A small signal transistor can be used for making many different electronic circuits. But newcomers to electronic are quite unable to exactly figure out how to connect these devices in an easy way and make them operate correctly.
How to use a transistor bc547 and operate it:
A transistor is basically an elctronic switch capable of toggling small electrical loads like LEDs or relays, in response to external triggers. These controls are done by appropriately configuring its three leads in conjunction with other basic electronic parts like resistors and LEDs or relays.
http://homemadecircuitsandschematics.blogspot.in/ If you take a transistor say for example a BC547,
Suppose you want to test the basic operation of this device as mentioned above by switching an LED, you can do it through the following steps:
Fix the transistor over a veroboard by soldering its leads.
Connect a resistor (10K) over the board by soldering so that one of its ends fuses with the base of the transistor.
Next connect the LED on the PCB by soldering with a series resistor such that the cathode of the LED solders with one of the resistor ends and the resistors other end solders with transistor collector.
Now connect the above circuit to a supply voltage of 3 Volts by applying positive to the LED anode and negative to the transistor emitter leg.
At this point the LED will show no effect, but the moment you connect the free end of the 10K resistor to the positive of the supply, the LED lights up, remioving the trigger again switches OFF the LED.