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This relay driving circuit is present on the body of the train.

Initially our trainís battery was of 3 volts (two batteries of 1.5 volts each) and was directly connected to the motor of the train,so,in order to introduce this relay driving circuit in the train,we had to cut the connection between the motor and the battery.

For normal operation of the train i.e. when no light falls on the photodetector,the thyristor gets no gate signal and is in OFF condition i.e. the thyristor part of the circuit is open circuited. In this condition when the switch is closed the current reaches straight to the SL100 transistor,makes it ON and then drives the the motor and the train starts moving.

Now,when there is a red signal at at the posts connected to the stoppers,the LED of stopper also starts glowing. When the train having the above circuit crosses the stopper,the LED light falls on the photodetector of the relay driving circuit and as a result the thyristor becomes ON. Now,this part of the circuit behaves like a short circuit and hence no current reaches the motor of the train and as a result the train stops. As the stopper is kept just before the stations,so,the train stops outside the station when there is a red signal at the post.

Note: There is an extra 1.5 volt battery in the circuit,this is because 3 volts battery was not enough to drive the motor of the train because of presence of the thyristor in the circuit.

And the two capacitors have been provided to bypass the internal capacitance of the thyristor in order to avoid unwanted triggering of the thyristor, the capacitors do the function of snubber circuit in this circuit.