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Topic: RGB SMT LED Cube, resistors, drivers, and shift registers. (Read 24 times) previous topic - next topic


How about these little guys? they look good enough for 1 amp at 5volts.

Mosfet Type: P-channel
Current Rating: 2.6A
Rds (On): 40 mOhm
Voltage Rated: 12V
Package: SSOT-3

Mosfet Type: N-channel
Current Rating: 4A
Rds (On): 100 mOhm
Voltage Rated: 60V
Package: SOT-223

They are much smaller than the ones I actually ordered (from the previous post). but they look like they exceed my needs, take up less space, and are even less expensive. These seem much better than what I ordered, but maybe they are not?


Im trying out TLC5940 chip on the SMT RGB LED cube, I have it wired up with 3 Chips, each have a 2k resistor, and I used 1 10k resistor, but the schematic didnt show any decoupling caps, so I didnt put any in.

Im trying it with the BasicUSe example software (just one plane of of 16 RGB LEDs, no multiplexing), and on Usb, it starts out really bright all on full, and after about a minute, it dims, and flickers each individual LED, but looks weird, and im pretty sure, wrong.

When I run it from 5v power supply, its similar to on USB, but its not bright at all, just flickering each individual led.

I know that I have some solder issues, since all the LEDs are not working (I buggered them up while soldering the wiring on the backside of the board.

Im using the info at:
and I wired it up like the example.

Heres a picture of the wiring, I havnt done the stuff for the planes, but I run a wire from the 5v to one of the common plane wires.



I worked on this again tonight. I repaired the some wiring on the middle chip, now all the LEDs light up in order, but they are pretty dim. I also put .1uf caps on each 5940, and its a little brighter, but nowhere near as bright as they should be.

Im just running one plane at a time with a 5v wire to the plane, and the software running the 5940s. The cubes gotten kinda beaten up being knocked around my bench for the last month or two. I will need to replace some LEDs I think :(


This is sort of an old topic, but I straighten my wire for cubes by grabbing both ends with needle nose pliers and stretching it. You will feel it stretch and then its perfectly straight


I tried that too. I used a vice, and vicegrips. The copper wire (24 gauge) breaks really easily, then springs out of control. The steel wire (19 gauge) did get straighter, but it was very difficult to stretch. I was able to do it, but when relaxing the wire, it still had curves in it. I also tried pulling real hard on the wire while smacking the wire with my hand, it didnt work either. I got straighter wire, but I couldnt get the curve out entirerly. I was working with about 6' of wire at a time, trying to get the longest section of straight I could get, but it just never got straight enough for me. I have some 22 gauge steel wire, but I havnt tried straightening large sections of it.

How were you doing it, how long and what gauge wire?

There is also a technique where you twist the wire, I actually got the best results with that technique, but when you look close, you could see the twist, even from 10 feet away, you could kinda see the twist. What you end up with is a wire that has very consistent tight twist to it that is roughly straight and rigid.  After a few days of trying things, I ended up going back to the original gentle uncurving metheod.

I was looking at the music wire that I had used on the charliecubes, and its got some rust on it, so its not stainless steel, its hardened steel.


Yea i was using a vice and needle nose pliers. And I think 22 gauge copper wire


I found the copper to be the most difficult. Thicker and harder were easier to get straighter, and I was spending a lot of time bending wires. My copper was a little smaller than yours, and my steel was a lot thicker.

How long of sections did you do? did you cut 4" sections, or many feet at a time? I wasnt able to completely remove curve, but if you spend enough time with it, it gets fairly straight. When using the music wire, its very straight, and doesnt get bent while working with it. But its very difficult to solder, comapred to copper or gavlanized steel.


I re-uploaded a new sketch, and hooked it up again, and its doing something!  :D

Right now I am only controlling one plane by connecting it to 5v.
The ICs are wired up like the example in the TLC5940 tutorial:
except that I have decoupling caps on each chip.


