Electric sliding door

My house is mostly open plan, so the lounge, dining room, hallway and the first floor landing/study areas are all open to each other. This is fine most of the time, but music played in one room is heard all over the house whether you like it or not, and it can get chilly on a winter’s night so I needed to be able to close off part of the space.

I built a partition across the end of the dining room, with a sliding door to cover the last metre or so. To allow plenty of light into the hall, the partition was built with a wooden frame and large glass panels – great, except that the sliding door weighed a ton and was hard to both move and stop as a result, and I was concerned that my small grandchildren might squash fingers and/or other body parts in it. I decided to motorise it, and this is a description of how I did this. I havn’t included any details of the construction of the partition itself, if you are interested in knowing how this was done, let me know

I started with hanging the door on a standard sliding door gear, a simple track at the top with trolley wheels attached to the door, and a plastic block screwed into the floor to guide it. The bottom of the door had a slot machined with a router, and a length of aluminium channel screwed in it to allow the plastic block to slide smoothly. This picture shows the door initially hung on the door gear and before the glass went in - it would have been too heavy to move around if the glass had been installed first:-

I originally planned a fully automated solution where the door opens automatically when a warm body approaches, but quickly realised that this kind of sophisticated solution would be pretty complex – for even the most basic automatic operation the door would need a variety of safety features to stop it at the end of its travel and detect obstacles in the way. I wanted the operation to be simple and foolproof, so opted for a more manual approach using readily available parts.

First, the drive motor. This needs to be fairly powerful to shift the weight, with a high starting torque, but it doesn’t need to run fast. A rummage in my junk box produced a 12 volt electric car window winder motor which is ideal, and a bit more digging produced a set of electric window switches out of a Renault 25. The motor runs in either direction simply by reversing the power, and the switches are designed to do the reversing of the power connections. I obviously needed a switch on either side of the door, and I also used the ‘window lock’ switch mounted high up so that I could turn the whole gubbins off to stop the children (and some of the grown-ups) playing with it.

I needed a robust mounting for the motor so I welded up a small frame from steel tubing and screwed it rigidly into place onto the framework in the space above the partition. The motor is mounted on an aluminium plate which has a short alloy tube fixed into the corner angle with short screws and epoxy glue (I know, it sounds like a lash-up, but it works fine). The tube has a pair of ball bearings pressed in and a short spindle which carried the cable drive at one end and a coupling to the motor at the other. See picture at right

Connecting the motor to drive the door was achieved by means of a length of thin steel cable which runs in a loop along the top of the door – I used a gearchange cable from a mountain bike, but you can buy cable like this by the metre from a DIY store. To drive the cable, the motor was fitted with a drum (mine came from a long forgotten photocopier, but it would be easy to make one on a small lathe) which the cable was wound round several times. The same copier produced a set of small guide pulleys to steer the cable.

Both of the cable ends are clamped to a plate screwed to the door using the ‘bolts-with-holes-in’ used on the brakes on bikes, these allow a neat fixing and also allow you to pull all the slack out of the cable so that it remains tight at all times – if there’s any slack in it, the drive pulley will slip and the door won’t move.

The initial trial run proved that this system worked, but the motor ran much too fast and the big heavy door took off like a tank with a baboon at the wheel, a bit unnerving to say the least. I bought a cheap motor speed controller kit from Jaycar and used this with trial and error to get the speed to a point where the door moved at a sensible speed and the weight was properly controlled, with major benefits to both my blood pressure and the structural safety of the house.

The last thing was a fail-safe method of stopping the door at both ends of its travel, even if the operator keeps their finger on the button. A simple micro switch is mounted at each end of the sliding door track, with an adjustable plate screwed to the ends of the door, so that the power to the relays is shut off when either of the switches operates - see picture at left.

The operating switches are not man enough to handle the power on their own, so they need relays to do the heavy duty switching for them. These are standard 12 volt car relays which you can pick up from a junk yard or buy new from a shop like Halfords (in the UK). Because of the need to reverse the power to run the motor both forwards and backwards, and to be able to operate the door from two switches (one either side of the partition), I ended up with four relays - I had plenty in the junk box, but it may be feasible to use less.

The switches have built-in tiny bulbs, but on mine most of these were blown, and they use too much current anyway so I carefully prised the switches apart and replaced them with 3mm LEDs. Remember that my house is very dark at night, so these show you where the switches are. Once all the wooden cappings were fitted around the partition, I added a pair of decorative alloy plates to the switches to give a neat finish.

Then I started to think about how it should all operate safely. Obviously the door should only work when one of the switches is held down, and second it must not trap the occupants if the room if either the power or the mechanism should fail. I tackled these issues in multiple ways to give a belt and braces solution.

To solve the power failure problem, I opted for driving the system with a small sealed lead acid battery of the type used in burglar alarms. This drives the motor, speed controller, and LEDs using 12 volts DC without the need for a mains transformer, and it keeps the door working for days without any mains power. To keep the battery topped up at all times, I installed a battery charger which is permanently connected to a power socket installed in the space above the door.

Other occupants of the house were understandably nervous that not just the power but the whole contraption might fail, leaving them trapped for ever behind the door – a valid point, as we’ve had one such failure when a poorly attached battery terminal came unhitched. The metal plate that clamps the ends of the cables is attached to the door with a pair of 10 mm diameter threaded rods that extend right through the door, with a slot cut in both ends so that they can be removed from either side of the door. When the screwed rods are removed, the plate is left hanging loose on the cables and the door is free to slide on the track. In the room 'inside' the partition, the space above the door has a couple of lift up flaps supported on short gas struts and contains a panic box containing a screwdriver for removing these screws as probably not many people watch TV with a toolkit in their pocket.

The panic box also contains a spare motor and a set of spare switches as insurance in case any of the parts fails and I can't remember where I put the spare bits, as I rather doubt that you can still buy window switches from a 1991 Renault 25. As a last resort there is also a spare door key so that if everything jams up and none of the emergency systems work you can always bale out of a window and get back in the house by more traditional means.

There is a small control panel above the door with LEDs to show that power is present, and although this was not its original purpose this has the massive advantage of shining a dim light down on the face of the door at night to save you the inevitable broken nose caused by walking into the closed door in the dark.

I originally thought it would be neat to shine LEDs into the edge of the glass panels to give them a greenish glow and spent an awful lot of time drilling holes and running wires to achieve this, but the end result is that the glow is very dim and only visible when all the lights are off, so I never switch them on. Brighter LEDs would work better, but if I was doing it all again I wouldn’t bother. These LEDs are operated by a switch on the panel above the door, with another small LED to show when they are on or off.

Conclusion

The door mechanism works really well, in spite of being made almost entirely from junk. Anyone can operate it, even when carrying an armful of stuff as the switches can be operated with an elbow, a nose, or any other sticky-out bit. It keeps working during the regular power failures that we get, and so far we havn’t managed to squash anybody, small or otherwise.