Power outage emergency lights (2)

The switch S in the figure is usually in a closed state. The 220V AC power is stepped down by the transformer, rectified by the diode VD and
filtered by the capacitor C, and the DC working voltage is output for use by the transistor circuit. Due to the biasing effect of resistor R3. VT is in a saturated
conduction state, and its collector, that is, the gate of the silicon-controlled vs is at a low level, so vs is in an off state and the lamp H does not light up. At this time,
the light-emitting diode LED lights up, indicating that the mains power supply is normal. Once the mains power suddenly fails, due to the small capacity of the filter capacitor C,
the transistor VT is immediately cut off, thereby releasing the blockade of the thyristor vs. The gate of vs can obtain a forward trigger current through the resistor
R4, vs is immediately turned on, and the light H stands for luminous lighting. At this time, the light-emitting diode LED indicating normal power supply from the mains also
goes out.