AC and DC darkroom safety light circuit

This darkroom AC and DC dual-purpose lamp has a contactless automatic transfer switch function. Uses orange light-emitting diodes. During normal power supply, AC 220 volts is rectified by bridge type of transformer, filtered by capacitor, and outputs about 4V DC (5V when no load). Because the battery voltage inside the machine is 3V, diode D is turned on, and the light-emitting diodes are powered by the rectified power supply. Since the BG emitter potential is lower than the base potential, BG is turned off, and the battery E in the machine is automatically turned off, and there is no current output. During a power outage, the BG emitter potential is higher than the base potential and is in a conductive state. Battery E supplies power to the light-emitting diode via BG. If the mains power is restored, diode D will be turned on again and BG will be automatically cut off.