Posted by: @prez

If you were going to power the relay from the GPIO, then the ground for the relay would go to one of the GPIO ground pins.

However, I would recommend instead buying a 24V -> 5V buck converter (eg. https://www.amazon.com/gp/product/B07HRGV7XD) - then connect the input of the buck converter to the VCC (+) 24V (red cable) input to the Prusa.  I would recommend the one for the electronics, not the heat bed.  The ground (both for the buck converter AND the relay board) can go to the ground (black cable) for the Prusa too.  That way the relay works regardless of the state of any GPIO pins, etc.

This would then mean that 1) you would not be using up a GPIO pin to just power a relay, 2) you would not need to make sure that GPIO pin is enabled for the relay to work, etc.

 

Preston

However, I would recommend instead buying a 24V -> 5V buck converter (eg. https://www.amazon.com/gp/product/B07HRGV7XD) - then connect the input of the buck converter to the VCC (+) 24V (red cable) input to the Prusa.  I would recommend the one for the electronics, not the heat bed.  The ground (both for the buck converter AND the relay board) can go to the ground (black cable) for the Prusa too.  That way the relay works regardless of the state of any GPIO pins, etc.

This would then mean that 1) you would not be using up a GPIO pin to just power a relay, 2) you would not need to make sure that GPIO pin is enabled for the relay to work, etc.

Preston

Some of the tutorials I've seen talk about an Arduino 5V pin, but from what i can see on the Prusa GPIO page, there's no similar power pin, correct?

Posted by: @prez

It should, yes.  Assuming the relay is separately powered from input voltage.

There ARE relays out there that power themselves from the same line they are powering - those will generally only have 4 wires (input, output, ground, sense).  They will, when they sense voltage, close the input to the output so that the switched device gets the same input power.  For these, the input voltage needs to match the voltage of the device you're switching, and the relay needs to match that voltage.

However, a properly isolated relay has 5 wires (power, ground, sense, input, output) - or more correctly, 2 wires (board power/ground) + 3 wires per relay (sense, input, output).  The board power is the 5V, 3.3V, 12V, 24V, etc.  That's not a range usually, it's very specific.  The input is often more a max capacity (like the AC 250V or DC 30V at 10 Amps I mentioned before).  This is the kind of relay you usually want.

Either way the sense wire can usually have any voltage on it DC (I am sure you could overload it with a massive voltage, but that's beyond this scope), so the floating voltage output from the GPIO (which can be anything from 0V to 3.3V, depending on line noise) is sufficient to be sensed by the relay.

There ARE relays out there that power themselves from the same line they are powering - those will generally only have 4 wires (input, output, ground, sense).  They will, when they sense voltage, close the input to the output so that the switched device gets the same input power.  For these, the input voltage needs to match the voltage of the device you're switching, and the relay needs to match that voltage.

However, a properly isolated relay has 5 wires (power, ground, sense, input, output) - or more correctly, 2 wires (board power/ground) + 3 wires per relay (sense, input, output).  The board power is the 5V, 3.3V, 12V, 24V, etc.  That's not a range usually, it's very specific.  The input is often more a max capacity (like the AC 250V or DC 30V at 10 Amps I mentioned before).  This is the kind of relay you usually want.

Either way the sense wire can usually have any voltage on it DC (I am sure you could overload it with a massive voltage, but that's beyond this scope), so the floating voltage output from the GPIO (which can be anything from 0V to 3.3V, depending on line noise) is sufficient to be sensed by the relay.

Posted by: @prez

No, the sensors are for any voltage.

 

I used https://www.amazon.com/gp/product/B00KTEN3TM - a 4x relay (5V).  The power for the relay board itself is 5V, which is convenient for me since I also power a few 5V devices too (so I can split the 5V to the relay board AND to the devices via. the relay itself).

The power you are switching (ie. the fans, lights, whatever) doesn't generally matter except that they are within the capacity of the relay itself (which in this case is DC up to 30V or AC up to 250V at 10 amps).

The sensor wires just detect if there are 0V or some volts.  It doesn't matter if it detects 3.3, 1.2, 5, etc.  It's either grounded (0V) or not.  If not grounded, then the relay closes, if grounded the relay opens.

 

That said, if you want to use a 3.3V relay, that's fine - or a 24V relay.  Any of them will do, as long as the power you supply to the relay itself is at the 5V, 3.3V, 24V or whatever the relay board itself needs.

I used https://www.amazon.com/gp/product/B00KTEN3TM - a 4x relay (5V).  The power for the relay board itself is 5V, which is convenient for me since I also power a few 5V devices too (so I can split the 5V to the relay board AND to the devices via. the relay itself).

The power you are switching (ie. the fans, lights, whatever) doesn't generally matter except that they are within the capacity of the relay itself (which in this case is DC up to 30V or AC up to 250V at 10 amps).

The sensor wires just detect if there are 0V or some volts.  It doesn't matter if it detects 3.3, 1.2, 5, etc.  It's either grounded (0V) or not.  If not grounded, then the relay closes, if grounded the relay opens.

That said, if you want to use a 3.3V relay, that's fine - or a 24V relay.  Any of them will do, as long as the power you supply to the relay itself is at the 5V, 3.3V, 24V or whatever the relay board itself needs.

Posted by: @_kaszpir_

Use MOSFET with octocouplers such as HW-532, or electromagnetic relay switches which can be controlled via octocouplers via 5v  https://www.explainthatstuff.com/howrelayswork.html

I actually use relays.  You can buy cheap relays on amazon.  Wire the power directly to the fan via. the relay (be sure to convert the 24V to the appropriate voltage of the fan), then wire the relay signal pin to the GPIO, so if the signal is high it will close the relay and activate the fan.

What kind of relay do you use? What specs should I be looking for? I have 2 24v fans (for enclosure) that are currently powered by the PSU and activated via a switch. I'd like to use the GPIO to turn the fans on when a print starts and stop when it ends.

Nonetheless I was thinking about using relays and stuff because I don’t like how turning fan on from GPIO disables the front buttons. But since Core One has been announced this whole GPIO/enclosure/whatever is obsolete now, since most likely it will not be reused for that upgrade. Money and time wasted, starts to become a theme with Prusa recently.