ohms total divided by 6 LEDs = 50 ohm resistor to each LED leg (related to Question 1: which leg, positive or negative leg, gets the resistor)? Or does this not work?

Thank you all for helping a stranger.

#### Nigel Goodwin

##### Super Moderator
Electrically it's all very simple, the construction obviously is very time consuming, but it's the quality of the etching that makes the project - the dragon and sword is stunning.

Some people obviously have FAR too much time on their hands

#### JimB

##### Super Moderator
I have the following information regarding my LEDs:
Green LEDs:
Forward Voltage Typ (if=20mA): 3.2
Reverse Current Max: (if=20mA): 10
I do not have any direct info on max foward current, but that "if=20mA" in parentheses makes me think the maximum forward current is 20mA?
Difficult to predict the maximim current from such information, but I would guess 30mA.
These specifications are ambiguous, the forward current (If) has no bearing on reverse current. Also, no units are specified.

Question 1: The resistor calculator website showed me a schematic where one resistor is placed on the (+) side and another on the (-) side--does it matter which side gets the resistors?
No it does not matter where the resistor goes in a series chain.

Question 2: I read on the website you guys linked to me that I should not run LEDs in parallel without having a resistor to each individual LED. So, my question is: Assuming I go with two 150 ohm resistors, can I just simply add the total, 300 ohms, and divide it by the # of LEDs and attach a resistor to each individual LED? For ex/ 300 ohms total divided by 6 LEDs = 50 ohm resistor to each LED leg (related to Question 1: which leg, positive or negative leg, gets the resistor)? Or does this not work?
No it does not work like that.
If you want to run 1 LED from a 12 volt supply, you need a 470 ohm resistor in series with the LED.
And again it does not matter whether the resistor connects to the +ve or -ve leg of the LED.

Thank you all for helping a stranger.
We are each a little bit strange in our own ways around here.

JimB

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#### hansdals

##### New Member
Just saw the worklog of this project. Wow, this is really fascinating. I was first thinking of UV light, but it isn't. It's interesting to see how well the plexiglass does not leak light (and of course the great art).

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#### duffy

##### Well-Known Member
I know you got your answer but I just want to jump in here and tell you how awesome that edgelit dragon-knife in the door looks.

##### Banned
3.2 volts at 20ma's is what I read there. That's pretty easy.
Use three LED's in series, with one 120 ohm resistor, just add more strings in parallel for more light, you'll obviously need to add them in threes. You still have to determine the actual voltage of your 12 volt supply though, as I said it could be as high as 14+ volts if it's not a regulated supply.

#### Mr RB

##### Well-Known Member
Electrically it's all very simple, the construction obviously is very time consuming, but it's the quality of the etching that makes the project - the dragon and sword is stunning.

Some people obviously have FAR too much time on their hands
The log showed he engraved it by hand, very skillful since acrylic scratches so easy too.

I wanted to say that every "trophy engraver" these days has a CNC engraver and they do a lot of plexiglass/acrylic signs and glass trophies. You can design anything in Corel draw and they will engrave it for you, usually up to 20x15 inches or even much bigger.

They can also do front panels for electronic projects, the black/white ones you see in industry all the time which are permanent marked (ie won't ever rub off). It's really worth a trip to your local engraver to see the stuff he can do, and usually for just a few .