I found this project Led Cube he has only used 8 mosfets, 8 npn transistors and 8 74HC595 shift registers. I found couple of led cube projects that uses components than this. this guy don't have a video with more complex animations. I just wanted to make sure if this is capable of doing multiplexing. I have read about it and POV, and it's just the timings so before I order the parts(I have already ordered shift registers) I wanna know that.
thanks
Hi,
Not the best design I've seen:
74HC595 is not a good choice of shift register for multiplexing. It has a max sink of 70mA per chip, limiting the led brightness to around 8mA instantaneous current,or 1mA on average. OK in the dark, but... tpic6c595 would be far better.
high-side switches seem over complex. Since the supply voltage is 5V, a single pnp transistor, per layer, capable of switching enough current for 64 leds, would be simpler.
a 9th shift register to control the layers would make wiring between the Arduino and the cube far easier.
But yes, it is doing multiplexing.
Next time ask before you order any parts!
Paul
PaulRB:
Hi,Not the best design I've seen:
74HC595 is not a good choice of shift register for multiplexing. It has a max sink of 70mA per chip, limiting the led brightness to around 8mA instantaneous current,or 1mA on average. OK in the dark, but... tpic6c595 would be far better.
high-side switches seem over complex. Since the supply voltage is 5V, a single pnp transistor, per layer, capable of switching enough current for 64 leds, would be simpler.
a 9th shift register to control the layers would make wiring between the Arduino and the cube far easier.
But yes, it is doing multiplexing.
Next time ask before you order any parts!
Paul
hi, thank you very much for your explanation and the advices. I did only purchased the shift registers yet. it was inexpensive, but I will go for tpic6c595. when a whole layer get lighted up, a max current of 1920mA will have to go through the transistor. (30mA for a LED) I have no complex knowledge of electronics, but it would be a great help to me if u could suggest me a transistor to use, I was going to order the mosfests before.
and one question, is it a good idea to use capacitors? I thought that it'd light up the LED's much more time and will help to multiplexing.?
and one question, is it a good idea to use capacitors?
Where would you use them?
I thought that it'd light up the LED's much more time and will help to multiplexing.?
No.
SScattered:
u could suggest me a transistor to use, I was going to order the mosfests before.
Perhaps the suggested circuit is the simplest that would work. I was thinking of a pnp transistor in a to220 package, but I cannot find any with a current gain around 100. A gain of 100 would require a gate current of around 20mA to switch your 1920mA for the leds, and a Ardunino pin can sink 20mA. However, all the pnp I can find have gain of only 30 or 40. I did find some with gain 1000, but these are Darlingtons, and so have a voltage drop of around 2.5V. This would not leave enough voltage for your LEDs with a 5V supply.
Perhaps Crossroads knows of a suitbale pnp or p-channel mosfet?
SScattered:
And one question, is it a good idea to use capacitors? I thought that it'd light up the LED's much more time and will help to multiplexing.?
Yes, you will need some large caps, at least 1000uF, which could be 100uF x 8, to smooth the high currents that are being switched.
I used a TPC6108 P channel FET (4.5A absolute Maximum ) in a few multiplexed LED projects, here is one of them:-
http://www.thebox.myzen.co.uk/Hardware/Hexome.html
Grumpy_Mike:
TPC6108 P channel FET
Hmm... I'm sure it has excellent characteristics, but a through-hole package might be easier for a relative novice, like to220. Can you think of something similar that's thru-hole?
Mike, for my education, why do you think the builder of the cube (in the link on the original post on this thread) uses an npn to drive the mosfet? Would not the Arduino pin (or a 74hc595 output for that matter) be able to switch the mosfet on/off directly? If the supply was more than 5V, I could understand, but it isn't.
The +5volt for the LEDs could (should) come from a different supply.
The small transistor ensures that the mosfet stays off when the Arduino is off (or powering up).
If the gate would be directly connected to the Arduino, it will be pulled low through the pin protection diodes.
Leo..
Wawa:
The +5volt for the LEDs could (should) come from a different supply.
Why? That's not very practical...
Wawa:
The small transistor ensures that the mosfet stays off when the Arduino is off (or powering up).
Isn't that what the pull-up resistor does, and the npn pulls it down?
Wawa:
If the gate would be directly connected to the Arduino, it will be pulled low through the pin protection diodes.
I thought the protection diodes were there to protect the pins from high voltages. But nothing here is over 5V. What am I not understanding?
