Gordon H

I didn't say that is what I used it for, just that such a thing existed. I was using it as an actuator on a heavy duty camera panning mechanism.

These signals are not really suitable for proper automatic control because their state is non-deterministic, i.e. you don't set them to a defined state with the input (clear or danger), you just ask them to change from what they are now. With a simple system such as a reed switch it only needs one false pulse to make the whole operation appear incorrect.

Evidence points to a pull-up resistor to supply being more effective than a pull-down to ground in the majority of cases.

There is at least one digital servo that uses a rotary magnetic encoder rather than a pot, specifically the Hitec HS-M7990TH. I used one a while ago on a works project to prove to some doubters that commercially available servos could do the job.

Choice of mast size would surely be dominated by the rolling stock loading gauge rather than track gauge, so chances are HO masts would be a better starting point if you don't intend scratchbuilding the system yourself. Additional interest would be further enhanced if the wires were being used for power too.

'Lord President', surely?

Yes, I came to the MRC on a couple of occasions and showed Jon Hewitt and Brian Kirby how we had produced the headspans on 'High Gill', which had appeared at IMREX that year (1986). In those days we had the advantage of King's Cross models down the road who stocked all the brass sections needed. No doubt Brian got a discount, as I seem to recall him being one of their shop assistants. This discussion seems somewhat familiar, having cropped up a while ago in a different thread...

On a slightly pedantic note, a few things ought to be clarified: 1) The curve the 'suspension' wire forms is a hyperbola, not a parabola. 2) As such, the hyperbolic wire is known as the catenary wire. 3) What you have referred to as the catenary wire is usually known as the 'contact' wire. However, it is good to see someone else doing it themselves and encouraging others to have a go. I have been doing the same for over 30 years - though I take it a stage further and insist the system conducts too. I wish I could show you a video of our club layout 'High Gill' working this way on a BBC TV programme 29 years ago, but it's not my copyright.

Fair enough, I couldn't find it in my haste at the time. However on looking at the document, it is noticeable that there are plenty of words and warnings regarding supply voltage, but no mention of the current the system is likely to draw from the power supply. This may be why DCC users are likely to get better results, as most DCC systems can provide several amps. Try using a 12V 0.5A DC 'Wall Wart' instead and the results are likely to disappoint. They really should give a recommendation for the power supply current as well as voltage.

It's a pity Peco don't seem to provide a downloadable version of the Smartswitch instructions, so that potential customers can evaluate the system beforehand - and the quality of the instructions that come with it. It is fairly obvious they have tried to produce a system which can work with a DC or DCC system, otherwise why would they have produced a special decoder to go with the system? If it was going to be usable with DCC only, surely the decoder would have been included in the main board? One of the main considerations with any multiple servo system is the power supply, which needs to be capable of providing plenty of current without dropping out when several servos move simultaneously.

In my view, this is one of the largely ignored consequences of the 'Arduino Generation'. It's all very well being able to buy a few general purpose modules on the cheap, but then to be able to turn them into a reasonably presented and practical product is an entirely different matter. Most of the posts about Arduino and the like seem to come from people who have no end of good conceptual ideas as a result of bit of background programming knowledge, but then lack the hardware experience to take the next step. This is not a criticism, merely an observation based on what I read on forums like this, and see at work. Discipline specialisation may be good for productivity on a commercial basis, but for hobbyists it tends to limit the options. Personally, I work the opposite way round. As a mainly hardware engineer I design from scratch from the ground up, on the basis that the shape, size and arrangement of the eventual hardware construction is equally, if not more important than being able to program the thing later on. For me, it is rare to use any ready made modules except where there is an obvious advantage in doing so. This is especially so where such a design is expected to be supported for a considerable length of time. Manufacturers are renowned for making items obsolete at the most inconvenient times! Far better to include and assemble just the bits you actually need for your own requirements than to have to disassemble and rearrange someone else's general purpose items which are not necessarily designed with that in mind.

When you say 'turned down', I presume you mean the voltage? Usually it is not the voltage that is inadequate, it is the current that can be drawn from the supply. Servos can take quite a bit of current each, especially when driving a load. To minimise build up of the effect, it is critical that the servo endstops are set so that it is not continually trying to drive against an immovable object at each end. You will soon know if this is the case because the servo will seem to 'buzz'. One way round it is to disable the pulse drive after the servo has moved to its new position, but this requires the servo controller to have that facility. I don't know if the Megapoints driver has that capability. On some (e.g. MERG Servo4), it is an option you can select.

Sounds to me like an inadequate power supply - a common problem with multiple servo systems. Twelve servos can take a lot of current, especially at switch on.

