How do I control a device running from another power source using arduino?

I have board of LEDs (about 16 x 10mm bright LEDs) that i made which is running off a 9v DC850mA adapter salvaged from an old Casio keyboard. Basically I want to flash the lights in sequences determined by my Arduino. I have plugged the LEDs directly into the board (which, at the time, I hadn't realized was potentially hazardous to my board) and it worked just fine. Problem is, when multiple boards are plugged in they can't get bright enough, and I want to use ALOT of these LED boards. so yeah, I am just looking for some advice on what to research, or even a schematic you may already know of. I know very little of transistors. And i know nothing about relays (not even sure if thats what might be used). Any help is greatly appreciated. thank you. 

mkanoap6 years ago
I just published an article that might help you a little bit. Check out step 4 at

I talk a little bit there about how relays work.

This circuit controls 120volt stoplight lamps from an arduino which is a bit of overkill for what you need, but includes bits that control a separate 9 volt circuit to drive the relays that control the lamps.

You could either treat your large strings of lights like the 120volt circuits, having their power be completely separate and controlled by relays, or substitute them for the relays in my circuit and have transistors turn them on. It depends on how much power needs to go through the transistors.

Another one to look at is which does something similar and isolates the relay part of the circuit to it's own little diagram.
frollard6 years ago
Well, you sniffed to the right place -
The arduino can supply very little current (I believe 40-50ma per pin) safely, so you need to use a transistor to act as a beefier switch to turn on your gadgets.

There are hundreds of led instructables that involve transistors, so I won't go into huge detail here...
I bet searching 'arduino led transistor" will get you on the right track.