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Combining sources

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Burningmace

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I'm modifying an old PS/2 keyboard to have a red glow from LEDs under the keys, but this is requiring the use of around 100 LEDs. The current consumption of each is around 20mA, which means the total consumption is a whopping 2A. As the PS/2 standard is limited to 275mA, I'd like to combine batteries with the PS/2 supply from the PC. I'm wondering how I might go about doing this.

My initial thought was to run some of the LEDs on batteries and some from the cable, but this would result in some LEDs going dimmer than others over time. I also considered switching between the two sources every 200ms and using a few capacitors as a reservour, but I get the feeling that this is a pretty complicated way of doing things, and it could go wrong and overload my motherboard. Not good.

One other problem to take into account is that I'm modifying an existing keyboard, which means I have limited space available. I can slot 6 or 7 AAA batteries into gaps inside the keyboard, and there's a couple of places big enough to fit a 14-pin IC, but that's about it.

Keyboards such as the Logitech G15 and many other keyboards with all sorts of back lighting features (I know of one on ThinkGeek with over 400 LEDs) have no problem powering their masses of LEDs on USB's 500mA limit, so how do they do it?
 

MikeMl

Well-Known Member
Most Helpful Member
How about running groups of LEDs in series. That way, each group requires only 20mA. What is the starting voltage?
 

Mr RB

Well-Known Member
And use high intensity LEDs. 2000 mCd leds are brighter at 1mA than old style LEDs are at 20mA.

You can get bags of 2000 mCd dirt cheap on Ebay, of course you can get much brighter but you pay more (and how bright do you need for keys?)
 

Burningmace

New Member
I'm running on the PS/2 standard, so it's a 5V supply limited to 275mA. I'm going to consider that the keyboard's existing logic takes around 25mA (it's only a small controller chip).

I calculate that if each LED pulls 20mA of current, and I give each one the bare minimum of 1.65V, I could wire them up in 12 sets of 3 and get 36 LEDs. That's a 3rd of what I need. :(

I could possibly cut my LED count down that far, but it wouldn't look so great. It's an opaque black plastic keyboard, so there's not going to be much diffusion going on to carry the light. If I could get the count up to 50 somehow, I'd be happy.

Using batteries won't be that useful either. If I make up the LED count with 16 more split into 4 sets of 4 on 6x 1.5V AAA batteries, I'll be pulling 80mA. That means I'll get at most 100 hours of operation before I have to replace all 6 batteries. That's going to sting my wallet a little. I could always buy a DC jack socket and put it in the back of the keyboard and power it with one of those 9V laptop-style power cables. SAeems a little overkill to be powering my keyboard separately though.

Any more ideas?


Update:
Found these on eBay:
http://cgi.ebay.co.uk/50-x-3mm-Red-...in_3?hash=item1c0b3bcceb&_trksid=p3286.c0.m14
At 9p per LED it's a little expensive, but I could light up my keyboard like the Eiffel Tower at only 5mA per LED.
 
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BrownOut

Banned
I could always buy a DC jack socket and put it in the back of the keyboard and power it with one of those 9V laptop-style power cables. SAeems a little overkill to be powering my keyboard separately though.
That's the way I would do it. I wouldn't think of it as overkill if I were trying to create a bitchin' effect that required that much energy to run. If you use a switched power strip, just plug it into the same, and it won't be drawing power when not in use.
 
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Burningmace

New Member
I've purchased the LEDs, not too bothered about the cost. I've decided to cut my LED count anyway, it's down to just under 50. I can get the same effect with 50 that I can with 100 just by simply spacing the LEDs out appropriately. I'm also going to spray the panel under the keys with gloss black spray paint to get better reflection.

I'm going to run 16 sets of 3 series LEDs in parallel, so that each LED has a supply of 1.67V. The current usage should be 1A at normal load, but I'm going to use a pot to bring the current down to 12.5mA per set of LEDs (thus 200mA total).
 

tcmtech

Banned
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Pretty sure white reflects far better than black. But then I have not been in a high school science or shop class in 18 years.
Apparently they changed that part of how light works now. :rolleyes:
 

Burningmace

New Member
Smart ass :p

I know white would work better, but they keyboard is matte black and having a white panel would look silly. I want to spray it gloss black, so then I get a nice shiny surface instead of a dull one.
 

tcmtech

Banned
Most Helpful Member
I figured so much.
I just on occasion work around high school kids that have no clue about reality.;)
Man is it a hoot to find out what kinds of garbage they have in high school text books now! :D:(
 

blueroomelectronics

Well-Known Member
Lighting up a keyboard from behind the keys will only show the keycap outlines (not the characters)

A simple LED light shining on the keyboard should be enough for low light thru darkness. Of course backlit keyboards are pretty cheap these days.

