for the lurkers- Normally transistors need the base(switch) to be connected to a common ground (in some way). One way around this is to use an opto isolator which is just an emitter and photo transistor. The base is for lack of better words "removed" and light turns on the transistor which can isolate one part of your circuit from another given there are two power supplies.
My project is using a 1284p with several components hooked up to it. (humidity, temp, sd, rtc, encoder counting, and 3 reflective IR sensors)
Currently I have 4 relays on the board, I bought a second supply but completely forgot that the grounds still had to be common, which could still cause noise on my power rails and potentially reset the MCU if the voltage drops too much on the active low reset pin.
The relays operate on 5V and draw 50mA. At most I should be turning on 3 relays at a time. The supply (9v cell phone charger through a 5V 7805 regulator) supports 500ma max but I really don't want the relay coils wreaking havoc on my power rails.
My question is this, am I over reacting with the need to opto isolate 4 relays, or should I just risk the safety of the rest of my circuitry and use transistors?
Thanks for your time,