Chapter-4e-1 Page 21
A Train Shuttle
A loco and wagons moving along a side track or through a mountain pass can add to the overall effect of a layout. This shuttle controller is simple to setup and install and provides a layout feature point. Train shuttle operation needs a DC throttle as with DCC power the train direction can not be sensed electrically, nor will a blocking diode stop the train. However a shuttle could be incorporated within a DCC layout as a separate scene. This circuit is simple and does not allow for intermediate station stops. Speed control (stop and start) is fast as power is cut immediately it enters the stop track or starts up but these areas could be hidden behind other scenery. Speed control is provided by your throttle. If you have grade changes then you could use a throttle with feedback to maintain speed. Because the detection circuit is looking for a loss of track voltage any dirt on the track or loco wheels may cause an unwanted direction change to take place.
While the loco is drawing power from the main track there is a voltage developed across the bridge rectifier in the schematic. This 1.2 volts activates one of the optical couplers, depending on the train direction. The tied collector outputs are at zero volts while the train is detected and this holds the Schematic and operation: 555 timer at 'reset', When the loco The blocking diodes control the runs onto the stop track no current stopping tracks and will only allow is drawn and the voltage is lost. the loco to move back onto the main line. There is one at each end. The reset (pin 4) is released and the timer trigger 'TR' goes positive
to start the timeout. The timer output at pin 3 goes positive. The capacitors are to filter loco motor and wheel contact noise from the input and provide a delay to the trigger. At timeout the timer output goes to 0 volts, this turns the transistor on and the positive change at the 'CD4013' flip-flop clock input 'Cp' triggers it. This is a digital circuit and the output at 'Q' will change to the state of the input at 'D' on receiving a clock pulse. This is tied to the inverse output at 'Q/' and so the 'Q' output toggles on and off as each stop track is reached. This changes the state of the relay driver transistor and the relay is either picked or dropped. So every clock input will reverse the direction of the loco. The LED indicates when the timer has been triggered and is timing out. The timeout delay is controlled by the 2 megohm (2m0) potentiometer and the 4.7 ufd capacitor and can be varied from 9 seconds to 120 seconds with the values shown.
