Exercise ten, “Transistor Switching”. This exercise introduces a very important component which has characteristics similar to the relay. Whereas both components can switch the flow of electrical current. That said, the documentation explains the ins and outs of transistors, in that they are naturally off until turned on; considered a limitation. Relays on the other hand offer more switching options, i.e., can be normally open, normally closed, or utilize a double-throw switch. When it comes to determining which component to choose for your needs it will always come down to the requirements for your application.
Through observation you will note that the LED is off (not lit). Voltage travels through R1 and arrives at the collector pin of the transistor. There is a small amount of voltage leakage but not enough to light the LED. When the push-button is depressed voltage also travels through R2 and reaches the base of the transistor. The voltage that arrives at the base causes the transistor to open its solid state switch – current flows down to the emitter of the transistor as a result. R3 is in place to prevent the LED from receiving too much voltage and burning out.
Overall, this was another fun chapter that covers the necessary basics of transistors (NPN and PNP). In addition to topics related to current. The author continues to do a great job making the information provided accessible for those willing to attempt understanding.
Also, I was able to simulate this circuit demonstrating the LED lighting up when engaging the push-button. In the video, the “Green Arrows” represent the LED receiving voltage.[wpvideo nH0TcgaT]
-AC Adapter, breadboard, wire, and meter