I've got three relay circuits, each connected to a different digital out (pins 2,7 and 12). The project runs perfectly when I power the Arduino via USB but not when powered from an adaptor (12v, 1.5a).
Why should this affect my project? I thought the Arduino could take between 7 and 20v. What's the best voltage to use?
Each relay is connected to a switch on a VCR (PLAY, STOP, REWIND). The relay circuits are separate apart from a shared 5V and GND from the Arduino. After the first relay triggers the play function, the VCR keeps trying to rewind, even though the relay connected to REWIND hasn't yet clicked.
7V is ok, 20V is much to high. The higher the Input Voltage of the arduino is,
the lower the current, the arduino voltage regulator can deliver is.
I think, 7~10V supply is optimal.
Simple 12V adaptors often deliver up to 17V, because they are not regulated.
Maybe the arduino resets if the relay triggers?
Are you driving the relays from the I/O pins without any type of driver? Are you certain each relay draws less than 20mA?