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Wednesday, 4 February 2009

Automatic - Railway Grade Crossing Circuit

The Grade Crossing circuit uses six - visible and infrared light sensitive phototransistors to control the operation of the flashers.

The protected section of track can be as long or as short as desired and does not have to be the same length on both sides of the crossing. The circuit works in both directions and can be connected to other grade crossing circuits to provide fully automatic protection for multiple track crossings.

The Grade Crossing circuit provides prototypical operation of the signals at level railway crossings and can supply up to 100 milliamps for LED flashers, small incandescent lights or other circuits such as crossing gates or sound units.

The circuitboard has been designed to be easy to build and install. Widely available and inexpensive components are used in its construction. None of the parts are static sensitive.

An upgraded version of the 2004 circuit is at this link - 2008 - Automatic Railway Grade Crossing Circuit. The new circuit designed to prototypically control crossing gates.

Printed circuit boards and parts are available for this circuit.

One Track Crossing - Circuitboard Connection Diagram

The following diagrams show how the circuit is connected for a crossing with 1 track. A diagram of the circuit board showing the terminal block positions is used.

The next diagram shows how the phototransistor sensors are placed along the track and the actions that occur as each is covered by the train. The sensors that start the flashers can be placed any distance from crossing desired.

Railway Grade Crossing Flasher Operation

  1. When a train traveling in either direction covers one of the "START" sensors, the crossing signals will start to flash.
  2. The signals will remain ON until approximately two seconds after the last car has passed completely through the crossing, uncovering both of the "STOP" sensors.

    (The grade crossing will be protected until both of the "STOP" sensors are uncovered.)

  3. As the train leaves the protected section of track, the "DISABLE" sensors prevent the flashers from being turned ON again by deactivating the "START" sensors.

  4. The "START" sensors are reactivated approximately 5 seconds after the "DISABLE" sensors have been uncovered.

Variable resistor R15 sets the flashing rate of the crossing signals.

The next diagram show the changes in the output terminals of the crossing circuit as a train passes through the protected section of track.

Grade Crossing Circuit Operation Notes

  • The flashers will turn OFF if a train enters and then backs out of the crossing.

  • The circuit is ready for the next train in either direction approximately five seconds after the "DISABLE" sensors are uncovered. If the departing train is still covering a "START" sensor after this time the flashers will turn on again.

  • Manual controls can start or stop the flashers as desired. The START push button could be replaced by a toggle switch in order to keep the flashers activated during switching operations.

  • Normal room lighting is used to detect the trains. If night operation is needed the circuit can be controlled by other circuits or by providing infrared light for the sensors.

  • The circuit is designed to use phototransistors but can also be controlled by CdS photocells by changing the values of resistors R1, R2 and R3.

  • The Crossing Circuit requires a regulated 12 volt power supply. The current draw is about 3 milliamps when the flashers are OFF and about 35 milliamps when they are ON.

  • Crossing gates and bells can be controlled buy using the MultiTrack terminal as an output to control these devices. The MultiTrack terminal is also used to connect the circuitboards together for crossings with two or more tracks.

Automatic Grade Crossing Circuit Schematic

The following diagram is of the 2004 Automatic Grade Crossing circuit showing conections for a single track crossing including manual controls and signals.

WARNING - If the polarity of the power supply for this circuit is reversed or the circuit is connected AC or DCC source, the circuit will be damaged. The maximum supply voltage is 15 Volts DC.

