LED lightning for my studio desk

[vendettav]

Hey guys, I've been wanting to make this since I came to this room (it's not my studio but it's like the room I stay in while i'm abroad here in Bulgaria). The idea is simple: 10 LEDs to light up my desk, with a USB stick to go to a powersupply that uses a USB jack and outputs 5v and 500mAh.

now the questions are:

I dont know the USB jack (male) pinout so a diagram would be helpful
I dont know what LEDs to get and what resistors to use
I plan on wiring a on off on switch with different resistors so that I have two settings, bright (for writing papers and all) and dim (for mixing and mastering  )

help is highly appreciated

V

[Seljer]

The pinout: http://pinouts.ru/Slots/USB_pinout.shtml basically, the outer two pins are ground and +5V, use a multimeter to be sure which is positive (if you rip open a cable the red and black wires typically)

For wiring it all together: http://led.linear1.org/led.wiz
With a white LED you're probably going to around 3.3V voltage drop, and you're typically going to want 20mA of current for a good brightness level.
The voltage drop on a LED is roughly constant, you have to add a resistor in series to limit the current to not burn the thing up.
5V-3.3V = 1.7V
1.7V/20mA = 85ohms = you could probably use 100ohm resistors without any drastic difference in brightness

Each LED will need its own resistor, and each LED+resistor series combination would be then hooked up in parallel.


To get different brighness settings I reccomend of trying a simple PWM dimmer circuit, here is an example with a common 555 timer chip http://www.reuk.co.uk/LED-Dimmer-Circuit.htm , much more flexible

[vendettav]

I think i'd rather go with a simple pot dimmer than a circuit, I dont have all the equipment here so dealing with ICs and all would be kind of a hassle

as for the wiring, can you tell me why i'd need resistor for each led? I was thinking of hooking it up in series then having a resistor and a pot

[deadastronaut]

hi v. joule thief?....run lots of leds off a 1.5v AA battery....even a dead one for days... .....

http://www.youtube.com/watch?v=kPCBwD7l9u0&list=UUGP0eO8ADt0H9FY5UcHTf8A&index=3&feature=plcp

http://www.aronnelson.com/gallery/main.php/v/chickpea/joule+thief1.jpg.html

http://www.aronnelson.com/gallery/main.php/v/chickpea/P200112_13_20.jpg.html

if you took the usb voltage and droppd it to 1.5v  , you'd be in permanent retina destroying bliss..... you gotta love leds eh!.....

ive only ever done this with 1.5v...  not 5v...


btw the toroid (ferrite ring) can be as small as you like...break open a cfl bulb, those ferrite rings are ideal...   don't bust the glass though..nasty stuff in there...

[vendettav]

Hey rob, that thing looks quite interesting and neat! but I think i'd leve it for some experimenting later on when I'm back to Armenia...

I want something really reliable and running off my power supply or my comp.

I fugred I need 2 of the chains connected to the same thing. each chain is going to have the following: 10 LEDs (200mA all together) a pot, switch and

[Seljer]

You cant hook it up in series because LEDs have a certain forward voltage, to get current to flow through the LED you need to apply a voltage higher than that (here is a diagram of the voltage-current characterstics for different colours http://www.telatomic.com/images/electricity/pepc5.jpg ).

If you take that a white LED drops about 3volts of voltage, and if you were to wire two of them in series you'd need a supply voltage of at least 6 volts to get them to light up. As your supply is only 5 volts you can't really do much here (other than getting into various voltage booster circuits like the joule theif)

You also can't wire all the LEDs in parallel and use just one resistor, each component has slightly different tolerances and the LEDs with slightly less voltage drop will shine brighter than the ones with higher voltages and such.


Using only a potentiometer to do the dimming is a viable option for your setup, however each LED would still need its own resistor to limit the maximum current. Try a couple of hundred ohms in series with the whole series/parallel array.
The issue with potentiometers is that wanted to dim a higher load, the amount of waste heat in the potentiometer would quickly go over its rated power.

[deadastronaut]

@v:  fair enough...

heres another example for those interested in such  'joule thief' experiments..

http://www.youtube.com/watch?v=rv6H7jxCvNQ

[vendettav]

You cant hook it up in series because LEDs have a certain forward voltage, to get current to flow through the LED you need to apply a voltage higher than that (here is a diagram of the voltage-current characterstics for different colours http://www.telatomic.com/images/electricity/pepc5.jpg ).

If you take that a white LED drops about 3volts of voltage, and if you were to wire two of them in series you'd need a supply voltage of at least 6 volts to get them to light up. As your supply is only 5 volts you can't really do much here (other than getting into various voltage booster circuits like the joule theif)

You also can't wire all the LEDs in parallel and use just one resistor, each component has slightly different tolerances and the LEDs with slightly less voltage drop will shine brighter than the ones with higher voltages and such.


Using only a potentiometer to do the dimming is a viable option for your setup, however each LED would still need its own resistor to limit the maximum current. Try a couple of hundred ohms in series with the whole series/parallel array.
The issue with potentiometers is that wanted to dim a higher load, the amount of waste heat in the potentiometer would quickly go over its rated power.

ok so the diagram that you gave me in the first post... 10 leds with 100ohm for each. then may be a 1k pot wired as a variable resistance, before the whole circuit. as in 5v>pot>leds

would that work?

@Rob, that thing is nice