hey guys.. wondering what suggestions or what you have used in your projects as far as SMD transistors go..
I want to make a simple PCB that has a certain amount of leds in a line across the pcb (pcb is roughly 1.5" long x .5" wide)..
anywhere from maybe 4 leds to as many as can fit across.
most likely the leds will be smd variations of some kind.. (most everything I have looked at as been around 3.1-3.3v and fV of no more than 20)
I want to be able to PWM control this 'strip/pcb' of leds (all at once) from an Arduino pin.
Knowing the current draw will be too much for a single Arduino pin to handle... I know I need something 'else/more' to achieve this.
Figured using a simple transistor to 'bridge' this gap, (having the transistor supply/control the direct power supply to the leds...andhaving the Arduino PWM the transistor)
I have a few questions that will hopefully help me make a better decision.
1.) the leds are NOT set-in stone yet...... that being said I was thinking about using 5050 SMD leds (one color).. maybe 4 of them..
the 5050 leds are really 3 smaller leds in one package/housing.. (ie; there is 3 GND and 3 POS pads on each 5050 led)..
I dont have a problem 'bridging' all GND pads and all POS pads together in the schematic/board layout.... more or less treating it as 1 led
my question is... if I use 1 resistor to limit the current it gets (as a whole).. let say to 30mA for discussion sakes.. (normally it would be 60mA,... 20mA per led x 3-leds in each 5050 housing = 60mA total).. but if I put on a resistor that limited this to around 30mA and knowing inside the 5050 unit it truly is 3 separate leds... will they all split up that 30mA equally? or will one of the leds in the 5050 unit be brighter?.. pulling a full 20?mA while the others split the remaining 10mA? hope that makes sense!? LOL
I know I'll need more current than what the Arduino will be able to provide.. but I dont if going with 5050's @ 60mA each is too much?
I'll only have 1 battery pack.. another pcb like this.. (the Arduino itself).. and a servo or two to power..
battery pack will more than likely be a 7.4v li-ion pack of some kind..
so... with all the above explained.. what SMD transistor do you guys recommend? (I cant seem to find a 2N2222 transistor in SMD package in Eagle?) can I use another SOT23 package? like the BC847 transistor series??
I have a question about mine.. and others I have seen..
1.) I have seen some diagrams have the battery+ pad/line (in my schematic) be the 3.3v or 5v lines coming FROM the Arduino?
2.) I have seen some diagrams have the GND line from the Arduino be used and tied into the GND line from the battery source/pack?..
Why? Im failin to see what part the Arduino circuit plays into this (powering the leds) if it can not give enough current.. 'and' a transistor is being used instead? (maybe it was showing the Arduino GND to imply they needed to be joined together?)