A Signal Circuit To Display Occupancy
By Trains In Multiple Blocks.

Bill Payne and I have been experimenting with logic circuits for trackside signals for use on our Nottawasaga Model Railroad Club (NMRC) and our home layouts. Commercial circuits by IrDot, Logic Rail, Dallee, etc are possible solutions, but we wanted to see if we could make our own and save some money.

On the Internet we discovered a 3-light signal circuit with approach lighting by R. Paisley. Our first task was to test it on an Evaluator board that I own.

Initial test set-up

Side view of set-up

We made the circuit cover 5 blocks so we could see what happens when moving the ground wire that represents the detector. The circuit, as shown in his diagram, has a train detected in block 2. Block 2 should then be red, block one should be yellow and block 3 is green. This represents one-way direction of travel, not bi-directional.

The choice now is to photo-etch signal circuit boards, find someone to do it, or use commercial stripboards as are often shown in wiring circuits in the NMRA's Scale Rails magazine. In the latter method one cuts the traces where needed. You can see the layout of the prototype board in the photo below.

As designed, this circuit uses LEDs. However, if the 470 ohm dropping resistors are bypassed, this can be used for 12 volt bulbs.

On the club layout we have a section of single track mainline. This will need to be bi-directional. That calls for a "coincidence" circuit. We haven't found one yet. The LogicRail boards I have been using on my Utopia Northern allow for bi-directional travel. We need to find a solution.

The other task is to develop a detector circuit that doesn't interfere with track current, like the one that Dallee sells. We've found a circuit using opto-isolators and are about to mock it up as soon as we locate all the parts. R. Paisley shows a Transformer-type, DCC only, block occupancy detector using a 555 timer.

Another set of circuits for block detection and signals is outlined in an NMRA clinic by Richard Schumacher. We haven't tried his yet. There's a lot of good information about assembly of the circuits in the articles.

At a recent Chapter meeting of our Canadian Association of Railway Modellers (CARM), one of our members, Peter Hughes, directed me to the MERG site in Britain. If you're interested in model railroad electronics (and you must be if you've read this far), check out The Model Electronic Railway Group. This may ultimately supply us with the circuits we need to build.

Planning the layout

Stripboard component positioning

Bill then used a stripboard planning sheet to design a master signal circuit board for the components. The sheet was downloaded from www.kpsec.freeuk.com. This shows where to cut the traces on the horizontal rows. A multimeter is used to confirm the traces are cut. Bill used a drill bit to ream out the holes. I was using an X-Acto knife.

Bill then built a prototype circuit board using a stripboard that he cut apart with a hack saw so we can make three boards from one sheet to save money. He mocked up the set-up so we could demonstrate how the lights change as a train moves from left to right through the blocks. If this were a loop of track, block 5 would be connected back to block 1.

circuit board master with LEDs

Overview of test set-up

The green masking tape at the bottom of the photo on the ground wires shows the block number. Touching the wire to ground simulates a detector triggering the LED. This is what the prototype mockup of the circuit looks like when using LEDs. The first block on the left is unlit to simulate approach lighting. Block 2 is yellow as a warning to slow down. Block 3 is red because that's where the train is at the moment. Block 4 is green and the last block on the right is unlit because the train hasn't reached the detector for that signal yet.

If you have also experimented with any of these circuits or have other sources, please email my from my Contact page. Bill Payne and I would appreciate your input.

Signal detection and wiring notes

During the summer of 2011 I was adding signal detection to four hidden tracks between Underhill South and Underhill North. I was installing signal bridges at both ends to keep track of what was holding or passing through the "tunnel". I ran into some problems with the green indication not turning off. I was taking the power from the original test installation about 20 feet of wire away. The power was coming from the same computer power supply: 12 volts. The new section's track is within the same power booster district from my Lenz system. Bill Hudson, from whom I buy the circuit boards, has been trying to help me through the problem by email. He had modified his circuit for the current transducers (CT) I am using as noted above. He had some interesting observations about what could be causing the problem. His musings include comments about wire sizes and conductive ballast. I have posted his comments on a separate page: signal detection musings.

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Go from "signal-circuit" to track occupancy using CTs and other wiring considerations.

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