Using a stepper motor to operate a window vent

I want to automatically open and close a greenhouse window vent (weighs about a pound) using a motor of some sort. What I have tried is using a spare motor connected to a belt which turns a 5.5 inch diameter pulley, and a three inch long arm on the pulley shaft moves the window (see picture below). Problem is the motor just doesn't seem to have enough power once the the belt is tensioned. It is a 12v bipolar motor and I am using four MOSFETS to run it. Is there a smarter way to do this? Or perhaps a bigger motor: http://www.ebay.com/itm/160746561872?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 ? And where might I find a suitable belt?

If you want to do this right - you either need a gear motor, or more appropriately, a linear actuator.

If going the gear motor route, a DC gear motor, and not a stepper would be best (while you can find stepper-based gear motors, they are not as readily available as DC gearmotors). You would likely also need to implement some end-stop limit switches into the design (very easy to do - see this thread: http://arduino.cc/forum/index.php/topic,6426.0.html).

Alternatively, a linear actuator can be used, but they can be pricey, unless you shop really carefully in the surplus market (even then, expect to pay around $30.00 minimum, at best). Most have built-in limit switches. Some have position sensors (potentiometers or other methods) as well. You could also build your own using a motor, some all-thread rod, and a nut or two (google "homemade linear actuator" and such for some hints).

Finally - go down to your local RV / camper shop and take a look at the various venting devices for campers; most use a simple crank attached to a combination linear actuator / scissor lift system (or parallelogram / pantograph mechanism) to open the vent. Almost exactly like what you want to do.

... or look for an electric window lifter from any local car breakers.

… or look for an electric window lifter from any local car breakers.

Ditto! Particularly the type that are arm worm gear driven in that the position of the vent will be held when the system is static. Come to think of it, any window regulator drive system will hold it’s position. - Scotty