The 12VDC is on all the time. The PWM output determines the fan speed.
You could use the exhaust fan’s PWM to drive a relay. The PWM output is not capable of driving a relay directly, though. You would need to use something similar to this to treat the PWM* as a logic signal to drive the relay.
*Normally, a PWM wouldn’t necessarily be suitable to use as a simple on/off signal, but Glowforge doesn’t modulate the exhaust fan. They just turn it on full speed, which effectively is a digital on/off output.
So I have a couple of these left-over from a RasPi project I did a while back…
I suppose I could power the 12v relay module off the exhaust fan 12v, set the module to Low trigger and use the GF PWM for the signal “IN”; thereby using the relay load side to control mains voltage to my fan? Again, I’ve never done anything with PWM, so this is new for me… Don’t I have to worry about the PWM pulse causing chatter on the relay? Or is the pulse fast enough that the relay won’t notice? I noticed that the GF exhaust does come on at a slow speed when the machine boots up; would I have to worry about chatter at boot?
I considered putting a simple RC circuit on the PWM “IN” line to smooth any chatter. But again, I don’t know PWM well enough to make an educated guess on the config. A desirable side-effect of the RC circuit on the “IN” could potentially hold up the optocoupler, and by extension keep the relay closed for a period of time after the GF cut finishes, helping exhaust any final smoke/fumes…
I’m going to experiment with one of my relay modules on an old Dell computer with PWM fans and see what I see. If you have any further input, I would welcome it!
This is what the “fine print” on my response was referring to.
Glowforge doesn’t do any speed control for the exhaust fan. They set it to full blast. For the PWM output, this means either 0% (off) or 100% (on) pulse width. You shouldn’t get any chatter.
You would want to set the IN to trigger on HIGH, assuming you want to fire the relay when Glowforge commands the exhaust fan to turn on. The output voltage level on the PWM is only 3.3V, IIRC. That should probably be enough to trigger those devices.
NOTE: Be VERY careful about what you hook up to that PWM line. There is ZERO input protection on ANY of the IO pins on their factory control board. If you accidentally put 12V into the PWM output, you WILL fry the microprocessor (I know this from personal experience).