Normally, I would have replied in that thread instead of creating a new thread, but OP didn't post back with the status of the proposed solution. And i have a few questions about the schematic proposed there.
and it has a single ground for the 3 colours. I found out the hard way that i can't use my normal N channel mosfet to control this guy because of the common ground instead of the common anode!
So, I did some research and built a schematic that should allow you to use an arduino to control each of the RGB channels. Here's a schematic i created that should work for this. Hopefully somebody here can confirm / deny if it will work :). [it's similar to OP's but used different / cheaper Digikey parts )
Now, the questions I had about my schematic and the proposed schematic (from linked thread)
will that schematic work?
P channel mosfets require that the source be more positive than the drain, and therefore require a negative voltage on the gate pin?
Even though the mosfet i want to use is a Logic Level device, i cant (or can?!) drive it directly with the arduino because the arduino can not output a negative voltage (needed to switch the Gate pin). The reason I ask is because the data sheet says that V(GatetoSource) is + or - 20 volts. Does this mean that i can trigger Drain to Source with a positive voltage?
Yes, though your P-channel MOSFET is a little old and I'm going to bet that cheaper MOSFET's with lower Rds(on) values can be found.
I win the bet!
P channel mosfets require that the source be more positive than the drain, and therefore require a negative voltage on the gate pin?
No. P-channel MOSFET's turn OFF when the gate is equal to the source (i.e., the gate-to-source voltage is 0) and turn ON when the gate is about 10V LOWER than the source (i.e., the gate-to-source voltage is -10V, or the source-to-gate voltage is +10V, stand on your feet or on your head: your choice).
So if your source voltage is 12V (which it appears to be in your schematic) then when your Arduino pin goes high and the 2N3904 turns on, the gate voltage will be near 0V so the gate-to-source voltage will be approximately -12V and the MOSFET will turn ON.
Even though the mosfet i want to use is a Logic Level device
You don't need a logic-level device in this application because you have 12V to play with. You need a logic-level MOSFET when you want to turn a MOSFET on with only 5V.
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The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected
Nice! Since you've pointed out that i was wrong in assuming i needed a logic level device, i can go back and look for a cheaper device (Digikey also has the really handy 'find me the lowest possible cost if I need N of this part' ; this is useful because the cheapest single qty part is not always going to give you the cheapest total cost!)
RuggedCircuits:
P channel mosfets require that the source be more positive than the drain, and therefore require a negative voltage on the gate pin?
No. P-channel MOSFET's turn OFF when the gate is equal to the source (i.e., the gate-to-source voltage is 0) and turn ON when the gate is about 10V LOWER than the source (i.e., the gate-to-source voltage is -10V, or the source-to-gate voltage is +10V, stand on your feet or on your head: your choice).
So if your source voltage is 12V (which it appears to be in your schematic) then when your Arduino pin goes high and the 2N3904 turns on, the gate voltage will be near 0V so the gate-to-source voltage will be approximately -12V and the MOSFET will turn ON.
Ah, ok. This is how i thought about it... the + or - 20 V from the data sheet had me thinking that i'd be able to send a +5 to the gate and have the circuit completed. I now understand why this isin't the case.
