Modular Signal System Implementation

Overview

This page serves as documentation for the circuits that I have built which make use of the Modular Signal System first documented by Gregg Fuhriman in Railmodel Journal, and further documented on the modularsignalsystem.info website.

This effort was an experiment on my part in designing and building a circuit board for the circuits involved. This effort documents two circuit board designs, one for cascade modules, where signals are located, and a second for detection modules, which go between the cascade modules. These terms are further explained in the link above.

The Modular signal system requires a combination of optical and current sensing detectors for reliable operation. This is required on a modular layout becauase we cannot assume that every participant will have resistor equiped wheelsetson all rolling stock. For this project, I used Rob Paisley's Block Occupancy Detector helper as the optical detector. I incorporated a BOD helper into each of the circuit boards, along with a 5vDC power supply for powering the detectors and the signals at cascade modules.

The two circuit designs are shown below, along with the data files needed to submit the boards for production using ExpressPCB

The Cascade Circuit

The cascade circuit includes BOD helper and outputs to connect signals control boards for each of the two blocks the cascade module influences. The circuit details are below:

• 
• The Schematic file for the ExpressSCH program
• The PC Board layout for the ExpressPCB program
• 

The Detection Module Circuit

In addition to the BOD helper and voltage regulator, the Detection Module circuit includes a connection for an NCE BD20

• 
• The Schematic file for the ExpressSCH program
• The PC Board layout for the ExpressPCB program
• 

Notes from Testing

My test configuration consists of one cascade board connected to two Atlas signal control boards, as pictured below

Testing has resulted in the following observations:

• I had to tune the LED output resistor (R5) of the BOD helper circuit to get acceptable results. The final resistor value was 640 ohms.
• The circuits work as they should when only my circuit boards are involved.
• The only testing I have done so far with other circuits has involved using one of Gregg Fuhriman's MSS testers This has resulted in the following important observation:
  • When the MSS tester is connected, the BOD helper is unable to drive the input signal sufficiently low to indicate the block is occupied.
  • The root cause of this problem is the current handling capabilities of the LM393 voltage comparitor in the BOD Helper circuit.
  • Operationally, this means there are configurations these circutis can be used in where the optical detector circuit MAY fail to keep a block occupied after the locomotives have exited the block. I am working on a solution to this problem.