26 May 2009

An optoelectronics novice

Because the weekend has been spent almost entirely in the garden, there has been no time for any work on the railway. So I have been thinking and planning, but not actually doing.

The current thinking topic is electronics and control in the storage sidings. These tracks will go in first and I want to put in the wiring at the same time. Because the storage sidings are less accessible than the top tracks, not to mention invisible in normal operation, I want to be able to (a) switch all the lower level points remotely, and (b) know where my trains are so I can set the correct route for a given train.

(a) is relatively easy, if not necessarily cheap. I can motorise each of the points, and if I use the standard Peco PL-10 point motor I can also attach an accessory switch that is operated by the motor itself. This switch can be used to light LEDs depending on which way the point is set. So when the point is changed, the appropriate LED lights up to confirm that the motor has actually fired. Combine this with a probe-and-stud panel showing the track plan, and you get a handy control centre that shows the available routes. Doing this is probably within my electronics ability, if I think about it for long enough.

(b) is rather harder. In the real world, low-voltage track circuits are used to determine where trains are. This is not easily possible in the confined lengths of a model railway - or at least, not without electronics knowledge I don't have. MERG do all sorts of stuff in this area using comparators, but it's not for the faint-hearted.

So I might use an optoelectronic approach. Maplin sell an inexpensive infrared detector kit consisting of one IR LED and one IR phototransistor, both mounted in "side looking packages". With appropriate circuitry, a lamp can be made to light up if the beam is broken. My idea is to have a bunch of these positioned along each of the storage tracks: LEDs opposite phototransistors, acting like security beams. When a train passes along the track it cuts through the beams and causes indicator lights to illuminate on the control panel. With enough such units along each track, you get a signalman's-eye view of the sidings and can see, to whatever accuracy you like, where the trains are.

The theory is simple, but execution requires a lot of components and a lot of extra wiring. I will see how my ideas develop. But before anything else, I have to complete the trackbed construction. Next weekend includes the promise of some time to work on this.

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