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Railroad Crossing Signal for HO Model Railroad DCC Layout

Updated on July 23, 2016

Railroad Crossing Signal for HO Model Railroad DCC Layout

I integrated my Walthers Life-Like track layout with my Walthers Life-Like Demolition Derby set in order to add to the excitement of near misses between the model trains and slot cars. I thought it would be cool to also have railroad crossing lights so I purchased two SceneMaster S/O Scales - Railroad Signal from Amazon. I was disappointed to find out that:1. The SceneMaster S/O Scales - Railroad Signal signs had incandescent bulbs installed which would increase the load on my Digitrax Zephyr Command station limiting the amount of DCC locomotives I could control at one time.2. The lights on the SceneMaster S/O Scales - Railroad Signal signs do not alternate back and forth like a "real" railroad crossing lights.

HO Scale Railroad Crossing Signs on Amazon

SceneMaster   S/O Scale Lights - Railroad Signal
SceneMaster S/O Scale Lights - Railroad Signal

This is what I used for my Railroad crossing lights.

 

Integrate your Life-Like Slot Car set with your Life-Like model railroad layout!

Life-Like Race Set and Train Set Combo Intersection Track
Life-Like Race Set and Train Set Combo Intersection Track

This piece allows you to integrate your Life-Like Slot Car set with your Life-Like model railroad layout.

 

Triggering the Railroad Crossing Signal

I already had a Digitrax BD4 Quad Occupancy Detector wired into my HO DCC Layout to determine a locomotive’s position. The BD4 has a Block Monitor board with LED indicators for troubleshooting purposes. I soldered two wires to the single inline pin header, one to pin 1 or DS 1 Indicator LED+ pin, the other to pin 10 or Ground for the Status LED. I used these wires to trigger my Railroad Crossing Signals.

The Problem

Although I replaced the incandescent bulbs with LEDs in my Railroad Crossing Signals, they still consumed too my current and would not be triggered by the Digitrax BD4 Quad Occupancy Detector.

The Solution

I determined the best way to trigger both Railroad Crossing Signals from the Digitrax BD4 Quad Occupancy Detector was to isolate the Rairoad Crossing Signals from the BD4 using a 4N38 optocoupler and deriving power for the Railroad Crossing Signals from the track.

Circuit Theory

The 4N38 optocoupler receives a 5 volt signal from the BD4 when a train is close the Railroad Crossing Signals. R9 is a current limiting resistor for the LED that is built into the optocoupler, pins 1 and 2 connect to the LED inside the 4N38 optocoupler. When pins 1 and 2 of the optocoupler have a 5 volt potential across them the internal LED lights causing the phototransistor between pins 4 and 5 to conduct, powering the Railroad Crossing Signals making them flash. Diode D3 and Capacitor C7 rectify and filter the DCC voltage on the track which can switch from +14.25 volts to – 14.25 volts. R10 functions as part of a voltage divider to reduce the voltage to the Railroad Crossing Signals. Each Railroad Crossing Signal (there are two) consists of two transistors, two capacitors, four resistors and two LEDs. Q1, Q2, C1, C2, R1, R2, R5, R6, LED1 and LED2 are contained in one Railroad Crossing Signal, Q3, Q4, C3, C4, R3, R4, R7, R8, LED3 and LED4 are contained in the other Railroad Crossing Signal. The components in the Railroad Crossing Signal form a common Astable Multivibrator Circuit which causes the LEDs to alternate like a “real” railroad crossing signal. Click HERE to read more about the Astable Multivibrator Circuit.

Construction

As always, when building a new circuit, I recommend breadboarding it before committing to perfboard or a PC board.Caution: Build this circuit at your own risk! You need a basic understanding of electricity and electronics before proceeding. I am not responsible if you:1. Damage your Digital Command Control station of Booster.2. Damage the decoders in your DCC locomotives.3. Cause the moon to go out of orbit (like in Space 1999)4. Cause a worm hole to open next to your model train layout.Each Railroad Crossing Signal contains a small perfboard with the parts for the Astable Multivibrator Circuit hidden under the lumber pile.

Construction Part Two

The 4N38 optoisolator, R10 R9, D3, and C1 are contained on a small perfboard hidden under the layout.

Video of Railroad Crossing Signal in Action

As a DCC Locomotive enters a block of track sensed by the Digitrax BD4 Quad Occupancy Detector it triggers both Railroad Crossing Signals. As the DCC locomotive leaves the block the Railroad Crossing Signals turn off. See my videos below:

Car and Train accident of my HO Scale Model Railroad!

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    • Tricia Deed profile image

      Tricia Deed 

      4 years ago from Orlando, Florida

      Very informative. Those cars are traveling very fast.

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