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.

See the update about the new signals on our club layout (July 2014) at the bottom of the NMRC page.

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
3-light signal circuit
This allows one to construct the circuit without soldering or using a circuit board. It is easy to change parts. It looks messy and it is, but it works. Each block set-up is identical. The interconnecting wires trigger the signal colors.

Side view of set-up
3-light signal circuit 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 Chapter meeting of the 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 planning sheet
Stripboard component positioning
stripboard layout

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
stripboard prototype board

Overview of test set-up
3-light signal circuit mock-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 me 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.

Update: April, 2012

Bill Payne and I have continued to experiment with the circuits as time allowed to try to eliminate the problem. Bill Hudson continued to guide us by email. In March, 2012 I ordered a DCCOD detector to see if the updated circuit by Bruce Chubb for DCC railroads would solve the problem. We replaced the CT detector we were using in Bill's circuit with the DCCOD. It worked fine for detection but did not eliminate the double aspects.

Circuit board installation at Underhill South
slc circuit boards

As you can see in the photo the boards are installed side by side in an upright position on two wooden rails. Hammond no longer sells the metal rails I had been using. It's getting very difficult to source the 22-pin boards we have been using. We sourced these from Bill Hudson. At this juncture we decided to reinstall a filter in the common return from the power supply. Bill's circuit can be referenced to the same ground as common rail wiring and the same line as used for power is referenced to the gapped signal rail.
experimenting with capacitors
slc capacitor
We tried various capacitors from 33µf to 220 µf (microfarads). The capacitor serves as a filter for the AC supply (in the DCC signal), but also to suppress spikes when boards for nearby blocks make a transition. We settled on 47µf. We also increased the value of the resistor on the wire to the green LED to make it dimmer. We went as high as 2k ohms. We also added 3 turns of Bell wire through the CT donut on the block we were testing. All of these changes appeared to have a positive effect. The capacitor made the biggest difference. The green still flickers slightly when the red or yellow turns on but is barely noticeable. We also tried doubleheading two diesels to check if the power draw would improve the detection. It helped so the number of turns is certainly a factor when a locomotive doesn't draw much power although turning on the red hasn't been the problem. It's been getting the green to go out. Swapping circuit boards didn't make a difference. There is a lot of wiring around these signal bridges at Underhill South so cross talk between wires could be making the problem worse. There's no way to decrease the number of wires in the bundles to the LEDs on the bridges. Most of the single 3-LED signals in adjacent blocks do not cause the problem. It has taken a lot of fiddling to improve the situation. At this point we seem to have it under control. Of course, built-up circuits from other suppliers such as Bruce Chubb's JLC Enterprises, Dallee or LogicRail probably would work better but at greater cost and without the personal satisfaction of doing it ourselves. After all, this hobby is all about learning how to do things you know next to nothing about when you begin.

As an aside, we also experimented with Paisley's circuit for photocell detection.
A photocell detector experiment
photocell detection
We mocked it up on a circuit evaluator test board. Depending on how it is wired the LED either turns on or turns off when covered (high or low logic). I don't plan to use it for block detection, but it could be used to turn on building lights or crossing gates. That's a project for another day!

Signalling on our club layout

In 2014 John Houghton took on the project of installing signalling on the loop trackage on our new end modules. He did an amazing job. He added a new twist to the electronics by installing an Arduino computer on the boards along with the occupancy detection and capacitor discharge units. There is a lot more about the project if you go to my Nottawasaga Model Railway page. I repeat John's documentation here for those who wish to jump into it directly. Here's a pdf explaining the signal aspects and the complete technical documentation with all the schematics and Arduino software code for the construction of the electronics for NMRC signalling operations.

The signalling system at the club was up and running for the 2015 train show season.

Return from "signal-circuit" to the model railroad wiring overview.

Go from "signal-circuit" to Roger Race's automatic signal circuit.

Go from "signal-circuit" to photocell train detector.

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

Go from "signal-circuit" to scratchbuilding a brass signal bridge.

Build a cheap HO dwarf signal.

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