About a month ago I made an interval timer for my aquarium (basically just tells a pump to turn on for 1 minute, then off for 5 and repeat all day. It is composed of a transistor amplifying an output from the board to trigger a 12VDC(trig voltage) relay that starts an 120VAC pump. The transistor is in there because the trigger draws more than the arduino board will put out. The thing seemed to be working fine for quite some time, but now it seems to trigger anytime there is a change in the AC current anywhere in the supply circuit. For example, if i turn a fan on or off and its plugged into the same outlet as the timer, it will start the pump. Even the act of the pump turning off at the end of the cycle is enough to turn it back on. So I assume some sort of back current is going through the circuit I build, but dont know where or how to prevent it. there is an appropriately placed diode in the trigger amp circuit, but thats not helping....any ideas?
Maybe you should think about proper decoupling:
maybe the turning on of other appliances makes a quick voltage drop that resets the arduino?