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Tom.G
Science Advisor
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@Next_Of_Kintetic, for the reasons you just discussed, I recommend you stick with your original latching relay approach.
What the 3.3V pins? No, these are always at 3.3V. They provide power (up to 500 mA if I remember correctly) for attached circuitry e.g. this neat little real time clock module. You could use them to provide power to the relay board, but 3.3V relays are hard to find, you would be better off using the 5V pins: see below.Next_Of_Kintetic said:Although, couldn't I use those as well to send signals to the relay board that you linked?
Yes: don't turn it off (I would power the relay from the 5V pins (2 and 4) on the GPIO connector so this was not an issue - you don't need a separate wall wart at all). I think recovery from a power outage is going to be more complicated than just having the Pi remember which feeder is feeding.Next_Of_Kintetic said:That's kind of the point I was making though. Because then what happens if I switched to the "NO" position last time before I turned off the 3d printer. If I also turned off power to the relay (or let's say a surge happens, power gets cut off), then the relay reverts back to its NC state. When power comes back on, wouldn't it remain in the NC state unless something tells it to switch again?
No, the principal point of opto-isolation is to prevent you from being electrocuted. It (or some other physical separation) is a requirement in the UK (and I believe in the US) when switching line voltages from a device with low voltage connections (the wires to the Pi from the relay box if this is in a separate enclosure) or exposed metal (the USB connector on the Pi if this is in the same enclosure).Next_Of_Kintetic said:Wait, and this is probably my overall confusion about the subject, but I thought the point of opto-isolation is to protect high voltage surges from other parts of the system from wrecking circuitry that is not resistant to that high voltage.
No, the wall wart already includes protection from line voltages appearing on the output, the same goes for the printer and its control lines to the feeders. You only need to worry if you are switching line level voltages.Next_Of_Kintetic said:I mean, do I not have to worry about a high surge occurring within the relay from say the wall wart and then "cascading" back to the Pi?