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Block Occupancy Detector with Coils

Dual – Transformer Type

Block Occupancy Detector For DCC

Please Read The Following

  The original J9119-A coils for the VT-5 BOD are no longer available through this site or through All Electronics.

It is not economical to supply the transformer coils through this site as I would have to charge much more for them than you would pay by buying them directly from Surplus Sales of Nebraska. Therefore only the circuit boards and other components will be available.

A suitable, and reasonably priced, transformer is available through Surplus Sales of Nebraska. The part number is Vitec Transformer – (TM) 57P1822 or at the same site Vitec Transformer – (TM) 57P1820

  There is a datasheet on the Surplus Sales of Nebraska web page lists these coils and both should work satisfactorily.

A similar coil in Europe is the AS-103 which can be found on the site.

  Another useable transformer is available from Jameco Electronics. The part number is 164718

  The MTC30101 transformer will be less sensitive than the coils listed above.

Printed Circuitboards for The VT-5 Detector

Circuitboards are available

  The price of the circuitboards is 2.25 dollars each. ($1.12 per block.)

NOTE: The circuitboards are not silkscreened as shown in the pictures on this page.

If you are interested in printed circuitboards and parts, please send an email to the following address: saying how many boards you would like.


Dual – Transformer Type

Block Occupancy Detector For DCC

Printed circuit boards are available for this circuit at this web site.

  Each circuit board has 2 complete occupancy detectors, each using one-half of a 556 timer IC to provide the release time delay and outputs. Each detector has an Open Collector and a Bipolar output.

Circuit Features

  •   2 Detector circuits on each circuit board.
  •   Instant detection.
  •   Timed release delay – Approximately 2 Seconds (Can be changed).
  •   1 Milliamp or less detection current sensitivity (Can be changed).
  •   2 outputs for each detector circuit – Open Collector and Bipolar.
  •   Large output current capacities (See Notes).
  •   5 Amps and greater track current rating.
  •   Complete isolation between track and output circuits.
  •   Very low voltage losses in the track circuit.

Why Use the 556 Timer As the Output Device?

  •   While the 556 timer is more expensive than voltage comparators ICs, the circuitry of the detector is simpler and the outputs have a much higher current capacity. (200 milliamps for the 556 timer Vs. 15 milliamps for a 393 of 339 comparator.)

What The Circuit Does Not Have

  •   On-Board indicator LEDs (Added cost and can only be seen from under the bench).
  •   AC power supply operation. (Cost and complexity and the circuit common to other circuits is through the rectifier diodes).
  •   The VT-5 detector cannot be used with conventional DC or AC systems.

Dual Detector Circuit

  The following is the schematic for the Dual – VT-5 detector circuit. The 556 timer IC provides two outputs for each detector.

VT-5 – Dual Detector Circuit

Detector Circuit Operation

  When no trains are detected the open collector outputs (Terminals A, and C) and the bipolar outputs (Terminals B and D) of the timers will be HIGH.

  1.   When a sufficiently large voltage is induced into the windings of the current transformers a pulsed current will flow through the Base to Emitter junction of the transistors. When the transistor conducts the voltage across the capacitors C1/C2 will be drained. (The capacitors are short circuited by Q1/Q2.)
  2.   As the voltage across the capacitors falls the voltage across the 1 Megohm resistors will increase.
  3.   When the voltage across the 1 Megohm resistors increases to 2/3rds of the supply voltage the Bipolar outputs of the timers will go from a HIGH state to LOW and the Open Collector outputs will turn ON.
  4.   When no trains are detected the transistors will stop conducting and the voltage across the 1Meg resistors will decrease as the capacitors charge. (The capacitors are no longer shorted.)
  5.   With the voltage across the 1 Megohm resistors decreasing, when the voltage at the TRIGGER terminals of the timers drop below 1/3rd of the supply voltage the Bipolar outputs of the timers will go from a LOW state to HIGH and the Open Collector outputs will turn OFF.

