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This design for a spot detector activated passage signal is quite old and today would be implemented using a small PIController which would give a reduced component count.
A train passing the spot detector puts a low level on the red input. This discharges the capacitor and gives the red aspect. As the capacitor charges back to the +12v the amplifiers are biased first to drive the LEDs alternately from the oscillator, giving Yellow, then back to Green. The total time can be set or modified by different values of component. The yellow time is also modified by the 100k trim pot.
This design for a spot detector activated passage signal is quite old and today would be implemented using a small PIController which would give a reduced component count.
A train passing the spot detector puts a low level on the red input. This discharges the capacitor and gives the red aspect. As the capacitor charges back to the +12v the amplifiers are biased first to drive the LEDs alternately from the oscillator, giving Yellow, then back to Green. The total time can be set or modified by different values of component. The yellow time is also modified by the 100k trim pot.
LINK TO IMAGE - sig-3dr.gif
Circuit Schematic
