General Purpose Block Occupancy Detector

  This page presents block occupancy detector designed for general purpose use on systems where the polarity of the power to the track is reversible.

  As with many types of detection systems this circuit senses the flow of current across the rails when a locomotive or resistance equipped wheel set is in the block. Whenever there is a large enough current flow the detector will produce an output indication.

  The circuit requires no adjustment and indicates the direction the train is traveling and can provide a steady state output for controlling block signals and interlocks.


General Purpose Block Occupancy Detector

General Purpose Block Occupancy Detector Schematic


Operation for General Purpose Block Occupancy Detector

  1.   When a locomotive or resistance equipped wheel set is in a detected block, a current will flow across the rails and through R1.

  2.   If the current flowing is greater than the threshold current level determined by R1 the output of IC 1 or IC 2 will go to a LOW and the EAST or WEST direction LED will turn ON. Which output goes LOW depends on the direction of the current flow through R1.

  3.   If either of the EAST of WEST direction LED's is ON the voltage across C2 will be bled off through D5. When this voltage is below the voltage between R10 and R11 the output of IC 3 will go LOW and D6 will turn ON.

  4.   As long as the voltage across C2 is below the reference voltage the D6 will be on in a constant state.

  5.   When a current flow is no longer detected the outputs of IC 1 and 2 will both be HIGH and C2 will charge through R9. When the voltage across C2 is higher than the reference voltage between R10 and R11 the output of IC 3 will go HIGH and the D6 will turn OFF.

  The output of IC 1 and IC 2 may be very noisy due to momentary interruptions in the current flow that are caused by dirty track or wheels. For this reason the EAST and WEST outputs are not suitable for applications that require a steady signal.

  The time required for the voltage at C2 to reach the reference voltage after the output of IC 1 and IC 2 are constantly HIGH is approximately 0.01 seconds. This means that as long as a current flow is detected within every 1/10th of a second the LED at the output of IC 3 will remain ON in a constant state. This will eliminate output noise from the detector.


G.P. Block Occupancy Detector Input Wiring Schematic


  The next schematic shows the use a 4 Amp bridge rectifier to replace two sets of 3 Amp diodes that are used to provide a detectable voltage when a train is in a given block. One bridge would serve two track blocks.

G.P. Block Occupancy Detector - Input Options

  Using a bridge rectifier may seem more complicated than using two diodes but it is electrically the same.

  While there is no significant advantage to using bridge rectifiers for this purpose, they may be cheaper if bought surplus and might be easier to wire if a large number of blocks are to be detected.

  The bridge schematic also shows an alternate location for capacitor C1.

  The LM339 detector section of the circuit would be the same as those shown in the previous circuits on this page


Output Options for Block Occupancy Detectors

Output Options Schematic

  The output of the comparator itself is an open collector NPN transistor that can sink up to 50 milliamps.

  The output of the comparator does not have to be connected to the same voltage level or current source as long as they share the same common.


General Purpose Block Occupancy Detector Notes


Input Vs. Output Results for Comparators

  1.   If the voltage at the PLUS input is lower than at the MINUS input the output transistor will be turned on. Current will flow.

  2.   If the voltage at the PLUS input is higher than at the MINUS input the output transistor will be turned off. Current will not flow.


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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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17 April, 2010