A simple switch circuit using a transistor. When the switch is closed, current flows through the base to the emitter (this junction acts like a diode). This switches the transistor on, so that current can flow from the collector to the emitter. A small base current can control a much larger current from the collector to the emitter.

The transistor wants the collector-emitter current to be 100 times the base current, but it can't, because such a large current through the 300 ohm resistor would bring the collector below ground. So, the transistor is in saturation mode; it brings the collector voltage as low as possible.

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