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General Purpose Block Occupancy Detector

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

  •   Power Supply – This circuit uses an unconventional dual voltage power supply arrangement.

    – For the detector to sense current flow in both directions a PLUS and MINUS power supply is needed.

    – Since the maximum negative voltage expected at the input of the detector is only 0.7 volts a full + and – 5 volt supply would not fully make use of the negative side of the supply.

    – The insertion of D7 into the common lead of the 5 volt regulator produces a floating common, the blue line, that is approximately 0.6 volts higher than the return to the DC voltage supply. This allows the input to be able to accept a negative input voltage.

  •   These circuits are designed to work with a 5 volt regulator. A 12 volt regulator can be used if the value of R5 is decreased to 270 ohms. This is in order to maintain the 0.3 volt reference used by IC 1.
  •   Each detector unit uses approximately 30 milliamps of current if the EAST, WEST and the time delay LED’s are all used.
  •   The voltage level of the threshold current through R1 is approximately 0.3 volts. This means that when the voltage across R1 is greater than PLUS or MINUS 0.3 volts the detector will change output states.
  •   The line shown in BLUE on the drawing is the INPUT common for the detector circuit. This line is common to all of the inputs from the R1and D1/D2 groups of all blocks.
  •   Resistor R1 determines the sensitivity of the detector. If R1 is 100 ohms then currents larger than 3 milliamps through the block will be detected. Increasing the value of R1 to 300 ohms will allow currents larger than 1 milliamp or larger to be detected. Conversely, decreasing the value of R1 will reduce the sensitivity of the detector.
  •   If the value of R1 is made too large the detector may give false indications due to leakage currents across the tracks and from the inputs of the comparators.
  •   Diodes D1 and D2 shunt the current around R1 when the voltage across them reaches 0.7 volts in either direction. These diodes require a current capacity large enough to handle the highest expected locomotive current load. A 3 amp rating should be sufficient in most cases.
  •   Increasing the value of R9 will increase the time delay required for the output of IC 3 to return to the HIGH sate.
  •   Resistors R2 and R3 protect the circuit from excessive currents should there be problems in the block wiring system itself. These components are included to protect the detector circuitry and have no other function.
  •   The reference voltages that are between R4/R5, R6/R7 and R10/R11 can be shared by other detectors on the same circuit board. This will reduce the total number of parts for a given number of detectors.
  •   Inside the ovals on the schematics are the approximate voltages at that location in the circuit.
  •   For more information on comparators please refer to the “Basic Voltage Comparator Circuits” page listed in the Circuit Index of the Model Railroad Electronics page or follow this link. – Voltage Comparator information

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.