Every Arduino pin has two protection diodes. One to ground and one to VCC.
So when an Arduino is OFF, and the micro's VCC pin is 0volt, the pin can't be higher than 0.5volt.
That will turn the high-side mosfet on.
No problem ofcourse if it's a shared 5volt supply.
Leo..
Thanks Leo, especially for the info on those proection diodes.
What do you think of the high-side transistor arrangement on the link in the first post? Could it be simplified? If so, what transistor would you select? Bearing in mind that its not a requirement that the leds don't flash a little on startup, as long as no damage is done (the series resistors ensure that leds are not exceeding their max continuous current).
Circuit is ok, but there might be a problem with the switching speed of the 2N2222.
With a base/collector current ratio of 1:1, it could come out of saturation too slow.
You could use a schottky diode from base to collector (Baker clamp).
Or, maybe better, a 2N7000 mosfet. No gate resistor.
But then again, I never made a LED cube, and I don't know what the circuit's switch frequency is.
Leo..
Grumpy_Mike:
Where would you use them?
No.
one for a layer and total 8 of them for the cube
Wawa:
Circuit is ok, but there might be a problem with the switching speed of the 2N2222.
With a base/collector current ratio of 1:1, it could come out of saturation too slow.
You could use a schottky diode from base to collector (Baker clamp).
Or, maybe better, a 2N7000 mosfet. No gate resistor.
But then again, I never made a LED cube, and I don't know what the circuit's switch frequency is.
Leo..
so it won't be much problem if I made the LED cube as in the link and using 2N7000 instead of 2n2222? I found from the wiki 2N7000 is capable of switching 200mA max. is it enough to switch the mosfet? I downloaded the dataSheet of FQP27P06 and don't know what I'm reading. 
Grumpy_Mike:
I used a TPC6108 P channel FET (4.5A absolute Maximum ) in a few multiplexed LED projects, here is one of them:-
http://www.thebox.myzen.co.uk/Hardware/Hexome.html
unfortunately TPC6108 in ebay. I searched by the name 'TPC6108' and no results. In my country it's hard to find those parts, so I order from ebay. I think I will stick to FQP27P06 till some other transistor is mentioned.
thanks
Wawa:
But then again, I never made a LED cube, and I don't know what the circuit's switch frequency is.
Leo, the frequency will be less than 1,000Hz. High enough to switch each of the 8 layers on & off in turn, fast enough to avoid visible flicker.
If, in the future, Sscattered decides to implement bit-angle-modulation, that frequency might go up by a factor of 8, so still less than 10,000Hz.
Logic level, Low Rds, P-channel mosfet to switch on current for each layer
[http://www.digikey.com/product-detail/en/SPP15P10PL%20H/SPP15P10PL%20H-ND/2783608](http://"http://www.digikey.com/product-detail/en/SPP15P10PL H/SPP15P10PL H-ND/2783608")
PaulRB:
Leo, the frequency will be less than 1,000Hz. High enough to switch each of the 8 layers on & off in turn, fast enough to avoid visible flicker.If, in the future, Sscattered decides to implement bit-angle-modulation, that frequency might go up by a factor of 8, so still less than 10,000Hz.
I quite don't understand what you guys are talking about, I have a lot to learn. this 'frequency' story, I totally don't know about that. I will try to learn about them after my EXAM.
btw thanks for the help guys.
CrossRoads:
Logic level, Low Rds, P-channel mosfet to switch on current for each layer
NDP6020P onsemi | Discrete Semiconductor Products | DigiKey
[SPP15P10PLHXKSA1 Infineon Technologies | Discrete Semiconductor Products | DigiKey](http://"http://www.digikey.com/product-detail/en/SPP15P10PL H/SPP15P10PL H-ND/2783608")
thanks, will get 'em
CrossRoads:
Logic level, Low Rds, P-channel mosfet to switch on current for each layer
NDP6020P onsemi | Discrete Semiconductor Products | DigiKey
[SPP15P10PLHXKSA1 Infineon Technologies | Discrete Semiconductor Products | DigiKey](http://"http://www.digikey.com/product-detail/en/SPP15P10PL H/SPP15P10PL H-ND/2783608")
That second link should be http://www.digikey.com/product-search/en?KeyWords=SPP15P10PL%20H-ND
Thanks Bob! So those can be driven directly by Arduino or 74hc595 outputs, with no intermediate npn transistor?