Its running the Useprogmem example program, it just lights all 16 RGB LEDs in a plane, and turns them off in sequence. I think it may do some PWM while its turning it off too.

Its not as bright as I had hoped, but maybe with some experimentation, i can make some improvements. I ran the same LEDs on shift registers and resistors, and I seem to remember them being considerably brighter, but maybe that just my imagination.


The smt cube is going to wait until i can figure out a good way to source 5v to each plane. Im assuming a mosfet would be the best way, but I have to figure out what part I need, and order them, and wait for shipping...

I setup a test fixture with some charliecube spires, and I have 3 different resistor sizes. no resistor, 150, 100, and 50 ohm (I didnt have any 50 ohm, so I doubled up on the 100s). No resistor is visibly brighter, but the others light up pretty well. Im thinking of modifying one of my cube with 100 ohm resistors.

It seems to me that there are two viable ways to introduce resistors. I could put on resistor on each of the i/o pins, or I could put one resistor on each of the 64 leads on the 16 spires.

More resistors typically means more evenly controlled, but I think in this case it may not actually make any difference, because It seems to me that the LEDs are being lit ONLY one at a time. Since there will always be 2 resistors used (one on the high pin, one on the low pin), There will always be 2 resistors being used to light any single LED. If I use 100's then Im doing 200 ohms which should mean my LEDs never get anywhere near 20ma.

Measuring what is actually going on would be best, but measuring is going to be difficult to impossible for me, a few calculations only takes a few minutes, and is relatively easy.



If you want really straight wires try heating the wire to a low red heat while applying tension to the wire. Allow the wire to cool naturally and you will have annealed the wire.. No more bends or gentle curves in the wire. This also is useful to relieve work hardening of copper and alumininum as well.

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That seems a bit excessive, I would have to heat it, to soften it, to straighten it, then harden it to help keep the structure of the cube. I certainly wont be heating up galvanized wire till its glowing red.

If i were to heat wires, I think I would use music wire, but use a lot of heat to tin them (like with a solder pot, or small torch.)

I really like the look of the copper, but its too soft for cubes, they really think they need a bit more durability than copper or aluminum provide. The music wire is also small enough that you can barely see it.


I think i have worked out controlling the columns on the cubes, but I need some help with controlling the planes. One cube sinks(Shift register/resistor cube) the planes, the other cube sources(constant current cube). 

It looks like the mosfets that I purchased are totally not appropriate for what theses, which kinda sucks, but moving on, I need to figure out a good way to sink, and source 5v upto 1 amp.

Based on what i've read, it seems like there are basically 2 viable options, Darlington arrays and Mosfets. It sounds like mosfets are a better option, because they waste less electricity.


So, it seems I should get an N channel mosfet or maybe I could use a couple ULN2803 chips for sinking the planes on the shift register cube.

How about sourcing 5v?, should I use a P channel mosfet? What are other options?

I understand that I want a fet with low RDS, typically measured in millionths of an ohm, What are all the factors I need to consider when shopping for mosfets?

Crossroads posted a way to sort through the mind boggling inventory at digikey, That is a good start. I tried to apply the concept at mouser, but that didnt work out well. Ebay, Thaishine, and Tayda all seem too difficult to figure out what part I want. 

Newmark has a good looking search interface, but Im unsure exactly what things to select. I was able to knock the list down to 100 parts for both n and p channel at 1amp.

I think i also need the parts to be logic-level, that is to say, they can be switched with 5v.

Jameco has lots of parts, but the search interface is kinda difficult.

If I knew exactly what i was looking for, and exactly what specs I should be looking for would make it easier,

I am also uncertain about packages. for 1amp at 5v, I dont think i need any heatsinks, but I would rather buy a part that wont need one, if that is an option. Smaller  (as in takes up less space), is better than bigger. SOT23 isnt too hard to solder, so even that isnt too small.

