Why is my non powered arduino giving power to my lights?

I am running an arduino uno to control three lights for a stop light costume (yes I got the idea from sparkfun. Sadly there will be no X-Bee in this project). These lights draw a pretty high current, so I have some mosfets on a proto-board to make it a little easier on my arduino. The lights are connected to digital pins 5,4,3. I have a power source at Vin and the thing is grounded at the ground pin right next to Vin. I have a switch grounding the arduino when flipped. When the switch is flipped the other way it gives me manual control, where there is a three way switch giving me the ability turn on and off whichever three series of leds I choose (all the leds are switched on and off via cathode being connected, not anode). The problem comes in here. When the proto board is attached I have control of the leds manually and everything is fine and dandy. Then When I attach the arduino, all three of the series of leds light up, until I switch back to automated mode(I haven't programmed the arduino yet so it just runs the las code, i.e. the pin 13 led blinks). I checked for short circuits and there are none. I am just wondering if there is a way to keep the arduino from connecting these things or should I just use the arduino or just use manual control.

bwrussell5 years ago
You can't expect any sort of predictable or controllable behavior until you program the Arduino. When you do that you can control the HIGH/LOW states of all the pins whenever the board is on.
GenAap (author)  bwrussell5 years ago
Thanks, I made it so the arduino was always on and the problem just went away!
Schwuuuuup5 years ago
Just a Guess: Maybe it has something to do with floating pins and you need a PullDown Resistor...
What state are the arduino pins driving the mosfets ? I suspect they reset HIGH not LOW. Have you defined them in the Arduino code to do anything ?