I'm currently working on a Table-Tennis ("ping pong") Scoreboard.
I am designing my own (~5" tall) 7-Segment digits to be laser-cut, and I need to design these displays to have bi-color digits (meaning, each digit can be either: Red or Green).
Considering I needed bi-color, I ordered up a couple hundred: 3-lead, common cathode, Bi-Color LED's (from here)
Upon further investigation into large seven segment displays, it seems the most common LED arrangement is to run the majority of the LED's in series.
Each segment is made up of 2 parallel strings of 6 LED's wired in series.
Check out the schematic here.
Unfortunately, (3-lead) Bi-Color LED's can not be wired in series.
(just as 4-lead RGB LED's can't be wired in series)
Doh! (I should have seen that one coming)
Leaving me only with the option of wiring them in parallel.
from what I understand:
In series:
you need a higher voltage (to make up for the cascading Vf from the LED's)
you don't need much current.
you only need 1 resistor per series string to limit current.
In parallel:
You don't need much voltage (just enough to exceed the Vf of the LED)
You need a LOT of current.
You need 1 resistor per led. (in my case, I would actually need want 2 resistors per LED to attempt to even out the RED / GREEN brightness differences)
I was looking into multiplexing as a way to greatly reduce the current requirement.
My original plan was to build the circuit with:
resistors (for LED current limiting)
shift registers (for individual segment control)
Mosfets (for supplying LED power... because I will need more than Arduino pin's alone can safely provide)
Both RED LED's and GREEN LED's (packing in 2X the LED's only to light-up 1 type at a time... poor-mans bi-color(?)) 2. How would you suggest I control this?
Such large digits, why even use bi-color LEDs? Just place red and green LEDs side by side. With a decent diffuser (you do have one, right?) you can't visually locate the light source anyway.
aarg:
Such large digits, why even use bi-color LEDs? Just place red and green LEDs side by side. With a decent diffuser (you do have one, right?) you can't visually locate the light source anyway.
That would mean I could only lite-up half of whatever # of LED's I can cram into the segments. Essentially half the brightness, and not as good light-spread. A diffuser would help spread the light, at the expense of brightness.
joshpit2003:
That would mean I could only lite-up half of whatever # of LED's I can cram into the segments. Essentially half the brightness, and not as good light-spread. A diffuser would help spread the light, at the expense of brightness.
Well, it's your coin. If you use SMD LEDs (especially high brightness) there is room for more than you could easily drive. On the other point, it's aesthetics, I guess. I prefer solid digits to a bunch of bright points of light.
But the real point is, it would solve your problem...
aarg:
But the real point is, it would solve your problem...
Thanks. I may end up going that route after all. I can't seem to find a solution or a wiring schematic for large 7-segment displays that use 3-lead bi-color LED's.
joshpit2003:
Thanks. I may end up going that route after all. I can't seem to find a solution or a wiring schematic for large 7-segment displays that use 3-lead bi-color LED's.
Well it is certainly possible. I can't think of a way of multiplexing them right now. Except by digit, so you would have 14 (7+7) anodes and 1 common cathode per digit. Also a big pile of resistors, one for each LED.
If you make a PCB, you can probably get that under control.
None plexing, w/ Bi-Color LEDS: (requires parallel wiring all LED's)
20 mA x 12 LED's per segment x 7 Segments / Digit x 4 Digits
= 6.720 Amps
20 mA x 2 LED's per "match dot indicator" x 12 indicators
= 7.200 Amps (holy crap!)
I'm not sure I've ever seen a 5V wall-wort PSU rated for 8 Amps.
so it seems I would need to Charlieplex the display, (turning on only 1 segment at a time).
That way i would only ever be pulling 240 mA.
My concern with Charlieplexing, is that (especially with a display this large) you would see the flickering of the segments, which would drive me crazy. I've seen the flickering in other people's projects, sometimes even w/ multiplexing (not even charlieplexing).
Anyone have any first hand experience with this? (multiplexing / charlieplexing showing visible flickering)
If I were to ditch the Bi-Color LED's and go with double the # of LED's (one of each color), then I could wire in series and calculations would look more like this:
2 parallel stings of 6 LED's wired in series (that's probably how I would do it):
means I would need a minimum PSU V of 6 LED's x 2.5 max Vf = 15V.
20 mA x 2 parallel LED strings x 7 segments x 4 digits
= 1.12 Amps
20 mA x 12 indicator circles
= 1.36 Amps
So I would need to source a >= 15V, >= 1.5A PSU to power the device
even that's going to be a painful hit to the BOM.
I'm starting to see why multiplexing is so popular now.
Multiplexing cuts down on brightness tho.
If you want it brighter, use TPIC6B595 shift register to drive each digit.
Easy to code too, just shift out the data when something changes.
This board I offer will drive up to 12 digits at 150mA continuous per segment, using just a few pins for SPI.transfers and a PWM output for brightness control. Other pins are broken out for connecting to buttons, etc. FTDI header for programming. http://www.crossroadsfencing.com/BobuinoRev17/
Here's a panel we assembled with 3 and 6 LED segments per digit, powered from 12V, as an example.
These particular digits had a 2x8 header on the back for 8 segments and 12V, so I made an adapter board to have a place to plug in ribbon cables for them.
Wawa:
Remember that multiplexing also reduces brightness.
And with the LEDs you have, that might be an issue for a score board.
Leo..
Thanks, I was not aware that multiplexing drives down brightness. I guess it makes sense though... lower-duty cycle of light should equal a lower light level. My friend pointed out to me that monitors are typically flashed at 60 Hz, anything slower and your eye might be picking up on it.
CrossRoads:
...
If you want it brighter, use TPIC6B595 shift register to drive each digit.
Easy to code too, just shift out the data when something changes.
...
Wow! Quite the accomplishment of projects you have. The fencing scoreboard was most similar to what I'm doing. Very cool. I'm hoping this project of mine will result in me designing my own board though, so I'd rather not use one of yours. But thanks for the link.
I'll definitely look into high-power shift-registers... if that could cut-down on me having to use the 56 mosfets/transistors I think I'll need then that would be awesome!
A friend of mine suggested I go with the Bi-Color LED's, and even though I'll need to wire all of them in parallel, and use a lot of resistors (2 resistors per LED)...
I could get the power needed: ~3V @ 7A = 21W
By using a high voltage wall-wort PSU (like 15V 1.5A) and feed it into a BUCK-CONVERTER (aka: DC-to-DC converter) to get me down to 3V w/ 21+Watts on tap.
I guess another perk of wiring this many LED's in parallel will be that if one fails it doesn't matter. Were as if the LED's were in series, one fails and it's lights-out. (but I imagine LED's failing is so rare this is probably a mute-point).
Anyhow, any thoughts on sticking with bi-color in parallel and using a buck-converter?