I've modified one of my charliecubes to use 100 ohm resistors, and it is noticably dimmer, Im going to try to measure some LEDs with a few different resistors sizes and calc the current use, and try to get them closer to 20ma. Nearest I can tell, I will have to settle for less current on the blue/green, If I drop to 50 ohm, that should put the blue/green at about 20ma, but the red should be over 20ma. Because of the layout, you can only use one size resistor and keeping red within spec means putting blue and green considerably lower current than red.

If you are considering trying music wire, I have found that for this size cube, 015 is too delicate (you can do it, but its very difficult), and 025 is more than sturdy enough, so Im going to try 020, I think it could be the ideal size for this size cube.

Tom Carpenter

Jan 19, 2013, 12:36 am Last Edit: Jan 19, 2013, 12:45 am by Tom Carpenter Reason: 1

I understand that I want a fet with low RDS, typically measured in millionths of an ohm

I think that is mOhm, or milliohms, i.e. thousandths of an ohm.

These are what I chose for my 12x12x12 cube. I ended up with 72 of them, and each one supplies 2 rows of 12 RGB LEDs, totalling 24*3=72 diodes per transistor = 1.44A peak current.

I chose them for (a) price, (b) size, and (c) they are rated at 2A drain current at a Vgs of -4.5V making them suitable for 5v logic levels.

Ideally one with a slightly lower on resistance (these are 0.1Ohm at -4.5V) would be better, but these were a good price and I only lose 0.144v which would have been lost over the PWM constant current sources I have wired to the Cathodes. In my case theThe power lost is 1.44^2 * 0.1 = 200mW, which is well withing the power dissipation the transistor can support.


Ah, now that you mention power dissipation, I remember reading something about power dissipation, I think it was nick. He mentioned a bunch of things that were a bit over my head, and hard to comprehend. There is something about gate capacitance or some such thing that I should also consider?

I think nick also posted some nice tiny FETs that were rated at 4amps, which seemed very surprising to me that such a tiny thing could handle such current.

So, it seems i will get the best results with a mosfet in both circumstances?

I decided I should test and measure the current on the common cathode LEDs that im using on the charliecube. I would like to find ideal resistors for them, by my calculations 200 ohms should keep the LEDs safe, but you dont know unless you measure. I soldered some resistors on to some LEDs, and hooked them up to a 5v power supply. I found that the blues were running at 20.9mA. I measured the power supply, and found that it was actually 5.25v.

I tested the test fixture running off usb, and it was 4.17v on the Vin pin, but a little higher on the 5v pin. I tested the cubes running on 500ma, and 850ma power bricks, and they were both different too. and both well under 5v. The highest voltage reading I got was on the 850ma on the 5v pin at 4.8v

If the LEDs are getting 4.2, then they need much smaller resistors than if they are running at 5, but since every way that I power them, they run at different voltages, i cant figure out the right size resistors, other than,200 ohm more than enough. At 5v, 200 ohms (2 100 ohm resistors) should be limiting the current to 14mA on the red. At 4.2v, those same resistors are limiting the LEDs to 10mA. If I change to 50 ohm resistors(100 ohms), and the LEDs are only getting 4.2v, they should be at 20mA, and the blue and green will be well under 20ma. but if they are getting 4.8v, then 100 ohms will only limit them to 25mA, but the blue and green will be down to 12ma.

Somehow, i dont think these LEDs are going to ever see 5v, no matter what.


Yet another crazy cube idea. one of the nice things about this setup is that you dont use too much current, and you dont have to worry about mosfets, It does require more parts than the charliecube, but its only 16 transistors. Because it only lights up 12 LEDs at a time it only uses 240 mA, but it has a duty cycle of 4%, so it wont be very bright.

Here my modification of a 5x5x5 transistor cube (thanks crossroads), that I changed to 4x4x4 RGB. I didnt quite draw all the lines in, but hopefully enough is there to get the right idea.

At this rate, I may never get the original cube SMT constant current cube finished, or the shift register-resistor cube built.

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