This circuit is a simple 3 light signal that uses a BOD or an IR type detector to sense a train and timers to hold the signal at RED and then YELLOW for a period after the train has left the block or sensor.
The circuit is not designed to accept inputs from other signals or interlockings. The circuit provides a GREEN, YELLOW or RED aspect as a train passes through a block or over a light sensor.
A toggle switch or any detector that can simulate a closed SPST switch will work as an input for this circuit.
The delay times can be adjusted by changing the value of the timing resistors and capacitors. A 556 timer could also be used for this circuit.
The original of this circuit can be found on the 555 Timer Page, Item 43 at this site.
A modified version of the Common Cathode circuit above. This circuit will clear a YELLOW indication and set the signal to RED if the circuit is retriggered after the Red timer has run out.
The logic of this circuit is not ideal because the Red timer both triggers and clears the Yellow timer. However, as long as the Red signal's timer runs longer than a few seconds the circuit will function correctly.
The next circuits function the same as the circuits above but is designed to drive common anode connected LEDs.
The next circuits function the same as the circuit above but the GREEN LEDs are connected differently. The disadvantage of these circuits is that five conductors would be needed to wire a signal head rather then four for the three timer circuits.
A similar circuit for a 2 Light Signal can be made with the 4 Photo-Detector With Delayed Release And Bipolar Output Circuit and circuit board.
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.
18 November, 2019