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Need help w/ electronics (LED wiring)

thatguywhojuggles

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Feb 9, 2002
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I'm building a prop and part of it has a cobra head. I want to give it glowing red eyes using LEDs or something similar.

I know very little about electronics, and need some help. I went to Radio Shack, and saw a couple of nice looking red lights that were labeled 120VAC. I assume this means 120 volts Alternating Current. But I have no idea what kind of batter to hook up. And something tells me 120/9 nine volt batteries is not right.

Anyway, ideally I would like to light two medium bright red LEDs with a 9 volt battery. Anyone know what I need to do this? What type of LEDs? Resistors? Etc.

I know how to wire, I just don't know anything about volts and amps and all that jazz.

Any help would be appreciated.

Edited to add: I know this is a strange question to ask in a skeptic's forum, but this place is crawling with smart people... so... :)
 
Take them back and get some that run off of DC. That's what batteries produce. You need some complicated stuff to get AC out of batteries, and it is every ineffecient.

Look for stuff designed for automobiles. 12v DC is 9 AAA, AA, C, or D batteries.

And I'm sure someone smarter than me will have a better idea.

Jeff

PS - It's possible that the 120v AC thing you have really runs off of DC. If there's a large "brick" that plugs into the wall, it will list the real power requirments.
 
I get LEDs from Radio Shack that work on small 1.5 v batteries. Stay away from the 120 VAC lights.
 
you need your two LEDs, the battery, and a current-limiting resistor. Depending on the amount of current the LEDs need to achieve the desired brightness, this may or may not be feasable. But probably is. There are a ton of pages about this. Just for instance:

http://www.kpsec.freeuk.com/components/led.htm
 
thatguywhojuggles said:
I'm building a prop and part of it has a cobra head. I want to give it glowing red eyes using LEDs or something similar.

I know very little about electronics, and need some help. I went to Radio Shack, and saw a couple of nice looking red lights that were labeled 120VAC. I assume this means 120 volts Alternating Current. But I have no idea what kind of batter to hook up. And something tells me 120/9 nine volt batteries is not right.

Anyway, ideally I would like to light two medium bright red LEDs with a 9 volt battery. Anyone know what I need to do this? What type of LEDs? Resistors? Etc.

I know how to wire, I just don't know anything about volts and amps and all that jazz.

Any help would be appreciated.

Edited to add: I know this is a strange question to ask in a skeptic's forum, but this place is crawling with smart people... so... :)
Click to expand...


120VAC sounds like overkill for a project of this scale. If you want to run it off an ordinary 9V, it's dead simple: get a pair of 5V LEDs, grab one lead from each, twist together, and connect the two extra leads to the terminals on the battery.

They're in series, so if one croaks, the other will stop working, so test them independently with a AA battery to see which one died.


Now, if you want to make it *really* cool, sacrifice a pair of cheap laserpointers and rig them up!

Updated to include diagram:

Code: 
----------LED--------LED-------
|                                          |
|                                          |
|                                          |
|                                          |
------------BATTERY------------
 
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thatguywhojuggles,

Probably a good bit of this will make little sense without further research on your part.... But, it should be possible to follow the directions and get the desired results even if it doesn't make a lot of sense to you.

LEDs would definitely be the way to go for a couple of glowing red snake eyes (IMHO). A 9v battery would also make a fine power source for them as well.

To wire them up....

You need to look up 2 items about the LEDs (it should be right on the package).
1) The voltage drop across the diodes (assume 1.6v if it isn't on the package)
2) The operating current. It'll be the figure followed by 'mA'.

For this example, I'll say that the box indicates 1.6v drop and 10-30mA (or it could be just one figure, but in reality you can go above or below that figure, higher current will be brighter, and lower will be dimmer)

The wiring will be completely in series for this. It will go battery, switch (assuming you use one), resistor (we'll figure the value in a moment), led, led, and back to the battery. Actually, the ordering of the components in the circuit will make no difference at all. You will have to make sure the LEDs are connected with the correct polarity. One lead will probably be longer than the other, and the box will tell which lead is which. If the box just uses the diode symbol... the arrow points to the negative battery terminal.

