Economy boards and LED strings.

[SirRawThunderMan]

I plan on doing a bit more experimentation/practice with LED strings soon. The only sound boards I have on hand are economy boards: one 2010 Vader Ultimate FX, one ROTJ Luke Bladebuilder. Bit weak, I know, but I'm still starting out. besides, they're cheap, and disposable if the need arises.

I plan to make two LED strings, one per board. The Luke will have one string of parallel LEDs, and I will be using the "standard" economy wiring as described here 2010 Electronic Lightsaber w/ DVD Tutorial , as if it works with one high power LED, I can't see why it shouldn't work with multiple LEDs that draws the same total amount of power.

For the Vader board, I'd like to try and use the board's scrolling capabilities, so I plan on trying an LED string with a common ground, with everything going directly through the board's output (no PNP transistor setup)

Now, the question is this: how much can I safely put through the boards? I've used both to power a 3.7v, 1A Ledengin, through the economy wiring method, but 1A is only 50 LEDs (@20mA) and I plan to try for a 100led string on both, which puts that at 2A. I've got the appropriate resistors (1 ohm, 5watt) but I'm unsure if the boards can handle that much without dying.

Any feedback would be welcome.

[JakeSoft]

If you are driving your LEDs with FETs, then you aren't putting all that current through your sound board. It should be fine. Just wire your FETs directly into your battery power. The only thing that needs to touch your sound board is the gate pins, and those draw minuscule amounts of current.

[SirRawThunderMan]

Not gonna lie, I've heard of driving LEDs with FETs, but I don't have the faintest idea on how to do so. Didn't put enough points into electronics the l;ast time I leveled up.

[TheBaconWizard]

Not gonna lie, I've heard of driving LEDs with FETs, but I don't have the faintest idea on how to do so. Didn't put enough points into electronics the l;ast time I leveled up.

You need more than this, but there are youtube tutorials and other online resources/diagrams. Here's a quick primer that should explain why this is the right way to go about it.


A Fet is a kind of transistor. In fact, ordinary (non Fet) transistors could be used for this, but Fets are much more forgiving in all ways. Also larger, but that's changing.

The names vary, but all transistors have 3 legs. One of these called the Base, or Gate, is for receiving a small signal which acts as a switch. That's all a transistor is: a switch, but instead of pressing it manually, you use a small voltage. You can imagine, this can be very fast and is very useful.

The other 2 legs are basically positive and negative in a circuit using much higher voltages or currents.
Note, your small positive voltage applied to the Gate flows out through the same negative, as the other, larger voltage circuit.

So the idea would be that you connect your LED string up with the Fet in series, usually on the common positive of the LED string, and the appropriate battery power supply for the string.

You then connect the Base/Gate to your much smaller voltage signal that the economy board produces.

Hence, the small voltage signal switches the Fet to its ON state, and the high-voltage circuit is switched on.
Thus, when the econo-board tries to light an LED or small string that it used-to, the result is your much larger and more powerful string is switched on instead.

There are different transistors and fets for different situations, but the most approachable and most likely to be what you need, is an N-channel Mosfet. I hope that helps and wasn't too patronizing.

[SirRawThunderMan]

TheBaconWizard: Not patronizing at all, you've mostly covered the basics of what I need to know to research it further.

Gotta ask, what's the diference between using a FET and a PNP for driving LEDs? They're both transistors, and they're used in similar applications, is there much difference beyond the way they work?

So the idea would be that you connect your LED string up with the Fet in series, usually on the common positive of the LED string, and the appropriate battery power supply for the string.

Okay, so I can see that working for the single LED string for the Luke board, but how would that work with the Vader board, where the LEDs are connected to a common ground with five separate positives for the scrolling effect?

[TheBaconWizard]

Gotta ask, what's the difference between using a FET and a PNP for driving LEDs? They're both transistors, and they're used in similar applications, is there much difference beyond the way they work?

 how would that work with the Vader board, where the LEDs are connected to a common ground with five separate positives for the scrolling effect?

Not too sure actually. I know fets are much more tolerant of heat, of wider ranging voltages etc. But I'm sure you could use an NPN

With regard to using a common ground instead of a common positive, well fets come in flavours same as other transistors, they can be P channel (PNP equiv) for example. I think that might be what's needed there: The gate goes low (compared to your String's power supply) and the current is allowed to flow.

Some careful use of resistors may be needed: Because the voltages are comparative with each other, and the Fet has a range of operation (be in the datasheet for it) you may need to bias the gate with some of the voltage from your LED string's supply, via the correct resistors. Such that, normal situation is the voltage at the gate is sufficiently high (being the soundcard's voltage plus some that you've added from your battery) to keep the circuit OFF, until your card grounds it. You'd need a resistor to prevent your card having to ground too much current for its own good, now it has to handle extra from your LED supply. Alternatively, you might be able to find a Fet that just fits your available voltages nicely. That could be a long google sesh.

[SirRawThunderMan]

Cool. Thanks for everything so far, anyway. You've given me enough to get started googling on.