question on daisy chaining leds
#1
Posted 22 August 2012 - 01:26 AM
#2
Posted 22 August 2012 - 01:30 AM
#3
Posted 22 August 2012 - 05:43 AM
The white LEDs need 3.4V at up to 20mA.
The "high current" Netduino pins are only rated for 16mA - see CW2's Wiki page. Unfortunately I don't believe the PWMs are even 16mA.
So you are not going to be able to drive one of those LEDs safely (let alone 25!).
What is needed is a transistor, probably a darlington transistor.
Depending on your available voltage, you will need to make several chains of LEDs to give each chain the correct voltage. Then combine the chains in parallel connected to a darlington.
I did some tests using a darlington and put the results in this Wiki page. I also experimented with an opto isolator which you may also find useful.
Hope this helps - Paul
#4
Posted 22 August 2012 - 05:51 AM
#5
Posted 22 August 2012 - 11:05 PM
#6
Posted 24 August 2012 - 08:51 PM
#7
Posted 24 August 2012 - 09:19 PM
#8
Posted 25 August 2012 - 02:03 PM
Yes the TIP120 looks fine. The data sheet says its a 5Amp darlington pair.
Figure 2 shows that at 100mA the Voltage accross the base emitter junction is 1.25V, and the voltage accross the collector emitter junction is 0.7V. It also says Ic = 250Ib
What does that mean?
The power disipated by the transistor will be your 80mA x 0.7V = 0.08 x 0.7 = 0.056w = 56mW
Figure 4 shows this is well within the safe operation area of the device!
The current into the base will be 80mA / 250 = 0.00032 = 0.32mA
The resistor fromm the Netduino should be (3.3V - 1.25V) / 0.32mA = 2.05 / 0.00032 = 6406 Ohms
Nearest preffered value 6K2 (6200 Ohms)
I think that's everything
Just before I throw this thing accross the room at a hard wall, I would like to say that trying to write a forum post with an android tablet is the most frustrating thing I have ever done.
A rather irritated - Paul
#9
Posted 25 August 2012 - 09:05 PM
It's not easy on an iPad eitherJust before I throw this thing accross the room at a hard wall, I would like to say that trying to write a forum post with an android tablet is the most frustrating thing I have ever done. A rather irritated - Paul
Regards,
Mark
To be or not to be = 0xFF
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#10
Posted 26 August 2012 - 08:57 AM
It's not easy on an iPad either
Regards,
Mark
I was finding it very hard to edit anything with the on screen keypad due to the lack of cursor keys. Its very hard to get the cursor where it needs to be!
So I paid out for a mini keyboard (about ten pounds - I just realised this keyboard has not got a
pound symbol. Thats pounds as in sterling, not pound as in "#"define).
I am now finding that firefox for Android does some very strange things with a USB keyboard. e.g. enter does a delete then a newline. It does it with a full size USB keyboard too. Sometimes the delete is on a different line to where the cursor is!
And don't talk to me about cut and pasting a link to a data sheet - I had to type it in manually.
sorry to go off topic. I needed a rant.
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