Stop-signal Override for Model Railways:
Parts List: R1,R2 = 10K R3,R4 = 6K8 R5 = 330 ohm (see text) C1 = 100uF/40V C2 = 1uF/25V IC1 = CD4013 CMOS IC IC2 = LM7812 volt regulator, 12V (see text) Q1 = BC547B NPN transistor, 2N2222, 2N3904, etc. D1-D4 = 1N4001, or small Bridge Rectifier D5 = DUS, 1N4001 D6 = Led 5mm, Red Ry1 = Relay, 9V S1,S2 = Toggle switch, SPST TR1 = Small Transformer 15-18VAC Maximum current the BC547B and 2N3904 can provide is 100mA. The 2N2222A can provide up to 800mA with a small coolrib, but a little overkill.Description:
Model railway enthusiasts know the problem: stop-signals disconnect the supply voltage from a section of track as soon as the signal indicates stop. Any train approaching the signal is obliged to stop on the dead section of track. It can only continue its journey when the stop-signal indicated line-clear or reduce-speed.
The problem arises when a train is supposed to approach the stop-signal from the opposite direction during shunting or at rural stations. This is not possible with the usual stop-signal circuit which disconnects the supply voltage, thus preventing traffic in both directions.
What we need is a circuit that allows the stop-signal to operate almost like a diode: in the normal direction the signal stops trains but allows trains to travel in the reverse direction.
As usual, at Elektor's Electronics Lab we have found a simple solution.
In addition to the stop-signal we need two rail contact at the two ends of the controlled section (see figure 1). These are contacts A at the end and contact B at the beginning, when viewed in the normal direction of travel The train traveling in the reverse direction reaches contact-A first. The contact closes and sets flip-flop IC1. The Q-output of the flip-flop goes to logic '1' and energizes the relay via Q1. The relay contact closes and bridge the circuit that was originally disconnected by the stop-signal. The train can travel in reverse along the controlled section of track. As soon as it reaches contact B the flip-flop is reset and the original state is restored. A train traveling in the normal direction first reaches contact B, thus causing the flip-flop to be reset and enabling proper functioning of the stop-signal.
LED D6 (drawn with dashed lines in the circuit) lights up when the signal override is effective.
The power supply of the override circuit is generously rated and can power several circuits of this type. The current drawn by one of these circuits depends on the relay used. Transistor Q1, the BC547B, 2N3904, or the 2N2222A (and others) can supply a maximum of 100mA which is enough to activate most small relays, and up to 800mA for the 2N2222A.
Instead of using separate transformer for the power supply, the AC voltage can also be obtained from the 'lighting-output' of a model railway transformer.
If a 12V relay is utilized the AC voltage required is 15 to 18Vac and a 7812 volt regulator is needed for IC2. With a 5V relay a type 7805 regulator must be used for IC2, in which case the transformer secondary voltage should be approximately 8-12V (although the 5V regulator may get hot if the AC voltage is too high). With a relay voltage of 5V, the value of R5 should be 120 ohm.
Redrawn and rewritten by Tony van Roon.
Source: "Elektor Electronics" magazine, Copyright © Elektor
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