Please briefly describe the circuit principle of light-controlled music doorbell.
When there is light, the resistance of VT5 is very small, VT2 is turned on, VT3 VT4 is turned off, and relay K is not energized. When there is no light, the resistance of VT5 is very large, VT2 is turned off, VT3 VT4 is turned on, and its normally open point is closed. The C 1 C2 filter, R4 V3 and R5 V4 divide the voltage to make the IC work, and the input music signal is driven by VT5.