Monday, 9 September 2013

Electro-mechanical reversing switch...

Combining modern electronics and old motors (that are capable of drawing high current and creating all sorts of electrical 'noise') is not impossible, but sometimes it is far easier to revert back to a simpler solution. The display in the cabinet it the Eltham Centre (see previous post) has an electronic controller that employs a touch pad to to start a display cycle of 30 seconds. This miracle of modern electronics was designed and built by myself with the assistance of Tim Sertell. That of course is a bit of journalistic licence, as Tim built it and I just stuck my two-penneth in here and there - but is is my blog so I have to be a little bias...

...anyway the microprocessor controller did the job last year with no trouble at all. However that was before we were presented with the problem of reversing a couple of kilos of narrow gauge loco on a short piece of track. Initial experiments by DT and his team, carried out at the Henley Gathering, proved that you can make very pungent smells and destroy the electronics  with one  push of the button. Further experimentation at DT's electronics research facility (the living room floor) culminated in the discovery of a new way to weld relay contacts together. After this appalling display of ineptitude by the whiz-kids, I thought I should step in and help them out before they got too upset and needed medication.

The finished switch. Click on the picture to enlarge the image

I can't take all the credit for this idea as it is based on a reversing mechanism build but my good friend, Chris Shute. He worked out the MO motor switch could be mechanically operated by adding a control arm to it and capturing the lever in a rod and strip connector. By linking this to a motor the lever of the switch can be repeatedly moved from one position to another. The rate of movement can be varied by changing the gearing or altering the speed of the motor.

A narrow strip and rod connector fixed to the switch lever

The automatic reverser consists of an MO switch with the centring spring removed mounted on a flanged plate. A channel bearing, with a double arm crank attached to the upper row of holes, is bolted to the top of the switch. A bush wheel fixed to a short axle is journalled through it. a cranked arm made from a pair of 1 x ½ inch brackets attached  the bush wheel to the  additional operating leaver that has been added to the switch as described above. The rest of the gearing can be altered to suit the application. On my version, the double bent strip is there to stabilise the meshing of the plastic gearwheel to the worm.

Rubber feet!

Small rubber wheels/tyres are bolted to the underside of the flanged plate to reduce vibration and to give it some grip on a shiny surface. There is a vast range of Meccano tyres available now and many of the smaller rubber ones have lots of 'non-wheel' applications.

Next job is to get it wired up...

The rebuilt switch before fitting (Photo: DT)
So I sent DT off to fit the device, and guess what, he had trouble with the Meccano switch. They can be a bit temperamental so after a bit of messing about he gave up and built his own switch with the contacts much wider apart - worked fine (even if it is ugly!) Now all we have to do is see how reliable it is, hopefully it will do the job for the next few weeks! I must give him a lesson on how to make Meccano look pretty!

The photograph shows the the rebuilt switch and a pair of diodes, that have been fitted to angle brackets, in the foreground. The angle brackets slip nicely into the gap under the rails, where the tinplate rail is formed, making good electrical connection. The loco will run towards the end of the track until it reaches the a gap in the power rail. This gap is bridged by one of the diodes (Diodes will only allow the current to flow in one direction). The loco will stop at the gap and will not move until the motorised switch reverses the current being supplied to the track. The diode will then allow the current to pass and the loco will set off in the opposite direction. A similar diode arrangement exists at the other end of the track.

If you are in the area pop in a take a look - if it is not working - call DT!

Ralph.

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