Railway Crossing Circuit Parts List

Qty Description Part Number DigiKey Part
1 - LM339 Quad Comparator 14-DIP - IC 1A, B, C, D - LM339NFS-ND
1 - LM393 Dual Comparator 8-DIP - IC 2A, B - LM393NFS-ND
1 - LM555 Timer 8-DIP - IC 3 - LM555CNFS-ND
1 - 2N3906 Transistor PNP TO-92 - Q7 - 2N3906FS-ND
6 - 3mm IR Phototransistor - Q1, 2, 3, 4, 5, 6 - 160-1030-ND
6 - 1.0M Ohm 1/4W Resistor - R1, 2, 3, 4, 5, 8 - 1.0MQBK-ND
4 - 100K OHM 1/4W Resistor - R6, 7, 10, 16 - 100KQBK-ND
1 - 33K Ohm 1/4W Resistor - R9 - 33KQBK-ND
3 - 10K Ohm 1/4W Resistor - R11, 12, 14 - 10KQBK-ND
1 - 3.3K Ohm 1/4W Resistor - R13 - 3.3KQBK-ND
1 - 500K or 1M TRIMPOT - R15 - D4AA55-ND
2 - 4.7uF 50V Miniature Aluminum Electrolytic - C1, 3 - P10315-ND
1 - 2.2uF 50V Miniature Aluminum Electrolytic - C2 - P10313-ND
1 - CAP 0.22uF 50V Ceramic Radial - C4 - 495-1106-ND.
1 - 1uF 50V Miniature Aluminum Electrolytic - C5 - P10312-ND
3 - 2 Position Terminal Block - 5mm - - - ED1601-ND
2 - 3 Position Terminal Block - 5mm - - - ED1602-ND

The DigiKey part number for the phototransistors is 365-1066-ND. A better choice might be Jameco - "LPT2023". (Look for a phototransistor with a - clear - case colour if florescent lights are used.)

Printed Circuit Board And Parts Information

If you are interested in a circuit board and parts for this circuit please send a message to the following email address:

The following picture is the printed circuit board for the Automatic Grade Crossing circuit. The board is two inches by four inches and has been commercially made and pre-tinned. (An assembled example of the board is also shown.)

Railway Grade Crossing Circuit PCB - Bare

Railway Grade Crossing Circuit PCB - Assembled

The price for 1 Printed circuit board is 6.00 dollars U.S. plus postage.

The price for 1 - Unassembled printed circuit board and all of the parts is 25.00 dollars US. - Plus postage.

The price for 1 - Assembled printed circuit board and all of the parts is 28.00 dollars US. - Plus postage.

NOTE: - The above prices include 6 visible/infrared sensitive, 3mm diameter phototransistors. The phototransisitor have 5 inch - Red and Black leads soldered to them and are tested before shipment.

Extra phototransistors can be purchased if needed.

- Accepted Methods Of Payment -

Paypal. International money orders. Personal cheques are OK but will have to clear before shipment.

If you are interested in a circuit board and parts for this circuit please send a message to the following email address:

2004 Grade Crossing Circuit PCB Parts Placement Diagram

Save and print this diagram to aid in assembling the circuitboard.

Circuit Board Parts Placement Diagram

Circuit Board Assembly

When installing the components on the circuit board start with parts with the lowest height and work up to the tallest parts. For example starting with the jumper wires then diodes then resistors, IC's, transistors, capacitors and terminal blocks.

Recommended Tools


Multiple Track Crossing Circuitboard Connections

Fully Automatic - 2 And 3 Track Crossings

The following diagrams shows how two and three circuitboards are connected for multiple track crossing with full automatic protection for each track. Any number of tracks can be protected using circuitboards for each track at the crossing.

The circuits use only terminal number 8 of the circuitboards as the common to avoid forming "ground loops" with terminals 5 and/or 12.

The diagrams above show one circuit board for each protected track at a crossing. It is possible to use one circuit board to protect multiple tracks but fully automatic control is not possible when this is done.

2 Tracks - 1 Circuitboard

Two tracks at a crossing can be protected by a single circuit by placing additional sensors in series with the sensors of the first track. This is a cost saving measure.

Because only one circuitboard is used, fully automatic operation is not possible as with the two circuitboard - two track crossing but if only one train uses the crossing at a time, the system will give prototypical operation.

1 Track And A Siding - 1 Circuitboard

The grade crossing circuit can also be used where there is a crossing with a siding.

Because only one circuit is used, fully automatic operation is not possible as in the full two track crossing but if only one train uses the crossing at a time, the system will give prototypical operation.

Extra phototransistors can be purchased for multiple track crossings using only 1 circuitboard.

Additional Circuits

Railway Grade Crossing Test Circuit

Before installing this circuit on a layout this simple circuit can be used to test the circuit board and demonstrate its operation.

SPST switches are used in place of the phototransistors to simulate their function. An open switch is the same as a covered or dark phototransistor.

