Direction-Sensitive Light Barrier

Model railway hobbyists who want to trigger train-controlled switching operations depending on the direction of travel often have things easy with systems such as Märklin HO or Miniclub Z gauge, since the corresponding switching track sections can be actuated individually in both directions. Things are different with other systems, such as the Märklin Maxi/1 gauge. Here it’s necessary to use selective switching hardware for this purpose. A popular approach is to use reed switches together with permanent magnets fitted to the bases of the vehicles. Unipolar and bipolar Hall switches can also achieve the same objective. However, a permanent magnet must always be present on the train to act as a trigger element.Although normal light barriers cannot be used for direction-dependent control, with a bit of additional effort and expense they can be built such that trains selectively trigger switching operations in the desired direction. To prevent the gaps between carriages or wagons from generating undesired pulses, an adjustable dead time is added after the sensor.

Resistors:
R1,R2 = 10-100kOhm (see text)
R3,R4 = 100kOhm
R5,R6 = 47kOhm
R7 = 1MOhm5
R8,R9 = 27kOhm
P1,P2 = 1MOhm preset
Capacitors:
C1-C4 = 10nF
C5,C6 = 100µ F 16V radial
C7-C10 = 100nF
C11 = 4µ F7 16V
C12 = 470µ F 35V
Semiconductors:
B1 = B80C1500 (80V piv, 1.5A)
D1-D4 = 1N4148
IC1 = 4093
IC2 = 556
IC3 = 7812
T1,T2 = BPW40
Miscellaneous:
RE1, RE2 = 12V relay (400Ohm ), 1× changeover contact
K1 = 2 solder pins
K2,K3 = 3-way PCB terminal block, lead pitch 5mm
PCB, order code 030333-1 from The PCBShop