Automatic Door Opener – Part 1

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We got a dog and with it quite a bit of hassle. Dogs are a hassle. The constant-eating-stuff thing is manageable, and well documented.

We have an okay-sized house and an okay-sized garden and I took it on myself to somehow convert our back door into an automated slider, perhaps with some kind of dog detection, perhaps with some sort of remote operation functionality too; so the dog could have free realm of both out and in.

I plan to document the develop in three posts:

    1. (This post) Basic description and the hardware.
    2. The Arduino-based electronics.
    3. The software, using an old Android phone, and a roundup of the in-place system.

Basic System

The basic system consists a 3D-printed unit fixed up into the door jamb which houses a geared DC motor. The motor is connected into a gear/sprocket assembly that drives a chain belt which runs along the head of the doorway. The chain loops around an idler sprocket housed in another printed unit screwed up into the door frame.

The chain is fixed off to the mullion of the sliding door so when the motor is powered up,the door moves with the chain. This is all pretty similar in principle to commercial sliding door openers you see in shops.

The interesting part is an Arduino micro-controller wired up to an old Android phone which is used control the DC motor.


Parts List

  1. A black and decker battery-powered screwdriver.
  2. 2x Shimano 14T gears from rear cassette of bicycle chain drive
  3. 1 and-a-bit Shimano bike chains linked together to make a longer one
  4. 3x “608” size bearings – skateboard bearings by another name. I actually recovered mine from a suitcase left on the roadside. Useful things.
  5. Assorted nuts bolts and washers, mostly M8 (for the driveshaft and the idler shaft, to match the bearing) and M4 nuts for the fixings holding the two housing assemblies’ parts together
  6. Wood screws for fixing into door frame
  7. Handmade aluminium “key” and a filed M8 bolt for the drive assembly – see below.
  8. Assorted 6mm alu. plate elements for the drive assembly and the base plate to the idler. I cut these to size in my garage.

The Door

In New Zealand a a bog standard residential sliding door is known colloquially as a ranch-slider. Don’t know why. They are lightweight aluminium fully-glazed framed sliding doors.

When we first arrived in New Zealand our landlord, when showing us around the house, noted that the deadlock on the ‘ranch-slider’ was broken. To lock the door you dropped a cutdown broom handle into the sill channel.

Good solution. Smart.

Having been in New Zealand 8 years now I have realised that this was not my landlord’s ingenuity. Everyone has these ranch-slider doors, all the dead locks are broken and everyone uses a broom handle.

I am going off track.

‘Power Train’

The first hurdle to overcome was to make sure system had enough oomph to open the door. I used a luggage weight scale to find the load needed to move the door. Scale read around 4.5kg, so 45N, with a reasonable tug to get door moving.

The 14T sprocket gears have radius of around 28mm. This equates to a torque requirement of around 1250Nmm (11 lb-in in american) to the driveshaft.

The Powered Screwdriver

This was a bit of a find – I was debating using one of my valuable CNC-to-be stepper motors which would have been a real waste.

The screwdriver cost 10 dollars from the Warehouse. It takes 4 AA batteries (so 6V). The manufacturer’s data sheet lists a torque of 20 lb-in (so around twice that needed).

Really easy to open up the tool and have a look what is going on inside. Beside the batteries there is a high speed DC motor and then a neat, compact rotary gearbox.

The Black and Decker Screwdriver gearbox

I could have designed and printed a gears system (I did actually for the stepper motor setup I started with) but it would never have been as tidy as this; so for 10 bucks, why bother?

Plus, as long as I can live with Black and Decker’s loud orange livery in the living room, I believe I can stick an Arduino Micro and the other electronic gubbins in the screwdriver’s handle (which normally houses the batteries).

The batteries go into the handle – should be able to electronics in there


Bike chains and sprockets were the cheapest way to go – they were free. I found that asking round the local bike shops, they invariably had a skip/ bin out back full of bits they had ripped of bikes they were working on. And the bike guys were happy the bits were getting a new home. I just soaked the parts in turps overnight and they were sparkling new in the morning.

The fourteen tooth sprocket (14T) was the smallest useful one.

The 13T, which seems to be the generally the smallest on rear cassettes, would have been better mechanically but it has a weird flange on it which would have just added hassle.

Splines and Driveshaft

The splines are the little bits on the inside of the sprocket that engage with the bike wheel hub.

Shimano 14T (Ultegra I think). Note the splines in inside ring.

I measured the splines with callipers to design parts that transfer torque from the driveshaft to the sprocket. Image below with an aluminium key is what I same up with – was just an excuse to invest in a pillar drill to be honest (yes, the cheapo automatic door opener is starting to get expensive!).

The aluminium key and sprocket assembly. A worm screw goes into the tapped hole in the alu key that locks it off to the driveshaft

The aluminium key and sprocket assembly. A worm screw goes into the tapped hole in the alu key that locks it off to the driveshaft

The Black and Decker has a female hex socket at its end – I connected this into the drive shaft simple by filing around the thread end of the main M8 bolt (slowly and carefully) to an approx hex shape until it fitted into the socket.

Using the bolt’s hex head is useful for this – you put the bolt head in your vice and file on horizontal plane. You then rotate the head a turn (60 degrees) and do it again etc. Photo below show a pretty job, but it works okay.

For the idler end I used a similar design but the sprocket just sits around a bearing, spinning freely.

3D Printed Parts

These were all designed in Inventor and printed off on my RepRap. Parts are not pretty in terms of their design or finish, but the good thing about my RepRap is that the parts it spews out are solid and dimensionally accurate. I ran out of PLA so had to take the plunge with the highest temperatures of ABS – this meant an overhaul of the RepRap midway through the design. ABS is more stable and harder wearing than PLA so environmental factors aside I might stick with ABS.

Render below shows the various parts assembled in Inventor. I tried to add chain to the model and animate it but my laptop spat the dummy – memory issues I think.

The power drive housing. sorry about the render, got carried away in Inventor

The power drive housing. sorry about the render, got carried away in Inventor

The idler housing

The idler housing




GIF (wow, I found a use for them with not cats in) shows the printed parts all assembled in a mocked-up door frame. Next step is the electronics.

GIF of mockup

GIF of mock-up