The BD243C is a 6A, 100V NPN Bipolar Power Transistor is designed for use in general purpose amplifier and switching applications. BD243B, BD243C (NPN); BD244B, BD244C (PNP) are complementary devices.
The IRF740PBF is a 400V 10A 125W TO220AB Package Third generation power MOSFETs from Vishay provide the designer with the best combination of fast switching, low on-resistance and cost-effectiveness.
The BC548 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.
The YX850 Power Failure Automatic Switching Standby Battery Lithium Battery Module DC5V-48V Emergency Converter. It is an automatic switching module. When the battery is dead or there is a problem with the battery, it is powered by the power supply.