Re: [tlug] [OT] Any electricians / electrical engineers around?

[Dave M G <dave@example.com>]

Josh, Curt,

> > There is no real need to take it off list, I think. Just stick to this
> > thread, and the non-interested parties can ignore it easily, while the
> > people who are (even casually, like me) interested can follow along.

Okay.

So far, this interchange took place off-list.

I said:

Andrew, Godwin,

Thanks guys for providing a little guidance.

I'm just a little fuzzy on where to start with this project, so forgive 
me if my email rambles a little. I think there's essentially three 
questions here, but I at the level of confusion where I don't exactly 
know how to phrase them.

That said...

The ring light link that Andrew provided is very much like what I'm 
after. If I could find a device that I could re-purpose, that would be 
great.

Basically, I want to place about 48 LED lights in a ring that will go 
around a camera lens. The lights will all be green.

I'm pretty sure I can figure out how to rig up some kind of mounting 
harness and whatnot. I just want to make sure that whatever I put 
together electronically won't be a fire hazard waiting to happen.

I thought I could be clever and use Christmas lights to accomplish what 
I wanted. I bought a set of green LED Christmas lights, which came with 
a switching mechanism. However, all the excess cable has proven to be a 
hassle for both organizing the lights into a circle, and placing them 
around the camera.

Even if I can still use the green LEDs that came with the string of 
Christmas lights, I'll still need to hack off the cables, mount the 
lights on some kind of board, and then wire them back together in some way.

Okay, so, first question... let's say I mount the lights onto some 
board, can I just daisy chain them together? Have power start on light 
#1, then light #1 links to light #2, and so on until I come around to 
light #48, which then terminates at the power source. Or is there more 
to it than that?

The second, and big question, is what kind of power switch can I use for 
this? I want to use AC power, so I need something to regulate the power. 
 A simple on off switch would be good, but a dimmer would be ideal. A 
guy I spoke to at Tokyu hands said that you can't dim LED lights. But 
the Christmas lights I have do fade in and out on one of the preset 
blinking patterns. What specifications do I need to look for in a switch?

Third question... what should I mount the lights on? This may be a 
little silly as questions go, but I really don't know. In Akihabara the 
other day, I looked at circuit boards with hundreds of little holes so 
that one could solder in circuitry. While that seems like the way to go, 
 it seemed like I could complicate my life trying to cut it into a 
circular shape in such a way as to leave the grid pattern of the holes 
still accessible for laying down my led lights.

They actually had circular boards for putting LED lights on, but they 
were all way too small. I'm pretty sure I should go with 48 lights, 
because the "pro" version of what I'm building uses that:
http://www.reflecmedia.com/

Okay, so, in summary, I just want to plunk the lights down on some 
surface so I can arrange them in a circle, link them up so they have 
power, and then hook them up to a switch so they can be plugged in and 
turned on and off.

So heading down to Akihabara... do I need a specific rating of LED? Can 
I just cannibalize Christmas lights? What specifications of switch 
should I get? And what would you recommend for placing these lights on?

Thanks for any pointers.

And Andrew replied:

Hmm, where to start...?

Q1) Yes and no.  LEDs run on direct current (dc) and consume about 20
milliamps (mA) at about 2 volts (V).  If you power them from alternating
current then they will flicker at the ac frequency because they only 
light up when the current is in the right direction and go off when the 
current is reversed (which is what ac does).

Let's assume that you do have 48 LEDs consuming 2V at 20mA.  Let's also
assume you have a 12V power supply (for safety, and because they are 
often easily available).  What you could do is make 8 groups of 6 LEDs. 
 Each LED drops 2V, so 6 LEDs in series drop 12V and draw 20mA.  If you 
make 8 groups of these and put them in parallel then the combined 
current will be 160mA, which is a small amount of current and you should 
be able to find a supply capable of that.

This link might help:

http://www.theledlight.com/ledcircuits.html

But there are a bazillion other pages on the web that might also help.
Look carefully at your Christmas lights and see if they are wired in one
long loop (two wires only) or in little groups (4 or more wires).  Don't
forget, that with ac you can connect two LEDs back-to-back so that each
one lights up on alternate cycles of the ac waveform.  That way you need
half as many wires.

If you had 48 LEDs in series then each one would require a 2V drop, 
which is 96V.  You *could* hook them all up to the mains (which is 100V 
ac) and probably get away with it but actually that's potentially 
dangerous so I strongly recommend you don't do it.

Q2) Since we can probably power all these lights from a low current, low
voltage supply then you can use just about any switch.  Since you will 
be using a transformer you could just turn that on and off at the mains, 
or you could put a simple on/off pushbutton or toggle switch on the 
output of the transformer.  If you have a bench power supply (expensive) 
you could vary the output of the transformer and dim the lights that 
way.  Dimming circuits are not too difficult to make, but if it's your 
first project we should keep it simple.

Q3) Mounting- well, you've got 48 LEDs which is 96 soldered joints to
make.  Rather you than me.  One of the easiest things to use is
stripboard, which is probably what you saw in Akihabara, but you could 
use thick card or thin plastic and an awl to punch little holes in it. 
Then push the LED legs through and solder them together in the right order.

The camping light I suggested could be dismantled and the white LEDs
replaced by green ones (maybe even cannibalised from the Christmas
lights).  White LEDs have a higher forward voltage than green ones, so 
you could use only 2 AA cells rather than the three that are present in 
the camping light.  Alternatively, some green cellophane might do the trick.

You could also cut and shorten *all* the wires in the Christmas light 
set, but that could take a while.

Because you want 48 LEDs you are up for a fiddly task, but overall it's
not too difficult.  I suggest that you sketch out a few ideas first, 
then build something.

Sorry I haven't actually given you complete and detailed instructions, 
but hopefully there is enough information to get you unstuck and get you
thinking.  I hope Godwin has some time to make some suggestions too.  If
he agrees with me then you'll be off to a flying start.  If not then
you'll have to decide which way to go.  When I worked in the industry 
one of our engineering maxims was "Whatever works, dude", so do bear 
that in mind!

HTH,

Andrew