Now, to pick out a resistor for the project.

9v - 1.6v - 1.6v = 5.8v

That is just the battery voltage minus the voltage drops of the LEDs. This indicates how much voltage that we need the resistor to drop.

We'll figure the resistor value for both ends of the current range (10 and 30 mA).

5.8v/.01A = 580Ohms
5.8v/.03A = 193Ohms

We'll go with nice round numbers and call the range of resistor values could be from 200 - 600Ohms.

I would buy resistors at both ends of the scale (they are just pennies a piece) and couple in the middle. I would test them all and then use the one that gives you the desired brightness.

Hope that helps.
 
For 5 volts, the canonical resistor to use is 49 ohms. Use about 100 (120 is a good value) for 9 volts.

You can get some really big, really bright red LEDs.
 
epepke said:
For 5 volts, the canonical resistor to use is 49 ohms. Use about 100 (120 is a good value) for 9 volts.

You can get some really big, really bright red LEDs.
Click to expand...

100Ohm would give a current in the neighborhood of 60mA. Unless you are using jumbo LEDs, they'll have a lifespan measured in fractions of a second at that high a current.
 
I really think 440V 3-phase is the way to go.

Or you could use a real cobra.

Probably safer, so long as Peta don't find out.
 
scotth said:
100Ohm would give a current in the neighborhood of 60mA. Unless you are using jumbo LEDs, they'll have a lifespan measured in fractions of a second at that high a current.
Click to expand...

Gee; I wonder how I managed to build thing with LEDs and 120 ohm resistors and 9 volt batteries for 30 years without having any of them burn out, then.
 
epepke said:
Gee; I wonder how I managed to build thing with LEDs and 120 ohm resistors and 9 volt batteries for 30 years without having any of them burn out, then.
Click to expand...

The LEDs that are commonly available around here have a 20mA Max rating. They don't last long at 30mA and they go poof immediately with just a little more. You must be getting something different, if yours aren't blowing with a 120 ohm resistor.

I can get LEDs that don't mind 40-60mA driven through them. They are just bigger than I've ever needed in any of my projects.
 
And I wonder how I was able to wire LEDs with no resistor directly to a 1.5 v battery and have them never blow out?
 
Jeff Corey said:
And I wonder how I was able to wire LEDs with no resistor directly to a 1.5 v battery and have them never blow out?
Click to expand...

Since most LEDs drop 1.5 to 1.6 volts across them at max current, the current will self regulate without a resistor. That works just fine as well.


If he isn't going with a 9v because its fits the space available better, it would be the easiest solution.

Edited to add: A single AA battery has enough energy in it to light a pair of 20 mA max LEDs in parallel for 50-100 hours (according to duracell's capacity rating, but that might be a little optimistic)
 
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scotth, guess you're data is somewhat out of date. Check out this specs sheet for a commonly available 5mm LED. Max current 50 mA, life time at this current is still in the 1000s of hrs. though. Would run at 60 mA just fine.

BTW, just google "Luxeon" for what 'commonly' available LEDs are capable of nowadays...(you can get flashlights with those LEDs at Target, for the commonly available part). Forward currents of 1000 mA (= 1 frickin' Amp!) are normal nowadays, at least for me.

For the OP, here's a good resource, basically LEDs for Dummies.
 
Modern LEDs are often very bright, and will last a long time with a relatively large value series resistor. The downside is that these LEDs are often very directional viewing angle, using the molded lens. In a somewhat related project (a radio controlled rat), I filed the lenses flat, so that there was a matte non directional lens with a wide viewable angle. Worked great.
 
shecky said:
Modern LEDs are often very bright, and will last a long time with a relatively large value series resistor. The downside is that these LEDs are often very directional viewing angle, using the molded lens. In a somewhat related project (a radio controlled rat), I filed the lenses flat, so that there was a matte non directional lens with a wide viewable angle. Worked great.
Click to expand...

Yah. You can also just treat them with sandpaper.
 
I file a crude crossed "waffle" pattern into to the top of the LED with a small triangular file.
Instant Disco LED!
 
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