I recently purchased one of these relay boards ...
to use along with my Raspberry Pi to switch an AC outlet on/off to control a fountain pump. Since the relay board requires 5V for the control lines and needs to pull more current (IIRC) than the RPi can supply on 5V, I put together this little circuit to test out my system:
I know that my "To Relay Control" line is only going to pull down to ~.3V when the RPi GPIO is high, but it was enough to toggle the relay, so I went with it. I have software that turns the pump on 15 minutes out of every hour, and the system was functioning fine for a few hours. However, after a few (~8) hours of operating the relay is no longer switching -- even if I pull the "To Relay Control" line directly to ground the relay will not flip into the "on" position. I thought this might have just been a random bad part, but I switched to another relay on that board and had the same problem after running fine for a few hours. The LED on the relay board lights up when the line is pulled low, but that's it, I don't hear a "click" out of the relays anymore when I toggle them.
Is there something about my test circuit that is causing these relays to fail? In general, what would cause a relay to fail in the manner that I've described?