Process

• I thought I had correctly wired the transistor and diode, but it didn't work at all. It worked only when I ignored both elements. I suspected that the resistance amount of the transistor was too high, causing insufficient current. 

• With the help of TA Zhenfang, I learned that to reach the required current of 1.1A for the solenoid used in class, the voltage assigned to the transistor needed adjustment. Using a 1000Ω resistor, the solenoid current only reached a mere 3.3mA (3.3V/1000Ω). Meaning, I decreased the resistance to increase the entire current and reduced the voltage consumed by the transistor to increase the voltage allocated to the solenoid. 
  

• Even reducing it to 47Ω (70mA, Solenoid allocated voltage 4.8V according to Zhenfang), only faint sound was heard, and it didn't work. However, a 30Ω resistor worked. Referring to the collector saturation region graph from the transistor specification, 30Ω  allows 110mA (3.3V/30 Ω), the voltage allocated to the transistor subtly decreases resulting in increasing the voltage size for the solenoid.