Timed Station Stop
Timed station stops are a useful adjunct to any layout, especially at exhibitions, where they give the operator some breathing space (cups of tea, natural breaks etc.!) If a simple unramped “stop-start” is all you need, this circuit fits the bill. If used with a slow running railcar or tram, the abrupt nature of the starts and stops will not be so apparent.
The unit requires a 12V d.c. smoothed supply (the Voltage Regulator circuit described in these pages is ideal), and a standard loco controller, which is used to set the loco speed. The unit is triggered by a loco mounted magnet passing over a reed switch fitted lengthways between the rails, at the required stopping point.
The magnet is attached lengthways to the underside of the loco so that there is a maximum distance of 5mm between it and the reed when the loco is on the rails.
How it Works. At first sight, the circuit looks rather complicated, but it can actually be broken down to two almost identical stages. Both IC1 and IC2 are configured as monostables, i.e. standard timing circuits. The first monostable, IC1, is included to introduce a delay before the loco stops, to avoid the magnet being over the reed switch at restart (without it, the train would stay in the same place ad infinitum!).
At switch on, IC1 pin 2 is at virtually supply potential, but when the loco runs over the reed switch, this pin goes to 0v momentarily, triggering the monostable. The output pin 3 goes high for a few seconds after triggering, allowing the train to clear the reed, and then returns to 0v. This change from +V to 0V is transferred, via C5, to the second monostable, which is in turn triggered, producing a long, high output pulse. The pulse is passed on to the relay driver TR1, causing the relay contacts to open, stopping the train. The length of the “stopping time” may be adjusted by means of VR1. The approximate range is from a few seconds to around 2 minutes.
Construction and Testing. Layout of the components is not critical, but the interconnecting wires should be kept as short as possible, especially those connected to the trigger reed. Be careful when handling the reed switch, as it has a glass encapsulation, and is consequently very fragile.
There is no set-up procedure, just connect the unit to the power source and train controller, set the controller speed control and the unit’s waiting time control to one quarter of their full rotation, and switch on. The train should run normally until the loco runs over the reed switch, whereupon it should stop and wait for about 30 seconds.
If the train fails to stop, check that the reed is not too far away from the magnet when on the track. If necessary, pack up the magnet with a piece of plastic card or similar.