Automatic door opening system

[mr-cecil]

I'm starting to think about making an automatic door opening system for the polytunnel.

I'm wondering if I can re-purpose a greenhouse vent opener such as this: http://www.amazon.co.uk/Gardman-Greenho ... o_pC_S_ttl

What's not clear to me, is if the vent opener relies on having the weight of the window for it to work. If this is the case, then I guess it would not work.

I'd be interested to know if anyone has tried something like this before / or has any alternative ideas......

Thanks

[peter]

Vent openers cannot lift above a stated wright, but I think for closing they would need some weight, you could compensate with a spring to close the door, but where you could get one, apart from bungee cord?

[mr-cecil]

Good point, it might need the weight for the closing, but not need it for the opening.

In that case I could probably use something like this:
http://www.amazon.co.uk/Securit-Black-A ... K5RN67YBS8

[peter]

Depends how the strength of the spring translates into a moment (?), weight equivalent.