This circuit depends on the parallel port's internal pullup resistors to pull up enough to turn on the feed relay and not turn on the takeup. If need be, add a stronger pullup resistor if the internals are weak.

This alternating logic can also be done using an inverter.
The +5V supply must be regulated; since it's common with the IBM PC's parallel port, it must meet TTL specifications.

Crydom D1225 or equivalent panel mount solid state relays work well. But any solid state relays that can control inductive loads of 10 amps or more should work too.

When wiring ground to the parallel port, solder together pins 18 thru 25 together at the connector, and use all of them as your logic ground ("-" on the 5VDC supply in the above diagram).

Variac allows camera operator to control the overall power to the tension motors. Power resistors across the relay outputs (R) allows a small bit of power to leak to tension motors when relay is off, to ensure the film doesn't unspool.

In the opcsdefs.opc file, configure the tension command to control the port/bit mask on your parallel port, eg:

	tension a 378 01 00 0     # aerial projector tension: pin 2
	tension b 378 02 00 0     # main projector tension: pin 3
	tension b 378 04 00 0     # camera tension: pin 4
    

For more info, see this tension motor thread in the OPCS forum.