Arduino and Solenoid Water Valve
The problem is most likely EMI (electromagnetic interference) from the switching of the inductive load by the relays.

The fact that you have your setup on a breadboard means that each and every pin of the ATMega328 is connected to a little antenna. That massively increases its susceptibility to falling prey to EMI.  Add to that the fact that there is absolutely no decoupling of the power supply (thanks to Arduino producing shoddy worthless tutorials) makes the whole chip very fragile.

When running with a real Arduino board you have decoupling on the power which helps to reduce induced power rail bounce which can cause the chip to fail.

There are two things that can help mitigate this:


Add decent decoupling to the ATMega328


This means adding capacitors between each VCC pin and ground. At the bare minimum a 100nF capacitor on each VCC pin is needed. Additionally a 1nF capacitor can be beneficial. A 10µF capacitor should also be added on the breadboard's power rails to act as a board-level reservoir.


Snubber networks on the switching


A snubber network, in its most simple form, is a resistor and capacitor in series.  It is placed across the switching terminals of a device that is switching an inductive load to absorb the sudden spike of back-EMF from the collapsing magnetic field whenever the load is switched off.







simulate this circuit &ndash; Schematic created using CircuitLab

Note that the capacitors here should be rated for at least 400V since the voltages involved are quite high.