Audible Train Approach Warning Circuit

  This is a circuit that will give an audible warning that a train is entering a particular section of track. The detector also has a LED that indicates when a train is entering or leaving the block.

  This circuit is based on one that is installed at the London Model Railroad Group's 'O' Scale layout and used to warn the "Hidden Yard" operator that a train has entered his control block. Due to the location of this yard the operator cannot see a train that is entering or leaving his blocks.

  When a train enters the Hidden Yard block the operator will here a short warning tone and then see the LED is on. If a train is leaving the LED shows the yard operator when it has left his block and he can resume staging operations in the hidden yard.


Audible Train Approach Warning Circuit Schematic


Approach Warning Circuit Operation

Approaching Train

  1. When an approaching train covers the (A) sensor the 'D' section of the LM339 triggers the timer IC 1 which produces a pulse of about 1/3 of a second.

  2. When the output of IC 1 is high the IC 2 timer will oscillate and produce a short warning tone from the speaker.

  3. When the approaching train covers the (B) sensor the 'B' and 'C' section of the LM339 disable the (A) sensor so that the IC 1 timer cannot be re-triggered by the moving train.

Departing Train

  1. When a departing train covers the (B) sensor the 'B' and 'C' section of the LM339 disable the (A) sensor so that the warning will not sound.

  2. Approximately 5 to 8 seconds after the (B) sensor is uncovered the (A) sensor will again be active and the next approaching train will trigger the warning tone.

Approach Warning Circuit Notes


Return to the Main Page