just a very quick question: i am building a large scale led matrix (3 x 3 x 3 metres) and just wanted to make sure i have my calculations correct in terms of the necessary resistors.
I will be using an external 12v power supply for the 125 LEDs, the matrix itself will be driven by an arduino and multiplexed through two MAX7219 chips. The LEDs i intend to use have the following spec:
Luminous Intensity-MCD: Typ: 230,000 mcd
Reverse Voltage:5.0 V
DC Forward Voltage: Typical:
DC Forward Current:300mA
so am I correct in thinking i need to order :
3.0 Watt capable or greater 33 ohm resistor for each led?
I will be using 2N2222A transistors for each led as the arduino does not put out enough current to drive so many of them.
been putting off working on this for a while but all the parts have finally arrived and there are no excuses now.
thank you for your time
thanks for your reply.
I did not know that 300mA LEDs needed a constant current drive - what is the maximum forward current that you can use a series resistor for? i am keen to use the LEDs i have mentioned as they are really bright and will be just right for the job + they are fairly cheap (I found 150 of them for $70).
With regards to the MAX7219 chip - i have used it in the past to control 64 LEDs with an external power supply, the chip itself got power through the arduino but the LEDs were powered externally. I have attached a schematic of how I did it before (with great results), however I was using not as powerful LEDs.
with regards to the DC forward Voltage - the minimum is 3.0v, typical 3.4v and max 3.6v.
How would I go about providing the LEDs with a constant current drive?
