I'm interested in making a mini replica of the RGBy desk (RGBy desk - YouTube) using the Arduino board. Since the Arduino only has 13 digital pins, I was planning on using a few TCL5940NT led drivers to drive the leds. The TCL5940NT varies the voltage of each led according to the signal sent by the Arduino while keeping the current constant.
However, I estimate that I would need 144 RGB leds per sq foot, so I don't think that it would be doable to control each led individually (144 leds*3 colors=432 pins per sq foot).
I was rather thinking about controlling the leds by groups of 16 (4x4). I'd like to control each color of these group with one pin of the TCL5940NT (9 groups*3 colors=27 pins per sq foot).
16 leds at 20 mA take 320 mA but the TCL5940NT is only able to sink 120 mA. What kind of circuit would I need to sink about 400 mA while having the voltage controlled by the TCL5940NT. My fuzzy memory of school tells me that's some kind of transistor, but I can't remember more...
In your case it is probably smarter to use a Darlington transistor Array like ULN2003, it has 7 Darlington transistors in a single IC, each can handle 500mA.
There's another one with 8 transistors, i can't remember the part no. (2008???) it might be even better in your case.
In your case it is probably smarter to use a Darlington transistor Array like ULN2003, it has 7 Darlington transistors in a single IC, each can handle 500mA.
Thank you very much, that's exactly what I was looking for!
LEDs are current devices. You don't vary the voltage to control them.
Yes but an LED in series with a resistor will vary in brightness with voltage, and given that all sources of power have a small but finite output impedance even an LED by it's self with change in brightness as you change the voltage across it.
Yes but an LED in series with a resistor will vary in brightness with voltage, and given that all sources of power have a small but finite output impedance even an LED by it's self with change in brightness as you change the voltage across it.
In that circuit you describe, the voltage across the LED is constant. It's only the voltage across the resistor that changes, and through ohm's law and kirchov's current law, you are regulating the current through the LED, not the voltage.
In the case of the LED alone, it's a very, very narrow band in the range where conduction starts. The difference between the LED being off and blown is tiny. A tiny ripple on your power supply will blow your LEDs.
As for the internal resistance of the battery, that's like saying since the volume of the ocean is finite so you can vary the depth by taking out water with a drinking cup. Let's use a 500aH deep cycle lead acid cell and see you change the brightness of an LED by varying the voltage.
You're exploiting a silly quirk, and all you'll serve to do is throw confusion on something that is already confusing to newbies.
In that circuit you describe, the voltage across the LED is constant.
No it's not please, look at the characteristic curves of components before you simplify everything to incorrectness. Did you have a hand in the UK national curriculum? That's what they do.
So your saying just because it is complex let spin them simplified platitudes? Basically you would have been better off not pontificating with statements like "LEDs are current devices. You don't vary the voltage to control them." The two quantities are intimately linked and to suggest they are not is to mislead. Best to make no statement than an incorrect one. >:(
No it's not please, look at the characteristic curves of components before you simplify everything to incorrectness. Did you have a hand in the UK national curriculum? That's what they do.
I spent a rather interesting semester of third year physics looking at characteristic curves of various semiconductors with some pretty sophisticated curve tracers. I'm quite familiar with that band of a few tenths of a volt you're referring to on an LED.
As you can see if you've ever looked at the characteristic curve of an LED, attempting to limit the current by providing not quite enough voltage to go into full conduction is a horrible idea.
Why are you trying to bully this discussion with personal attacks and bring in your apparent dislike of the British School system into it?
So your saying just because it is complex let spin them simplified platitudes? Basically you would have been better off not pontificating with statements like "LEDs are current devices. You don't vary the voltage to control them." The two quantities are intimately linked and to suggest they are not is to mislead. Best to make no statement than an incorrect one. >:(
It is not at all a complex issue. LEDs, like all semiconductor junctions, are current devices, you don't vary the voltage to control them. There are countless project pages on the web where people totally miss this point and figure they can connect 4 3V LEDs to a 12 volt battery. And when it seems to work, they think they understand what's going on. But they don't understand that a fresh battery at 13 volts can fry the LEDs or a slightly weak one at 11 might not light the LEDs.