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controlling a led disk

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Newbie level 1
May 20, 2011
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hello guys,

I have a LED disk consisting of 20 (or so) LEDs and i want to conrol it with my ATMEL processor. Now is the program no problem but i cannot get the circuit right to control the LED disk with an circuit. In the attachment you can find the circuit that i though up. the LED disk need 19V and approximately will need 40mA. I just cant get the transistor to switch right.
i hope you can help me

greets ruud


  • naamloos.bmp
    576.1 KB · Views: 42

I also use Atmel MCUs (C51 series). Their pins are mostly open collector (lightly pulled up internally to give a state of 1 if not connected). To me, I prefer using a pin for active output by writing a 0 to it (CLR portPin). So if I need to drive a small LED, I connect its anode to Vcc and I add a resistor between its cathode and the pin. For example if it is a red LED (hence about 1.8 to 2.2 V forward) and 10mA is good, the resistor could be calculated approximatly in case Vcc=5V with the simple formula R=(5-2)/10 in KΩ. The actual LED current will differ a bit.
In your case there are many LEDs so, as you already thought, an interface circuit (using a transistor for example) should be added to isolate the LEDs supply from Vcc.
Your circuit is good looking but I am afraid it is not practical in this case.
The good one (in its simplest form) will need also a transistor and 2 resistors but connected differently.

I will try to draw it for you though you (or another friend here) might be able to do it more quickly than I.


Since you said 19V and 20 red LEDs, it seems your LEDs are arranged in two branches in parallel having 10 LEDs each. In this case I prefer to add a resistor in series to each branch. So now the number of resistors in the new circuit is 3 instead of 2.
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