The Darlington Bipolar Power Transistor is designed for general-purpose amplifier and low speed switching applications. TIP120, TIP121, TIP122 (NPN); TIP125, TIP126, TIP127 (PNP) are complementary devices.
The MT3608 Step-up converter (Output voltage higher than input). Supply voltage 2-28V, output max. 28V at max 2A. Avoid the state when the input terminals have higher voltages than those set on the output terminals. The drive chip may be damaged.
The BC547 is a NPN transistor hence the collector and emitter will be left open (Reverse biased) when the base pin is held at ground and will be closed (Forward biased) when a signal is provided to base pin.