Chapter-1c Page 7
This Spot Detector for a DC powered track will be activated when a locomotive wheel set is bridging between the isolated track and the powered rail. The DC Track voltage will be between 2.8 volts and 12 volts, plus or minus, depending on the motor speed and direction. It may be a constant voltage or pulsing from 0 volts to 12 or higher (some pulsed or PWM throttles can go higher than the 12 volts that the Construction motors are specified for). A small square of strip board or components and connecting wires The circuit is quite simple. As the train wheels make contact between the isolated track and the main track a circuit is completed and the track voltage is supplied to the rectifier. The voltage, rectified by the bridge, produces a DC voltage.
The Optical Coupler contains both an infrared LED and a photo detector such as a silicon diode, transistor or Darlington pair. With current flow in LED it shines on the Transistor which conducts and connects the two output pins.
4N25 Optical Coupler
This activates the optical coupler and the 'output' is connected to the 'Gnd' (ground) internally. This grounded connection can then be used by following low power circuits. 0 ma is the maximum current output If more current is needed then a transistor or a relay should be added. But the output is a pulse or series of pulses as the train passes the spot.
vero board, 14 holes by 5 can be used to hold the components. The bridge Rectifier type W04 (round) is mounted in 4 holes and will need the holes enlarging to 1.2 mm (#56). On the diagram the light holes and bars are the copper clad tracks which run horizontally on the underside. The dark holes are where the
are soldered and the 'X' marks where the copper is cut from around the hole to break the track circuit. The components sit on the topside.
The cuts on the copper side are in column 10 and at rows 1, 2, 3 and 5. Twist a 1/8" (#31) drill bit in the hole to shave off the copper and leave a break in the strip.
