Thursday, May 22, 2014

Preparing To Throw An Electrified Frog

Please note that no amphibians were harmed during the production of this blog post!

I'm afraid that now the disclaimer is out of the way, the rest of this post will probably turn out to be quite boring in comparison; the title is accurate but rather misleading.

While I may have finally settled on a plan for the new layout (and no one highlighted any obvious problems with it) there are still a few other things to sort before I can start laying any track. The main outstanding issue is the matter of how the points are going to be controlled.

On Jerusalem I used a wooden dowel as a manual point control (think of it as a poor mans wire-in-tube control). This approach is both cheap and simple but it does have a few down sides. Firstly it can't be operated automatically and on Jerusalem at least I can't reach the lever to operate the point while stood at the front of the layout watching the trains. While I could fix one problem by extending the control so it could be operated from the front or the back of the layout that still doesn't allow me to control it automatically.

The other problem with the points on Jerusalem is that they rely on the switch rails to transfer electrical power and I've found that this really isn't very reliable, especially once they have been painted and ballasted, no matter how well I try and clean them. Many layouts solve this problem by switching the power externally to the point, often by a micro-switch connected to the tie bar so that changing the points changes the polarity of the frog (the frog is the point of the vee where the two lines leaving a point meet).

Given my current plan for powering the tracks I decided to try and create a simple electronic circuit that would combine changing the points with switching the track polarity. To produce the mechanical movement necessary to change the points I've opted to use a small 5v servo although I have yet to finalize exactly how I'm going to mount them to the layout and connect them to the points.

If you switch the track polarity using a normal switch it needs to be a SPDT (single pole, double throw) and such switches are also easily available as electromechanical relays. I thought they should also be available as integrated circuits which would draw less current than a relay, but after a long search and a long discussion on the MERG forum (sorry, but that is only accessible to other MERG members) it turns out that if such an integrated circuit does exist it would be prohibitively expensive to use.

So what I now have is a simple Arduino powered setup that on a button press will switch a relay and move the servo arm. I'm actually using a double pole, double throw (DPDT) relay as that opens up a few options which I'll return to in a later post. Anyway here you can see the whole thing in action.

Each time I "press the button" (rather than a button I'm just pulling the pin low by shorting it to ground) the servo switches to the other position and the relay changes over; hopefully you can here it click in the video over the servo whine. I've currently got the servo switching between 45 and 135 degrees so there is obvious movement. Obviously I don't need that much movement to throw the point, so this will need configuring once the servo and point are connected. You can also see that the light on the Arduino goes on and off to signal the position of the servo and state of the relay, which will be useful on a control panel for showing the position of the point. At some point I'll remove the need for the Arduino (like I did for the flickering fires in the mill on Jerusalem).

While it might not look like much, I'm quite happy with how it all works as I can see how I can combine this with some other ideas into an integrated control panel which should give me both full manual control over the layout as well as the possibility of running some trains automatically. Next up will be connecting this up to an actual point and configuring the servo properly.


  1. Are you using your Raspberry pie in the clip. Dad

    1. No just the Arduino, I didn't need the extra processing power the Raspberry Pi would give me.

  2. I can't believe how mesmerising that was!

    1. Sitting there making it go back and forwards is highly addictive! You'd be surprised at how long I've sat playing with it.