a timer or a photo sensitive system, dimming capabilites mostly. i knew i shouldn't have said anything.. sigh (joke)
timer uses a simple 555timer (ic) and a few common components, a resistor and capacitor, to make a timing constant. good for timing jobs in the micro second to the days.
-= or =-
photo sensitive system (my favorite)
instead of a timer, you install a special purpose diode that is sensitive to light, and wire it so when lite is not detected to 'turn on' the moonlight. basicly it's a fancy switch. here's the system in bit better detail.. install the photo sensitive diode near the existing lighting fixture, when that gets turned off the photo diode detects no light and turns on the moonlight and when the main lights come back on, the photo diode detects the light and shuts off the moonlight.
and/or you can install a dimmer switch to fine tune the moonlight's output. like i suggested before finding an external medium is the best choice.. but it's very simple to do, however it's not a 'true' dimmer, but it works in that capacity. all you will need is a 50k wand potentiometer (or pot for short) which is nothing more than a variable resistor, it can change it's outpot from ~0 to ~50k Ohm, also remember that 5% tolerance thing i was talking about ealier, it's valid on pots too. radioshack sells them for 1-2 bucks i believe, you install that between the dc
transformer and the leds. i made up a pic for another aa member on how to install it. oh yeah bow down to my uber MSpaint skillz. lol http://12598.tripod.com/Pot.jpg
you will want connect terminal's A and C. if you connect anything else you will not have a variable output, it will be deadlocked at max resistance value of the pot, which happens to be 50k in this example.
if you hook up A and B = deadlock, B and C = deadlock
easiest addition is the dimmer pot, it's not a true dimmer, and i will save you from the theory of why it's not but it still works reasonably well as a dimmer. the timer is medium in the difficulty department, it's not that complex to build or do the math for, but can prove to be a pain if you exceed their max voltage/current requirements. and the photo diode idea isn't that hard it's just finding good ones, that's the problem. the ones radio shack sells have poor reactions (from personal experience) where it can take a full 75w bulb put 6inches from the diode to get it to react to the light and they weren't cost effective enough to buy from digikey.com either. (cost effective opinion is based on experiecnes from 2yrs ago, so plz don't quote me on that)
there are other things to you can do with led
's but they don't really fit into the aquarium too well. those are the 3best ones i can come up, if you have any ideas on additions i am all ears.