Part 4 referred to a DC current used to power Tortoise by Circuitron (“Tortoise”) barriers which provide some measure of safety by blocking the passage of rolling stock when the bridge is raised. While track power is removed from both wings of the NDM when the bridge is raised, in some situations this safety measure can be innocently defeated, such as:
- If one happens to be backing a lengthy train out of a siding on an adjacent module the rear end of the train could easily fall through the open bridge if the operator is not paying attention.
- If a multiple unit consist is being operated with motive power located mid-train some locomotives are sufficiently powerful to push the front unit into the void, again if the operator is not paying attention.
- If a locomotive is equipped with a Keep Alive or similar super capacitor the run time of the locomotive in the absence of track power may defeat the dead track section, again if the operator is not paying attention.
Notwithstanding these safety precautions, operator error is the fault of the operator and there is no way to prevent all permutations of “stupid”.
The 12-volt DC power used to operate the Tortoise barriers is supplied by the very simple use of a bridge rectifier which taps AC power from the Free-mo accessory bus and converts it to DC power. Bridge rectifiers can be obtained from several suppliers including Mouser or Digi-key (see my Links I Like page).
Before this project I had always respected the warning clearly stated by Circuitron, “Warranty is Void if Label is Damaged or Removed”. The only way to open one of these by unscrewing 5 Philips screws is to damage or remove the label (i.e., they don’t want you taking it apart).However, having used many Tortoises over the years for their intended purpose of operating turnouts and their very useful properties of not only containing stall motors which draw very little current when “stalled” while at the same time exerting considerable torque, I suspected that repurposing a couple of Tortoises would serve my purpose well. Besides, I like taking things apart to see how they work so this was the perfect opportunity to do so with a Tortoise.
Inside the very robust Tortoise is a simple plastic gear train, part of which consists of a strong shaft the cross-section of which is in the shape of a cross, as in a + sign. By drilling two small holes in this shaft about 1 inch from each other it was possible to bend a piece of piano wire to pass through each hole in the shape of a flat-bottomed “U” and firmly affixed in place with epoxy. With a little “Dremel surgery” to the green outer case I was able to have the two ends of the flat-bottomed “U” protrude out through the case. I bridged these ends with a couple of pieces of flat styrene sandwiched together. When 12-volt DC power is fed to the Tortoise in one direction the piano-wire mounted styrene flips in one direction and when the 12-volt DC power direction is reversed the styrene flips in the other direction. I made two such Tortoise barriers and mounted one on each side of the bridge opening so they both flip when the bridge is opened and down, out of the way, as the bridge is lowered.
While my Tortoise barrier design was very reliable and robust, in practice it had two drawbacks, as follows:
- The barrier located at the hinged end of the bridge gets caught between the bridge and the wing if the bridge is lowered too quickly. This is not a problem if the NDM is being used by my local Free-mo group because all its members are well aware that the bridge must be lowered slowly. However, when used in a setting with many operators such as Big Valley this becomes a hindrance. Therefore, I inserted an Anderson Power Pole connector in the circuit of this Tortoise barrier so it can be deactivated when desired. In hindsight, this barrier is not as important as the one on the open-close end of the bridge because the bridge itself provides a reasonable physical barrier by itself when open.
- The barrier at the open-close end of the bridge works very well at preventing rolling stock from falling into the chasm. However, when the barrier is up, and operators are walking through the open bridge, loose clothing or clothing ensconcing a “wide load” can get snagged on the barrier.
My intention is to redesign the barriers, so they are not located at the very ends of the bridge gaps. Instead, the Tortoises would be relocated to be underneath the wings on each side and raise and lower some sort of rod, gate or bar located under the track or on top of the module.
The remainder of the wiring of the NDM is quite straightforward. However, it made ample use of different coloured Anderson Power Poles as well as differing stacking of same in order to foolproof the connection of the many wires during setup.
Note that I did no adhere to the Free-mo standard of including a Digitrax UP5 in the NDM for the simple reason that, with power being cut to the NDM track each time the bridge is opened there seemed little point in including one of these. Besides, in my experience, few low-profile traditional duck-under modules include a Digitrax UP5 either.
To be continued...
