With this instructable surely you will get an idea about using a BC547 transistor as a switch.
Step 1: Getting to Know BC547 Transistor
The transistor we are going to use has three terminals collector,emitter and base.Out of these the base is for controlling the input across collector and emitter.The transistor has three operating regions which are
In these regions the transistor will work as a switch in saturation and cut-off regions and as an amplifier in active region.
Step 2: Recommended Current and Voltage Ratings
The above mentioned current and voltage ratings are very important for the proper working of the transistor.The main thing to keep in mind is the base current, it should not exceed 5mA. And you can connect upto a maximum of 100mA load to the collector emitter area.