In the output subsystem, the LED is connected to the power source via the transistor "switch". Now adjust the voltage divider components, to see the changes to the circuit when the transistor switches on... You should be able to see the larger blue current flowing from the positive side of the power source and entering the LED and transistor's collector. The current then flows out of the transistor's emitter, with the input subsystem's smaller green base current being added to it. Both currents flow back to the negative side of the battery to complete the circuits. The transistor is acting as a current switch and It is important to match its power capabilities with the actual current required by the output device(s). The FastTrack designers do this automatically for you. Further Reading:
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