Burningmace
New Member
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?
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?