Dual - Transformer Type

Block Occupancy Detector For DCC

Printed Circuit Boards Are Available For This Circuit

Please Read The Following Regarding Coils For this Circuit

  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.

  The 57P1820, FIS121NL and MTC30101 transformer coils listed below have been tested with the VT-5 detector circuit and have worked satisfactorily.

  A suitable 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.

  DigiKey also has a compatable coil 553-1835-5-ND at a reasonable price.

  A similar coil in Europe is the AS-103 which can be found on the http://nl.rs-online.com 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: rpaisley4@cogeco.ca 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

Why Use the 556 Timer As the Output Device?

What The Circuit Does Not Have

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.

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.

  NOTE: The boards are not silkscreened.

  Option 1 - 1 to 19 printed circuit boards - 2.25 dollars US each - Plus postage.

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

  Option 3 - Printed circuit boards with all parts except the current transformers - 7.00 dollars US each - Plus postage.

  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: DCC Block Occupancy Detector

  Your message will be answered as soon as possible.

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.

Parts Placement Diagram

VT-5 Detector Circuit Notes

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:

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

Recommended Tools


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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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27b June, 2016