I wanted to know if this was possible and maybe where I should start.
I want to take a Letterpress type drawer, put an LED in each compartment, then use photo resistors, to gradually turn the light on and off with the sun. With that I may need to add an additional power source so the 40 or so LEDs are bright enough. And if possible I might want to add a second set of LEDs, and a RTC, and have those light up on special days(my partners Birthday as this is a gift).
Sounds pretty reasonable. You will need a separate power supply/regulator for the LEDs, assuming 40 LEDs that take 20mA each. The arduino's regulator can only handle around 250 mA (I think), so don't try to power them the 5V pin.
Weedpharma: My goal is to fade the LEDs in with the sun going down, my electronics experience ok, I mostly use Arduinos to build sensors for research work. My coding abilities are getting better, I'm taking python classes this summer, and in fall I will be learning MATlab.
CWashburn: I've never used a separate power supply/ regulator before and I guess that's where I'd like to start. How should I add one in, that lets the Arduino still control the power supply
If you are controlling all 40 LEDs at the same time, you will need to use PWM and a MOSFET.
What you need is a logic level gate mosfet. The gate needs to be fully on or the resistance of the drain/source is not at its least so causes heat.
My favourite small mosfet is NTD5867NL. It works with 5v on the gate and handles a few amps with ease. They are cheap. I bought mine from RS Components.
There are others but make sure they are fully on at 5v on the gate.
You will also need a separate power supply. The size depends on the power of the LEDs. What LEDs are you going to use?
crossroads. I think, has made a board that uses a LED driver that can handle some vast number of LED's
uses MAX7219,,,,,
handles up to 64 individual LEDs.
I think I get how this works now, I need to connect the LEDs positive lead to the external power source, then run the negative from that power source to the GND on the Arduino. After that run from PWM to the Gate on the MOSFET, connect the MOSFETS drain to the LEDs, and its Source to the GND.
Then with a little coding I should have something that fades the LEDs on and off?
Then you need to wire all the LEDs in parallel with current limiting R for each or in series / parallel strings with current limiting R for each string.
What power supply are you going to use? What LEDs are you going to use? We can then recommend a suitable wiring system and current limiting R.
You can use any plug pack you have available, such as an old laptop charger. You simply connect the LEDs in series to suit the supply you have.
Your calculation of current is only true if all the LEDs are wired in parallel. The current can be reduced by wiring strings of 5 and adding a limiting R then using a 12 volt supply, you need about a 0.5A supply.