I am designing a PC Controlled 8 channel Relay using AVR microcontroller.
Here, the relays are on the right side of the board and are controlled by the AVR micro on the left side.
The eight TTL outputs from AVR are connected to a high current darlington pair IC like ULN2003A.
I may use either USB or RS485 interface to connect the board with PC.
Each relay is rated up to 7 A AC switching and there are eight of them. The relays are used to control AC electrical equipment.
PCB is double layered FR4 material.
The green layers in the below image represents the ground plane. The AVR microcontroller section has ground plane on both sides.
I want the circuit to work reliably in a high noise environment without resetting the AVR micro.
Both Micro and relay drivers are powered by a single 12 V DC adapter. 12 V DC to 12 V relays and 5 V to micro through a linear regulator.
Now my questions are:
Should I keep the ULN2003 close to the relays, so that the length of the high current traces going to the relay remains short and make the TTL 5V lines to the ULN2003 long or vice versa? Which is better?
Do I have to extend the Microcontroller ground plane under the TTL 5 V lines going towards ULN2003 as shown in the above image also or should I leave them without a ground plane (under the TTL 5 V lines only)?
Do I have to provide a ground plane under ULN2003A IC and the relay DC contacts?
Other Suggestions about proper ground designs are welcome.