DIY automatic pophole opener

I've long toyed with the idea of installing automatic pophole controllers and I was wondering if Santa might slip one into my Christmas sock.  Unfortunately this looks unlikely as with four houses to cater for, the cost of the commercially available units is prohibitive.  However as I'm an electronics engineer by profession and worked in the control and computer industry for most of my working life I reckon if anyone can design and make one I ought to be able to.  I've looked at photos of the Gizmo type and I reckon its not that sophisticated that almost anyone with a basic understanding of electronics can't produce one.  Sourcing the parts looks to be the biggest issue.

I've a question however for anyone who has one and has looked at how they work in a bit more detail.  How is the door held open once it has reached it's fully open travel?  I assume that the motor drive is switched off by a reed switch operated magnetically, but is the motor braked or is there enough resistance through the gearbox to prevent the weight of the door falling and winding the unpowered motor in the reverse direction through the gearbox? If the resistance through the gearbox is enough to hold the door open it makes the design a whole lot easier than if a solenoid latch needs to be used to hold the door open.

I know there's been discussions on this topic previously and the ingenious use of a car aerial raise and lower drive has been used, rather than a DC motor and string arrangment, but I guess this requires a good deal of vertical travel space which might be available for a shed type of house but is not so convenient for an ark type of house.

Any suggestions or comments greatly appreciated.

HF

If you are at the fed hillfooter, have a chat with my OH, we have always used auto pop openers and he may be able to answer some of your questions.. :happy:

Hi, Wish I had your skills; as I don't then it had to be the aerial route for me as it solved the power on/off at-the-end-of-travel problem. I'm halfway through making one (had to leave for a while due to other pressures). i'm using a photocell switch kit from Maplin and a standard electric car aerial. To avoid the aerial space problem I've taken off the actual metal aerial and have the nylon toothed drive strap with  electrical choc-block connectors at the ends to give a stop position whilst being flexible. The drive to the strap is a screw, so there is no need for a lock on the strap. The operating cord runs over a pulley above the door. HTH

hillfooter

Have you thought about a counter balance to help with the movement of the pop hole and also to make it easier to keep it in an open position, ie less stress on the motor, gears etc.

Not sure if pop openers normally have a counter balance as I don't have one and have only ever seen pics of them.

BB

Big Bird is spot on (and I forgot to mention). Most pictures I've seen and the commercial versions us very thin, light doors, plastic or aluminium. I've gone for a thick ply door suspended over a pulley and counterweighted. Much easier to get a straight drop without jamming and fox proof.  Aerial was new; it didn't seem sensible to buy S/H and have worn or corroded parts. It was £19 on e-bay and appears well made with solid working parts and a good, weatherprood casing. Total cost will be £30 all in.

I'll have to have another look at the aerial option as it seems this solves most of the tricky problems like limit stops and weatherproofing.
 
Yes I was thinking about the idea of a counter balance as it's useful to have a heavy door so it maintains a good closing pressure when shut thus detering foxes from lifting it.  I was toying with the idea of running the cord from the door up over a pulley turning the direction at right angles so the cord enters the house under the roof ridge passing under the ridge along the inside of the house and out the back wall over another pulley and down to a counter balance.  The lifting gear would then be mounted on the back wall, out of the way of the pophole, and act on the counterweight. 

Using a light weight door might be more vulnerable to being lifted.  To benefit from the extra closing force of a heavy door though the counterbalance has to be decoupled in some way when the door is shut.  So the draw cord would still need to support the counter weight in the closed position to remove it's balancing effect.  I can feel the engineer coming out so I'll stop now.

HF