Googled backlit keyboards and got this.
 

Burningmace

New Member
I know I could probably buy one pretty cheap, but the point of the project is that it's a fun mod.

Just as a thought, is there an easy way to prevent too high a current being pulled and my motherboard being killed? Some sort of fuse or something? I don't know if you can get them for 200mA, and even if you can wouldn't the delay be pretty long?

Update:
I found some 200mA quick blow fuses at Bits Box that might do the job. Do you think it's worth using them as a safety precaution?
 
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ericgibbs

Well-Known Member
Most Helpful Member
I know I could probably buy one pretty cheap, but the point of the project is that it's a fun mod.

Just as a thought, is there an easy way to prevent too high a current being pulled and my motherboard being killed? Some sort of fuse or something? I don't know if you can get them for 200mA, and even if you can wouldn't the delay be pretty long?

Update:
I found some 200mA quick blow fuses at Bits Box that might do the job. Do you think it's worth using them as a safety precaution?
hi,
Inside your PC case you should find a 'spare' CD/HD 4/5 pin flying lead molex connector.

This has +12V and +5V able to supply a fair amount of current, check the rating of the PC's psu.

Run a 2 core cable from this molex upto a 2.1mm jack plug and use a mating socket on your keyboard.
 
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Burningmace

New Member
Lol, that keyboard did indeed make me feel a little ill. My girlfriend smokes, but if she ever did that to my keyboard... *shudder*

eric - I'd rather power it from the PS/2 itself. I know I could run a molex's 5V line down through a jack, but it's extra cables and my PC has enough of them sticking out of it to hang myself a thousand times over.

Related smoking story:
I worked in a PC repair shop for ages, and we had a guy bring in a machine from his office that wasn't working. He said it'd boot up breifly, then just shut down with no error in mid-POST. It looked pretty old, and the white plastic had faded to that dirty yellow. I booted it up and it did exactly as he said, but it seemed to die quicker and quicker each time I tried. Then I spotted the CPU temp in the BIOS - almost 95°C. I opened it up and witnessed the horror of its contents. The entire inside of the machine was covered in a layer of thick black gunk. The fans couldn't even spin. What was worse was the smell. It was like the contents of a million ashtrays glued together with melting plastic. These guys must have been using the machine for 10 years whilst chain smoking 24/7. It's scary to think that if the PC was that bad, what did their lungs look like?
My boss was just as gobsmacked as I was, but I figured I could try and clean it up a little. He phoned the guy up and told him it'd cost extra because of the state of it. I took the machine outside (it was stinking up the place) and put on some rubber gloves, then proceeded to extract the hard drives and take off the CPU fan. There was no way that I could salvage the fan or CPU heatsink, they were both completely drowned in the tar. The motherboard was coated, so was the chipset heatsink and fan. The PSU had gunk inside it, I'm surprised it hadn't blown.
In the end the whole machine got scrapped. I had to take off the controller board on the hard drives, clean the gunk from underneath, and put them back together for use in the new machine. We actually put the old case and the components in layers of plastic bin bags just to get rid of the smell. The guy at the local dump didn't look too impressed.
 
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kpatz

New Member
Don't forget, you don't need to run the 7000 mcd LEDs at 20 mA, at 1 mA they'll be plenty bright enough, and 50 of them would draw... drum roll... 50 mA! :D

Running them in series groups of 3, and limiting yourself to 100 mA total, you could get 6.25 mA per group, which should make the LEDs more than bright enough.
 
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Burningmace

New Member
Yeah, that was what I had in mind kpatz.

Just as another current-cutting idea for future reference, I thought about using a PIC and a bunch of transistors to rapidly switch between having sets of LEDs on. If I wanted to run a few hundred LEDs on 50mA or so, I could light a set of 30 or so at a time for less than 1ms. This would mean that the actual current requirement of the circuit would be cut to a fraction of what it used to be. The current requirement of the PIC and the transistors should be much lower than the amount I save using this method, right?


Edit: Figured I'd link you to the project page - Keyboard mod project
 
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audioguru

Well-Known Member
Most Helpful Member
LEDs are not light bulbs and cannot be connected directly to a voltage source. They need current-limiting that is usually done with a series resistor. But the series resistor drops about 1V so you need 6V or more to power three "1.67V" LEDs in series.

Your cheap E-Bay LEDs probably do not have detailed spec's that list the range of their voltage. It might be from 1.5V to 2.2V each. If they are 1.5V then the 5V supply will burn them out. If they are 2.2V then three need 6.6V plus 1V for the current-limiting resistor and they will not even light with only 5V.

Your mother-board will burn out much quicker than a fast fuse.
 
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