dc42:
It sounds like a wiring or power supply issue to me. Post a photo of your setup and preferably a schematic too, then it will be easier for us to help.Back emf is generated when the solenoid is switched off rather than on, and can be tamed by connecting a diode in parallel with the solenoid, cathode to positive end, anode to negative end.
Attached is a photo of my barebone circuit. The wires are labelled in red. Please let me know if you need more info.
One correction to my previous posting. The serial connection drops when the relay/solenoid is turned OFF, not ON as stated in my previous posting.
Many thanks!
George