This circuit can be used for the assembled and unassembled circuit boards. The circuit board terminals are the same for both.

One Track With Two Crossings Circuit

The next circuit is similar to one in use at the London Model Railroad Group's layout where there is two grade crossings that are quite close together.

The circuit is set up so that as the train approaches, both sets of crossing signals start at the same time but they turn off separately as the train leaves each crossing.

Reverse Loop With Single Track Crossing

Using Block Occupancy Detectors To Activate the Flashers

Block Occupancy Detectors (BODs) can be used to activate the crossing signals as long as they have a steady output state while any detectable cars are in the protected section of track.

The signal to stop the flashers should still be phototransistors as a BOD would be too slow and not suited to sensing a section of track that is shorter than many cars. Also, every car in the train would have to be detectable not just the first and last cars.

Most BODs have a LOW output when a train is detected and would use Circuit 1 in the following diagram.

The input to stop the flashers should be phototransistors as a BOD would not be suited to sensing a section of track that is shorter than many cars.

The section of track that is sensed by the BOD can be any length on either side of the crossing.

Stall Motor Crossing Gate Driver Circuit

The next two circuits can be used to drive stall-motor switch machines to lower and lift crossing gates.

The printed circuit board for the 556 Timer Stall-Motor Switch Machine Drivers circuit could be used to make Crossing Gate motor drivers. See the "Additional Circuits" section of the page.

Crossing Gate Driver Circuit

'Two Wire' LED Connection

The circuit in the following link will allow the crossing flasher LEDs to be connected using only two wires instead of three as in the circuits above. This could make wiring of small scale signals easier.

Common Anode and Cathode Connected LED Adapters

Unconfirmed - The LEDs in the Walters # 933-2914 crossing signals are wired in a common anode circuit but the Black wire is the positive and the Red wires are the negative.

TOMAR - Common Anode LED - Adapter

This circuit can be used to connect common anode wired LEDs such as TOMAR Industries, crossing signals to the Automatic Grade Crossing circuit.

The 22 ohm resistors distribute the current evenly though the LEDs. The 1K ohm resistors limit the current flow through the LEDs and can be changed if the brightness of the LEDs needs to be adjusted.

Incandescent LED Circuit

This circuit when used with a 555 timer will cause light emitting diodes to turn on and off more slowly. This will make the LEDs appear similar to incandescent lamps.

Driving High Current Lamps

This circuit can be used to drive higher current light bulbs that are found in older or large scale signals. The adapter can be connected directly to the Automatic Grade Crossing circuit or through optoisolators if a separate power supply is to be used for the bulbs.

Adding Capacitors To The Circuit's Inputs

If there is electrical noise that causes the crossing circuit to trigger falsely, small capacitors can be added to the input terminals. This will make the crossing circuit slightly slower to activate but will not affect the operation of the circuit.

If the trains are very fast, the DISABLE sensor may have to be moved farther from the START sensors when using added input capacitors.

Grade Crossing Bell Ringer (LM555)

This is an updated version an LM555 timer based - Grade Crossing Bell Ringer - circuit that was built for the London Model Railroad Group. The circuit drives a warning bell that is shown on the page.

SOUND - The sound is reasonably loud and mechanical sounding, similar to actual crossing bells.

CIRCUIT MODIFICATIONS - The value of R1 may be lowered and the value C1 increased to give a more powerful coil current for a louder ring. Some experimentation may be needed to determine the best values for a particular situation.

BELL MODIFICATIONS - The circuit-breaking contacts of the bell's solenoid are not used and have been bypassed by connecting the blue wire directly to the coil. The armature has been bent so that less throw is needed for it to strike the gong.

Home Depot carries a bell in their electical section that is similar to the one shown on this page. The model number is 172C and the stock number is 106 419

The bell is made by Heath/Zenith and can be seen at this link - Model 172C electric bell.

Heath/Zenith also make one with a larger gong - Model 174C electric bell.

Controlling External Flasher Or Other Circuits

Electronic grade crossing bells at and

These devices have not been tested with the Automatic Grade Crossing circuit on this page but should work.

A DCC To DC - Accessory Power Supplies that could be used to supply the crossing circuit.

Return to the Main Page

Please Read Before Using These Circuit Ideas

The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.


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