Building the MODRATEC Lever Frame - Stewart McSporran

© The content of this article copyright 2005 Stewart McSporran / The 2mm Scale Association. Send email for Stewart McSporran via MODRATEC.

Late in 2004 someone posted a message to the VAG [Virtual Area Group - ed] (or maybe the Templot group, I can't recall) mentioning that a company called MODRATEC ( had released a new kit for building fully interlocked signal lever frames. So I took a look, liked what I saw and bought one. This article gives an overview of their system, plus some pointers, found the hard way, to building the kit.

Designing the Frame

A clever feature of the MODRATEC system is that they have released a small Java application, called SigScribe4, which enables one to draw a track diagram of the area to be signaled. The diagram allows the specification of turnouts, catch points, ground signals, facing point locks and a myriad of signal styles. Once the diagram has been drawn the user then sets up routes through it, and assigns signals to control those routes. At this point the signals may be interlocked with each other and the turnouts. Figure 1 shows the SigScribe4 screen with my completed track plan on it.

Figure 1: SigScribe4 Track Plan [Click for full size image]

The yellow dashed lines on the track plan show the currently available routes; you can see that both are blocked because the up and down home signals (numbers 2 and 17) are on. The letters under each of the levers, at the bottom of the screen, indicates if that lever is locked or free at this moment. SigScribe4 allows the levers to be pulled, enabling the interlocking to be tested to one's heart's content.

The square section to the lower right of the screen shows the selected signal and its arms. It is currently showing the up home signal (number 2). Other signal arms may be added to this area, allowing for the setup of bracketed signals.

Bear in mind that the SigScribe4 user interface is a bit idiosyncratic and requires careful reading of the help pages. You can't just pick this one up and run with it!

Once you're happy with the design, and provided you're connected to the internet, you can select a menu option to obtain a quote for a kit. The price for the example shown above is $136 Australian, approximately £56, including the optional micro switches for each lever. These kits are heavy and postal charges are high; mine weighed nearly 2.5Kg and cost £25 for delivery.

Prior to purchase you e-mail your SigScribe4 file to MODRATEC. They check it for glaring errors and use it to generate a custom construction manual. This is e-mailed to you as a very large html file. Mine came to forty printed pages!

Some Terminology

If you're not familiar with the terminology and workings of signal interlocking then you'll find the manual confusing until you've read it a few times. I've summarized the essential points here:

Drawing 1: Locking Parts Identified

Drawing 1 indicates the principle parts of the system. Levers are said to be either Normal, pushed back away from you, or Reversed, pulled towards you. Tappets are rectangular brass plates that have notches at critical points. The levers act on the tappets via the lever links to move them forward and back. The locking bars have pins in them that act with the notches in the tappets; the tappets may either be free to move, and may cause the locking bars to slide from side to side as the pins are pushed out of the notches, or they may be locked by one of the pins.

This is best illustrated by an example. In Diagram 1 all levers are currently free to move. If we move lever 3 to its reversed position then its tappet pushes the right hand locking pin to the right (by virtue of the notch in the tappet). This state is shown in Diagram 2.

Drawing 2: Lever 3 Pulled & Locks Lever 2

With lever 3 pulled we can no longer pull lever 2. In order to move lever 2 it has to move the left hand locking pin to the left. However it can't because the right hand pin is jammed against the tappet of lever 3. Just repeat this logic a few dozen times and you've got the full locking plan.

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