Need idea for motorized window blinds project

So my brother has a school project for making motorized window blinds.

The school says you can buy anything that will turn non motorized blinds into motorized for any price and they will pay the students back.

We got custom blinds on order for our small window and we have a motor setup for closing the blinds where the rod normally would be used but we ran into a problem.

How to rig a motor to pull the string which makes the blinds go up and down.

I want to try stepper motors but my brother thinks micro kill switches and brushed motors on a 9 or 12VDC setup would be better.

I was thinking of mounting a pulley to the motor shaft and attaching the string to it and when it winds one way the blinds go up and down to the other way...

Now that pulley would have to be fairly big to hold the amount of string for a set of blinds that will be going down 30 inches so its like, how do I get that to fit in the top part of the blinds where the mechanics all are?

Now the rules don't say it all has to be contained but we would like it to look clean.

Were taking that latch out that normally holds the blinds open so the motor will have to hold that string at one position.

So, any ideas on how to get a motor to pull a string and hold it in position while trying to keep it in that top section?

Thanks

it looks like we will be going with a 12vDC battery powered setup.

edit,
I forgot to mention although it isn't that big of a deal...these blinds must open by themselves during the day and close at night via a photo cell...we may go with a 24hour timer though and there must be a button to override this timer.

How to rig a motor to pull the string which makes the blinds go up and down.

I suggest a spool on the shaft (like thread comes on), wrap the string around the spool several times so it doesn't slip and hang a weight on the end of the string. That is the way the old radio dial cord used to work, except it used a spring instead of a weight.

Hi, how about using two plastic bottles and enough water to fill one of them. Use a fishtank air bubbler to compress the air in the full bottle to transfer the water to the other one via a tube. Tie one bottle to the open string and the other to the close string.
Advantages: Quiet!, No brake required to hold the blind up, automatic snag handling and recovery!, subtle action, easy to get the right amount of torque required, no limit switches required, can be manually over-ridden without damage, easily automated, extendable to other blinds.... (cheap as well)
Disadvantages: Heavy on the blind mounting, slow, hard to hide, water transfer tube routing is problematical, no easy way of stopping halfway if that was wanted, water will turn green with sunlight (add glycol antifreeze or such)...
Cheers.

The pulley and small weight sounds good..I might just get a motor with a good gear box that could pull the string strong and hold it in place to get the weight (another non-needed part) out of the build to keep it compact.