Input voltage range: 85VAC ~ 264 VAC external capacitor, recommended value 6.8uF / 400V output filter capacitor C2 is electrolytic capacitors. Ceramic capacitors C1, C3 can be film capacitors or electrolytic capacitors
The metering device uses silicon photocell as sensors, and light-emitting diodes as the light intensity indicator. The stronger the light, the more the number of LED lights. This circuit can also be used to check the ambient light intensity whic is suitable for reading and writing.
Working: Figure VT1-VT5, LED1-LED5 sows level indicator line, it is the use of the signal voltage amplitude. When both ends of the variable resistor RP receives no input signal voltage, VT1-VT5 base does not receive forward voltage, so the five transistors are in the OFF state, so, LED1-LED5 does not glow. When RP gets signal voltage input, VT1-VT5 base will get a positive voltage, due to the different value of the signal voltage, and the role of each diode drop VD1-VD4 (each tube voltage drop of about 0.6v or so), will VT1-VT5 each base forward voltage obtained by decreasing one by one. In some transistors, in saturation conduction region, some work in the higher input area, while others are in the OFF state. Therefore, the light-emitting diodes, some glow, some dark, some not lit, indicating the strength of the signal level output.
VT6 is silicon photovoltaic cell composed of photoelectric conversion lines GL, GL when light does not radiates, it does not produce a voltage at both ends, so that the transistor is at off state, RP there is no voltage output at both ends, then LED1-LED5 are off. When light shines on the GL, GL will produce a corresponding voltage at both ends, so make VT6 conduction. Conduction degree VT6 with GL output voltage magnitude, that is proportional to the light intensity of irradiation on the GL. So light is stronger, RP obtained across the higher voltage. or making more LED’s to glow,the adjustment value can be adjusted using RP, which lits LED1 witin a threshold level.