Hi. I'm relatively new to electronics. I just made a Arduino Uno with a ESP 01s and a relay board to control a 12V solenoid valve. Everything is working fine when the relay is turned on. However, once the relay turned off, the ESP 01 immediately stop working. I read that this is due to the back EMF from the energized coil. May I know how could I add a diode to the circuit? Thanks in advance!
You have two sources of the back emf - your relay coil and the solenoid itself :
There is a sticky on the subject and ..
Guide here :
Welcome to the forum.
Please read the post at the start of any forum , entitled "How to use this Forum".
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
"May I know how could I add a diode to the circuit?"
Across the relay coil, cathode to the more positive end of the circuit (it seems 'backwards', but that's how it works).
Websearch "relay diode".
There is a brief tutorial on this forum explaining exactly what you want to know:
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