Thanks for the confirmation Tim… I don’t really see how it works as described. I did find another example of a single transistor inverter but using a PNP transistor and that makes a bit more sense to me but if it does work it is obviously something I mis-understand so I will shut up!
PWM _In goes to FAN -ve, ground to RAMPS/Mega2560 ground , 5v to Mega2560 5v and TTL_Out to laser PWM_In