Start with a simple circuit that turns and LED ON. Note that the is a resistor which is to safe guard the LED by liming the current. It is called a current limiting resistor. In this circuit we are using a 100 ohms resistor (which is actually low, it should be around 200 ohms. the reason we are using a lower value is, our LED blocks already have a 100 ohms resistor.) Notes: LED has tow pins, Anode and Cathode, note the connection carefully. For a real LED, longer pin is the Anode. Shorter pin or the side that has the flat side is the cathode.Here we are using a simple slide switch. We are using one side of the slide switch, that is the middle pin and the one pin on the right side. When we shift the slide switch to the right side, the circuit completes and the LED turns ON.Here we are expanding the circuit by adding one more LED to the other side of the slide switch. This circuit can turn one LED ON at a time with the help of the slide switch.Let us add a transistor, in this case an NPN transistor (like BC547). A transistor has 3 pins as shown below.
