Making Of

To solve the equation (rotary_dial + transmission_chain + Ikea_mackis) = Wedding_present some more stuff will neeeded.

For example a motor to actually move things. Yes, weddings presents must have motors. They do – ask anyone!

The chain moves 6mm (one link) for every tooth of the sprocket. With a 24-tooth sprocket rotating at 5 rounds-per-minute (rpm) The chain would move 120 links per minute or 720mm per minute. That is 12mm per second or 10s for 12 cm. At 10 rpm this would shorten to 5s for 12cm – sounds good.

Opening for 12cm sounds about right. Maybe 15cm or a little more, we will see that. 10s opening time for a full move is not too bad either, maybe a little faster would be good. So the design calls for 5 to 10 rpm on the chain sprocket.

I had a motor lying around that does about 20 rpm at 12V (after its internal transmission, wich is a 1:200). So dividing that by 3 puts us well within the target corridor. So while I was shoppoing at Conrad I picked up some modul-1 gears that I wanted to use for the secondary transmission.

They are 10 and 35 tooth. The resulting 3.5 ration at 20 rpm input gives us 5.7 rpm for the sprocket or 13,7 mm/s opening speed – that is fine. Now a little trip to Bauhaus – a chain of home-depot like shops selling construction stuff and also some metal.

They should have the 8mm axles I need. But of course they don't. As usual, the stuff you need is sold out. In 3 shops. Bummer.

Well, a tube with 8mm outer diameter should be fine as well – and save on weight :-)

The gears have a 6mm bore of course. Well, that can be helped, we will see that later.

More Power!

The whole thing needs power, of course.

The usual hase-style packaging has internal batteries. Batteries are completely fine. Unless you actually want to use them.

On the Brio-box for Ela and Michi, Matthias an me used run-of-the-mill battery holders. The plastic type you find in a thousand battery operated devices. They broke on the transport from the workshop to the wedding. We had to fix the whole thing in a terrible botch-job on site.

The batteries for the Brio-box for Susanne and Toby were soldered on for this reason. Too bad that they had been sitting on the shelf for about 20 years: they worked quite well in testing, but then the inner resistance started to grow and two days before the wedding (on a final test) they were unable to provide even the 300mA the motor needed. So on the day of the wedding, I replaced them with rechargeable batteries that basically became the present for Toby (I knew he needed replacement accumulators on his sailboat anyway).

No batteries this time! More Power. Mains power. Bwahahaha!

Electricity bites...

There is just one small problem there.

The reason I switched from fiddling with radios and stuff to computers in 1981 was this: electricity bites you.

At least one radio had biten me again that day when I read an ad for the Sinclair ZX81. The idea of „9V power supply“ started to grab hold of my brain: all the can-and-will-bite-you voltages locked away in a power supply, no voltages above 12V in the whole device and it still can do something fun? Computers were for me!

... when used improperly

I applied the same principle here: make proper use of electricity: lock away the dangerous voltages in a power supply that is known-good. Or at least comes from a big manufacturer that will take the blame if something goes wrong :-)

I turned to Ikea again, since they have 12V lamps that come with external transformers. The transformers are not awefully efficient (touch one that has been plugged in for a day: its warm – all that energy you pay for on your electricity bill), but they are known to be safe. They do not sell the transformers separately, so you essentially get a free halogen-bulb lamp with it.Which can be incorporated to the overall design.

I chose the Ikea Korsby

• it has two lamps of 10W each, so the transformer is rated for at least 20W of power - sufficient for the motor.
• It has a builtin switch (to the left in the picture) The switch has 3 positions (left-middle-right) that I could use as an additional input
• The bottom plate is screwed on with 3mm screws. I could easily replace that bottom plate using longer screws.

Incorporating the lamp also makes the whole thingy an object of day-to-day use and will remind Anna-Barbara and Micky of their wedding day for as long as they keep it around.

Summary at this point

I think we have all the components together now.

• a power source (12V AC transformer)
• a motor to move stuff
• transmission parts to provide the necessary torque and slow speed
• transmission parts to apply force where needed
• a base frame made of easily-modded plywood

Seems it is time to get started on constructing the electronics that will control the whole thing and start putting together the mechanics as well.