LED dome

[geodesic]

I'm looking to make something conceptually similar to this:

I was wondering if anyone has thoughts regarding:

• How to implement PWM for ~200 channels.
• How to buy hundreds of Luxeon / Cree emitters for relatively cheaply..

Thanks for reading!

 

[anuragwap]

If you want bare emitters in 100+quantities, Mouser and Future are the best places for Cree and Luxeon respectively. For color leds, XB-D is cheaper than XP-E and Luxeon-Z is cheaper than Rebel. Also don't forget the latest color XM-L offerings.

No idea about how to control so many PWM channels but I'd like to know for sure. I searched about the Times Square LED Ball Project by Lumileds in 2007-08, several big lighting companies jointly designed it. It used the popular DMX512 protocol.

This is a product from one of the companies involved in the 'ball' project for controlling 512 channels. This is a very cool project done by another company that was involved. (I don't have much knowledge on DMX. Its too complicated for my non-engineering background)

Another lumileds case study revealed that there are some dedicated lighting specific MCUs in the market (e.g. Cypress EzColor). So I guess there are much more than PIC/Arduino/TLC5940 that we don't see much in the hobbyist community.

 

[jtr1962]

As far as implementing 200 channels, the best way to connect the LEDs together is to charlieplex them. If n is the number of pins you're using, then with Charlieplexing you can drive n*(n-1) LEDs. To drive 200 LEDs you would only need 15 pins. Of course, programming the microcontroller gets more complex, but you can simplify your wiring.

If you want to get large quantities of LEDs for not much money, you can't go wrong with either the XT-E or the XB-D. 100s of lower bin XT-Es can be had for not much over $100. 100s of XB-Ds are under the $100 mark. There's also the Lumileds LXM3-PW51-90 neutral white which LEDSupply has on closeout for $0.75.

 

[RoGuE_StreaK]

There's also the Lumileds LXM3-PW51-90 neutral white which LEDSupply has on closeout for $0.75.

Or on similar specs, LedRise has got Nichia 757's for Euro0.40 (~US$0.52)

[EDIT] Actually not overly similar, the Lumileds is putting out 150 lumens at 700mA, the Nichia puts that out at 200mA for warm white. CRI is the same (CRI 85) if warm white, but the cool white Nichia is a fair bit less (CRI 70). Nichia forward voltage is also higher, in the ~6V range.

[EDIT2] I thought at first it was all white LEDs, but am I seeing green and blue in there? Any details on exactly what it is you're trying to make, and/or what the "conceptually similar" device is at least?

 

[anuragwap]

As far as implementing 200 channels, the best way to connect the LEDs together is to charlieplex them. If n is the number of pins you're using, then with Charlieplexing you can drive n*(n-1) LEDs. To drive 200 LEDs you would only need 15 pins.

Do you know how to charliplex high power LEDs? Of course the MCU pins can't provide beyond a few mA's of current. I haven't seen how to use a MOSFET in charlieplexing. Also any kind of multiplexing will reduce brightness.

I googled OP's image, and found it in this page. Probably he's interested in white LEDs only.

 

[SemiMan]

First I have to ask why .... it is not the most attractive thing ... maybe if the LEDs pointed outwards?

I would not get too hung up on the price of the LEDs, I expect everything else will cost a lot more ... mechanical, heatsinks, drive electronics, etc.

Issue with "Charlieplexing" .. or multiplex drive, typically used for LED displays and the like, is that it is scanned. I.e. duty cycle is not 100%. That and you have to send the same "signal"/voltage to every LED. That almost forces you into say a 5V supply and linear drive.

You could do a modification on the above and use Charlieplexing only for control and run a 5V line to each LED.

Then again if I was building it I would likely whip up a custom board with the LED on it, simple serial communications with a dirt cheap micro, and power control. Not sure I would run 5V to every board .... lots of current but really simple, or use a higher voltage constant current drive for a grouping of LEDs.

Semiman

 

[geodesic]

The lights are inward-facing and white in order to evenly illuminate the center from all directions.

It looks like I can limit it to 350ma per light source. I haven't been able to find charlieplexing examples at that current, but I'll keep looking.

The store links are very helpful; thank you! This forum is fantastic

 

[geodesic]

Then again if I was building it I would likely whip up a custom board with the LED on it, simple serial communications with a dirt cheap micro, and power control. Not sure I would run 5V to every board .... lots of current but really simple, or use a higher voltage constant current drive for a grouping of LEDs.
Semiman

I had figured that this approach would be more expensive, but maybe microcontrollers are cheaper now...

 

[jtr1962]

Do you know how to charliplex high power LEDs? Of course the MCU pins can't provide beyond a few mA's of current. I haven't seen how to use a MOSFET in charlieplexing. Also any kind of multiplexing will reduce brightness.

You just need to have the MCU pins drive the gates of MOSFETs (choose MOSFETs which turn on fully at 5 volts). You would need both P and N channel MOSFETs for this project. To keep things simple you would also need a hefty ~5V supply. This avoids getting into voltage translation issues when driving the P-channel MOSFETs. Basically, if the source of a P-channel MOSFET is connected to 5V, driving the MCU pin connected the gate low will turn it on, driving it high will turn it off. For N-channel MOSFETs, the source is connected to ground. Driving the MCU pin connected to gate high turns it on, driving it low turns it off. The wiring would be exactly the same as if you were using the MCU pins directly except you connect the LEDs to the drains of the N and P channel MOSFETs. You may also need to include resistors to limit the LED current.

Of course, you could also wire all 200 LEDs independently, perhaps having each one with a controller and unique address to turn it on or off. To interface with a microcontroller you would need 8 lines for the address, and maybe one or two lines for control (i.e. turn the LED on/off, or PWM at a given duty cycle).

 

[anuragwap]

Awesome! Thanks a lot jtr1962 for that explanation of using MOSFETs. I wanted to know this for a long time (ever since I made this toy! Credit: Jose Pino)

 

[RoGuE_StreaK]

Do you actually need PWM? What do you intend doing with the PWM?
Do you actually need 200 separate channels? You say the idea is to illuminate evenly, surely that would mean you want all LEDs running the same?

Perhaps you could investigate the likes of the TLC5940 from Texas Instruments; 16x PWMable constant current sinks, each capable of doing 120mA, so by hooking up three pins per LED you get 5x 360mA PWMable constant current sinks in one chip, at ~$1.25 a chip. 40x chips, daisy-chain 'em all up to one MCU, and away you go?

 

[geodesic]

I was unclear. Although even illumination is a feature, it also needs to be able to light with different patterns for complicated reasons that I can go into if anyone is interested.

The TLC5940 looks like it will work perfectly for what I'm trying to do.

I'll update this thread once I get some pictures of the build.

 

[nickelflipper]

Another option is the TLC5947 which is a 24 bit cousin of the TLC5940. TLC's for PWM, inverting buffers, and inexpensive AMC7135 CC led drivers could be another avenue to explore.