NOTE: Due to the high rate of direction change (thousands of times per second) block occupancy detectors for DCC systems do not need to sense current flow in both directions . The time delayed release built into the circuit is more than enough to compensate for the time when the track current is flowing in the reversed direction. This applies to the zero stretching mode as well, which has a minimum frequency of about 83 hertz at maximum stretch.

Parts Lists and Digi-Key Stock Numbers

  The following table shows the parts list for the VT-5 Dual Detector circuit. The list shows the Digi-Key stock numbers as well as the schematic part numbers.

  •   The “QTY” column indicates the number of parts to be ordered from Digi-Key. As some of the parts are only available in multiples of 5 or 10 there may be resistors or diodes left over if a small number of detectors are built.
  •   The “Digi-Key NUMBER” column gives the stock number for the the components used in the circuit if they are purchased from the Digi-Key Corporation. (
    (The part numbers can be pasted into a DigiKey online order form.)
  •   The “DESCRIPTION” column gives a brief description of the particular parts used in the circuit. This is the same description used by Digi-Key on their online order form.
  •   The “SCHEMATIC NUMBER” column gives the part numbers that are used on the schematic drawing for the parts listed in this table.
Parts Available From DigiKey Electronics
2 2N3904FS-ND IC TRANS NPN SS GP 200MA TO-92 Q1, Q2
5 10KQBK-ND 10K OHM 1/4W 5% CARBON FILM R1, R3
5 1.0MQBK-ND 1.0M OHM 1/4W 5% CARBON FILM R2, R4
Parts Available From This Site

Printed Circuit Boards For The Dual Detector Circuit

  A printed circuit board is available for the Dual detector circuit. The board is 2.25 inches by 1.75 inches and has two complete detector circuits on it.

Dual Detector Circuit Board

  Placing two detectors on one circuit board was done to reduce costs as a Dual detector circuit board is about 38 percent less expensive to build than two single detector circuits. The four primary factors in the reduction are;

  1.   – The cost of a Dual circuit board is almost the same as a single detector board. (46%)
  2.   – A 556 timer is used in place of two 555 timers. (26%)
  3.   – One filter capacitor is used for two detectors. (50%)
  4.   – 6 terminal block positions are needed for a Dual board while two single detector boards would use 8 positions. (32%)

No terminal blocks were used on the input side of the transformers as these would only add cost and complexity with little benefit being gained.

The board has been commercially produced, is silk-screened and has large pre-tinned pads for easy soldering.

Printed Circuit Board Prices

Cost per block is 1/2 of the unit cost.
  1 to 19 printed circuit boards – 2.25 dollars US each – Plus postage.

20 or more printed circuit boards – 2.00 dollars US each – Plus postage.

NOTE: The boards are not silkscreened.

Printed circuit boards with all parts except the current transformers – 6.50 dollars US each – Plus postage.

If you are interested in printed circuit boards please send an email to the following address:

Your message will be answered as soon as possible.

– Accepted Methods Of Payment –


International money orders are good, a personal cheque is OK but it will have to clear before shipment.

VT-5 Detector Circuit Notes

  •   This circuit is not suitable for detecting trains on conventional DC train systems and is not sensitive enough for use with AC systems.
  •   Two outputs are available from each detector. The Bipolar outputs (Terminals B and D) could be used to operate track side signals while the Open Collector outputs (Terminals A and C) could connect to block control systems. (See the Output Options section of this page.)

    For each circuit, when no train is detected both outputs will be in a HIGH state.

    For each circuit, when a train is detected both outputs will be in a LOW state.

