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.
Operation for General Purpose Block Occupancy Detector
When a locomotive or resistance equipped wheel set is in a detected block, a current will flow across the rails and through R1.
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.
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.
As long as the voltage across C2 is below the reference voltage the D6 will be on in a constant state.
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.
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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