I just removed the low pressure valve portion of Amazon auto drain valve from the solenoid timer/coil assembly by loosening the nut and sliding the coil off the valve stem. I then removed the coil from the high pressure valve and replaced it with the timer/coil assembly. I then plumbed it to the outlet of the water separator.
Works perfectly. Both delay and duration are adjustable.
I did add a flow control valve to the outlet of the drain valve to restrict the air flow so It would drain slower (and not spray all over) and so it would loose less air pressure while draining. The noise level is similar to opening the tire stem valve on my car. ($12.00 w/free shipping)