Not so. If you look closely the final punctuation mark is a full stop, not a comma - so it is 12V expressed to the nearest nanovolt. It would take some doing to regulate your voltage to that level of accuracy!

Those are not cross arms, they are diamond frames. Almost completely different.

If your dates and magazine title are correct, then that was not my article. Mine appeared in 'Model Railways' in Feb '84. The reason I asked Michael as to where he got the dimensions for the kit was in the hope that he didn't blindly follow my dimensions from that article, as I did exactly what he suggested and tried to scale it from pictures and drawings at the time. No such thing as the ACLG in those days! On the upper arm 'triangulating' cross-braces, I seem to recall that some examples had them as a single brace, and some were formed as double braced (i.e. like an X). The other benefit of the cross-arm design was that not only did it not protrude over the cab roof (per 298's comment), but it also didn't protrude backwards over the switchgear area as much as a Faiveley would. The crossed nature meant that a reasonable height could be reached for much shorter lower arms than an equivalent diamond frame.

Slightly confused now... If the original cross-arm drawings are not available, what dimensions were used for the kit version?

There have been a number of articles written in the past about different ways to produce models of the AC electrics in 4mm, which if you can find copies might give you some starting points and ideas. The series I am most familiar with was published in 'Practical Model Railways' back in early 1986 and created by 'GWC1' of this forum. There seem to be a few copies available on EBay with a quick search.

Actually, the Lima pan heads and the springs were quite useful as spare parts. I used the heads for convenience when I was scratchbuilding a batch of cross-arms, and the springs found use on my scratchbuilt Brecknell Willis pans some time later.

The EM2 pantographs were very good representations, with quite fine metalwork. Unfortunately, it was a bit of a decline after that. The AL1 wasn't nowhere near as accurate, but that pales into insignificance compared to the abomination they came up with for the 86's when they arrived. Their '90' had a Brecknell Willis with extra arms to make it physically active, and was/is not really adaptable for fine scale operation. All of these at least had a roof switch to allow the user to try them out on the wires. However, none since the EM2 has really been usable that way. Even the EM2 needed its springs weakened, and ideally the head changed - as it was originally designed as a single rounded piece to ride over the wire 'fishplates' that their catenary system used then.

If you have ever seen 'Model Railways' Feb (I think) 1984, and a couple of 'BRMs' in late 98 you would know the answer to your first point. The first covers the scratchbuilding of AEI Cross-Arm pantographs, and the second covers my third rail shoegear implementation on the Class 508. My point really was that although I was able to do these things for myself, there are likely to be others who although they might be interested to do something similar, they could well struggle with the relatively intricate on-loco aspects for various reasons, and therefore give up at the first hurdle. If, as many Continental & Japanese manufacturers do, they at least provided the means to have their pick-up systems working as an option, that hurdle would be overcome. After all, the Tri-ang AL1 and EM2 had this facility over 50 years ago! I would be quite happy if a mainstream UK manufacturer took the trouble to make their pantographs and shoegear operational, if only to encourage others to take the plunge and give it a go. Thereafter, the catenary/third rail set up can be as accurate as you choose to make it.

On that basis, what practical use does running trains on a layout have at all? For me it's not the practical use that might come of such an effort, but the engineering challenge of making it happen that is of interest. If the manufacturers helped a bit by making some of these features a bit more workable from the start, others might be encouraged to give it a go too. There was a time when model railways was seen as a way to encourage engineering potential. Let's not try to kill it off by dissuading people right from the start.

Which is why I included your quote in my message to that effect. It is this certainty of knowing that it can be done that keeps me trying to encourage others to try to do the same, and not to simply accept what the nay-sayers might want you to believe. There are lots of examples of things that 'couldn't be done' until someone actually did.

Doing the whole task In software alone would certainly be difficult. That is why microcontrollers are used for this kind of thing rather than microprocessors, as they have the necessary hardware peripherals built-in. It just needs a bit of careful design and thought as to how these peripherals might be used to best effect with concerns like interrupt priorities etc. However, I am not convinced the ready-made PICAXE environment allows for that degree or resolution of low level control, instead using raw processing speed as a substitute for it.

Given that PICAXEs usually run a form of BASIC interpreter to make programming easier, it seems unlikely that code execution would be fast enough to decode DCC packets. Of course, they can easily handle the servo side of things, as that is the kind of application they are intended for. That said - there is a project here that claims to do DCC decoding using two PICAXES: http://www.picaxe.com/Project-Gallery/PICAXE-based-DCC-Accessory-Decoder/ However, note how high the clock rates need to be to do this task, and the fact that the job is divided between the two devices to make it work.