  •   Data sheets for the LM556 timer state that the OUTPUT and DISCHARGE terminals are able to handle currents of up to 200 milliamps but as the power supply will likely not be located close to the detectors, a maximum current of 100 milliamps would be more reasonable. The circuit has been tested at 110 milliamps continuously from both bipolar outputs with no problems.
  •   The sensitivity of the detector can be changed by increasing or decreasing the number of turns on the primary side of the transformer. (See the Sensitivity And Number Of Primary Turns section of this page.)
  •   The current transformers provides complete electrical isolation between the track and the detector’s output circuits.
  •   There are no manufactures specifications available for the J119-A transformer but prototype testing indicates that it is well suited to this application.
  •   The output section of the detector requires an external power supply of between 5 and 12 volts. A regulated supply is best but a well filtered supply will work.
  •   In this circuit the RC timing portion of the timers is used in an unconventional manner. This was done to allow the outputs to be in a HIGH state when no trains are detected. The function of the timers is otherwise unchanged.

    The output state of this circuit can be reversed by modifying the the circuitboard. This is shown later on this page.

  •   The release time delay of the circuit can be made longer by increasing the value of capacitor C1 and C2. (Release time is approximately R2 x C1 x 1.1 = Tseconds.) The recommended capacitance range for C1 is 1 to 25 microfarads.

    Resistors R1 and R3 limit the current that can flow into the THRESHOLD terminal of the timers. They have very little effect on the release timing of the circuit.

  •   The circuit board has two mounting holes and can be placed directly against wooden bench work without problem. Using a small piece of plastic under the circuit board would guarantee isolation. Squares cut from used plastic food container lids would be ideal for this purpose.

    More LM556 timer information and data sheets are available at this page. LM556 Timers – Circuits and Calculators

  •   If you are planing to use this detector with a computer control system, check with the manufacturer of the control system for compatibility between the circuits. The VT-5 detector is known to work directly with the “CTI Electronics” train control systems.

Sensitivity And Number Of Primary Turns

  The number of turns in the primary circuit of the transformer determines the sensitivity of the detector, more turns equals greater sensitivity. It is best to use the minimum number of turns that will provide the desired detection level.

The prototype detectors had one and one half primary turns and could detect a track current of less than 1 milliamp . Tests can be carried out to find the number of primary turns needed for a particular application. Some points to consider are:

  •   Too many turns create greater losses in the primary circuit.
  •   Too many turns creates greater currents in the secondary circuit. This is especially true when the primary current is high due to a short across the tracks.
  •   Turns that are closely wound are more efficient than loose turns. (Fewer turns are needed.)

Method Of Keeping The Primary Turns Tight

Making The VT-5 Detector Less Sensitive

  The VT-5 BOD is designed to be as sensitive as possible but there are situations where less sensitivity may be desired such as a long block with high leakage currents.

The next schematic shows a method of reducing the sensitivity by placing a resistor across the coil of the transformer . A typical value for RL would be between 10K ohms and 33K ohms.

Detector Sensitivity Reduction

  The values given in the table on the diagram are approximate as the number of windings in the primary also affect the sensitivity. The resistor would be soldered to the terminals of the VT-5 transformer on the underside of the board.

VT-5 Detector Output Options

  At the right side of the first two schematics In the next diagram is the equivalent outputs of the timers if they were toggle switches and not transistors. This may be helpful in planning how to use the circuit outputs.

Detector Output Options

  The VT-5 detectors can drive LEDs with common cathodes or anodes by using a 2 diode adapter circuit to convert these LEDs to bipolar operation.

The output diagrams do not show a value for the current limiting resistors. This value would be selected based on the current desired. The following link may help to determine these values.

LED – Current Limiting Resistor Calculator

Other Circuits

VT-5 Detector With Reversed Outputs

  The VT-5 detector is designed to have its outputs go LOW when a train is detected. This can be reversed by cutting the traces and installing jumper wires on the bottom of the circuitboard.

VT-5 With Reversed Outputs Circuitboard

  The circuit must also be changed according to the following schematic.

VT-5 With Reversed Outputs Schematic

  When a train is detected, the open collector outputs will turn OFF and the bipolar outputs will go HIGH.

VT-5 BOD – Diamond Protection Circuit

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 diodes then resistors, IC 1, transistors, capacitors, terminal block and lastly the transformers.

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