Automatic railway signals are meant for efficient movement of trains, without colliding. The track is divided into blocks or sections. At the entry of each section there is a signal. The signal consists of three lights-red, amber and green in colour. Red light means the train should stop totally, because another train is in the next block. Amber means the train is to proceed slowly as another train is in the tracks one block ahead. Green means the train can shoot straight ahead as next two blocks are unoccupied, i.e. the line is clear. When there is no train anywhere; all signals should be green. But when the train is in’ one particular block, the signal for that block should turn red while signal for the previous block should turn amber. And these red and amber signals should change as the train moves to the next block. As manual working of the signals cannot be efficient enough, an automatic signalling system is required. This circuit is built around TTL gates. Each block requires two inverter gates (7404 or 7406) and two OR gates (7432). Rail C is, the common rail for the entire track having electrical continuity all along the track. Bl, B2 etc are the block rails. B1 is not directly connected to B2, B3 or B4. C rail is connected to positive end of SV DC supply. Eachblock rail is connected to negative side of the supply through 22k (or 47k) resistors. The actual value of the current- limiting resistors does not matter so long as they keep the current to a low level. To B1 may be connected the input of two inverter gates. One will drive the red signal of block 1 and the other will drive amber signal of previous block, i.e. block 4. – To B1 one input each of OR gates operating green signalsof blocks 1 and 4 may also be connected. The other end of OR gate operating green signal of block 1 is connected to B2 and of OR gate operating green signal of block 4 is con- nected to BQ. The pattern is repeated for all other blocks. The output of OR gate drives green LED through 220- ohm resistor, and outputs of inverter gates drive the red and amber LEDs. Each gate has current sinking capacity of 16 mA. Hence separate inverter gates are used for red and amber lights so that about l0mA can be safely-sunk in each. When there is no train anyvvhere, all blocks (Bl to B4) are at ‘low’ level. So all OR inputs are ‘low’ and all OR outputs are also ‘low’. Therefore, all green lights will glow. And since all inverter inputs are at ‘low’ level, all inverter outputs are at ‘high’ level. Therefore, allred and amber lights Will be off. When train is in block 1, it will short C rail to B1, and so Bl will be at logical ‘high’ level. Thus one input each of OR gates for block l and block 4 will be at logical ‘high’, and outputs of these two will also be at logical ‘high’. Therefore, green lights Gl and G4 will be off. Also, inputs for inverter gates controlling A4 and Rl will be at logical ‘high’, and so their outputs will be at logical ‘low’. Therefore LEDs Rl and A4 will glow. When the train proceeds to blocks 2, it will switch R2, Al and G4 on, while G2 and Gl will be switched off. Thus the train will automatically go on changing signals while moving. Any power supply regulated by lC 7805 will do. As tht circuit does not require more than 150 mA, a 250mA trans- former will do. A Signal poists” may be constructed using a black bakelite strip l cm wide and 5 to 6 cm liigh. LED holders can be fitted on it for mounting LEDs. ‘ – Red, green and yellow colored wires may be used for wiring, and these may be neatly laid beside the track to add to its decorative value.
l. lt is convenient to lay a closed track; it allows the train to move continuously. But closed tracks available with the train are too short in length. To increase the length of the track, use ‘T’ sections used as guide rails for sliding shutters used in cupboards. 2. The toy train used was ‘Taj Express’ which has a man- ual start and stop system. 3. By adding one inverter gate to each block we can set up a cabinman’s train position indicator. For this fix an LED in each block and let theacorresponding added inverter gate drive the LED. Thus the lit LED will indicate the position of the train. ,