I'm building a control box to provide logical control of power outlets.
I'm using a 4 channel 5V optically isolated relay system to control 4 separate outlets.
To use optical isolation you connect arduino +5V to the VCC line, and logic inputs to the 4 channels of the relay unit, and connect a separate +5V supply to run the coils of the relays.
To energize one of the relays, you pull that control line LOW. To de-energize the coil, you drive the line HIGH.
If I power on the relay but leave the Arduino unpowered, all the relay coils energize.
Do Arduino pins float when it is powered down, or are they grounded?
Could I wire a pullup resistor to the inputs? And how would I do that without losing optical isolation? If I connect my pullup resistors to the +5V for the relays, the relay's +5 is connected to the Arduino's logic lines. If I instead use the Arudino's +5 rail, then when the Arduino is powered down, it's +5V line is dead.