Making up an LED light board

backless

Hi folks,

I have a basic enough understanding of circuitry (learned from technology class in school all those years ago) and understand the rudiments of circuit building and that but would appreciate if I could get a little bit of guidance on a project I'm going to undertake.

I want to make an LED illumination board, something like

what this guy has done. It's for the purpose of lighting up a box frame from the inside, with an opaque glass front. It'd be almost like a light box (for tracing in the artistic and design world) but smaller (6"x8") from that can mount on the wall and stay illuminated under its own self contained power source.
What I've thought about for this is two (simply to reduce the frequency of changes) 9V batteries. Would this work?

Applications aside, I need a bit of guidance in the resistors needed and I would like to know if there's a way to wire in a variable resistor with a view to being able to adjust the brightness of the light source in the back.

Any info is welcome and appreciated, and thanks!

H

Bruthal

While people think of LEDs as low wattage, since a single one only takes about 1/50th of an amp, 99 of them are going to take 2 amps from the 9v battery, if every LED is supplied through an individual resistor, which wont last long with a 9v pp3 style battery.

As for resistors to use, to get the resistor required, you can simply divide the supply voltage by the current an LED takes, so for a typical 20ma led, its 9/0.02 = 450 ohm which is roughly the resistance required. For exact resistance for a 20ma LED it would be (supply V - LED forward voltage)/0.02. The forward voltage of the LED would be in the specs, but the first method is good enough.

Them resistance values are for a resistor for every LED, like is done in the video you linked, but that is not needed, and the LEDs can be connected in groups of 5 in series for LEDs with a forward voltage of 2v (fv), for example to 12v (supply voltage sv), with each group having a 100 ohm resistor (12sv - (5 x 2fv))/0.02, and then make 20 such groups in parallel. Because the LEDs in series dont have a resistor for every one, each series group is now taking 20ma for 5 LED`s, and for 20 of these series groups in parallel, the current taken is now just 0.4 amps, 5 times less than when a resistor for every LED is used, because most of the supply voltage is uses, instead of some wattage being wasted in resistors for every LED. All 20 groups could also be connected in parallel, and use just one 5 ohm resistor of 1 watt.

For dimming, varying the resistance does not work properly for LEDs. A PWM setup is needed, which simply switches the LEDs on and off rapidly, for example, on for 10 milliseconds, and off for 10 milliseconds would give half brightness, on for 15 and off for 5 would be 75% brightness and so on.

A pwm circuit is easy enough to make, using either a 555 timer chip, or what I usually use myself for them, 12f675 microcontroller chip, and write a simple program on it to allow dimmer control.

Once the max off time is below about 15ms, no flicker will be seen, athough stroboscopic effect would be, if fast rotation items were being lit by it.

Stoner

also for matching your LEDs with your supply and current limiting resistors there are a number some threads here about this already look them up. there are array helper sites that will take the specs of your diodes and supply source and advise on the current limiting units.

backless

Any chance of a link to some helper sites? I did some searches but obviously not in the right places

Bruthal

backless wrote: »

Any chance of a link to some helper sites? I did some searches but obviously not in the right places

Just get the LEDs you want to use, and the specs for them. And come back here and it can easily be calculated.

But I just had a quick look, and here is an array calculator.

You can put in forward voltage of 2 volts, and current of 20 ma, supply of 12v, and number of LEDs in your array as 100, and it comes back with 2 circuit diagrams, one with 6 LEDs in each series leg and a 1 ohm resistor ( needed because the supply might be a little above 12v), and also another circuit diagram with 5 LEDs in series, as described in previous post.

If use 5 diodes in each series leg, it is showing the 100 ohm resistor as in previous post.

Notice again with the 6 diodes in series and the 1 ohm resistor, the overall current draw is lower, at 340 ma, where as with the 5 LEDs in series and 100 ohm resistor, it is 400 ma, which was also mentioned in previous post.

The forward voltage and forward current of the LEDs you get, are needed to be known, and should be in a spec table about them. I picked 2v forward and 20ma forward current, as a typical value. They might be a little different, particularly the forward voltage.

Stoner

I used more or less that helper a few years ago when i was building IR LED boards.

http://led.linear1.org/led.wiz (as above)

All the info is available on the components , just look them up in onlione first something like www.cpc.co.uk get the details and pop them in, I made some large and small ones
As for the driver for this, to be honest I'd buy one, unless you want to tell everyone that you made it, the components will cost you IMHO more money than a made up unit will. Work out the total load , say for example 20 number 0.25 Watt diodes say 5 Watts look around for a driver that can handle your needs.

trappedtony

Is there a resource out there who can make me a circuit?

I want a circuit that takes a signal input from a switch opening and closing and put it in a counter.

The counter should have 2 programmable alarms AL1 & AL2.

The alarms should be configurable, changeable and reset-able.

There are three LED's to be part of this circuit. GREEN, ORANGE & RED.

When the counter is counting up to AL1 the Green LED is on.
Once AL1 condition is met, LED goes from Green to Orange.
Once AL2 condition is met, LED goes from Orange to RED.

Simples!!