All three LEDs of AC mains voltage indicator are connected between collectors of transistors T1, T2, T3 respectively. Here potentiometer VR1, VR2, and VR3 are used to adjust the base voltage of transistor T1, T2 and T3 respectively. As shown in circuit diagram first AC mains is stepped down by 9V-0-9V transformer and then rectified by a signal diode D1 and smoothed by C1 which give output 25V DC.
This circuit is work on the principle, when AC mains vary DC voltage also varies proportionally and sensed by transistor T1 through T3. For setting the low level voltage, a manual AC voltage regulator (MVR) should be connected to the primary of transformer X1. Now set AC voltage of MVR to about 175V and slowly potentiometer VR1 adjusted until voltage across the base of transistor T1 reaches 9.7V and transistor starts conducting which glow LED1 and stop glowing when the base voltage drops below the preset value.
This process is repeated for 200V and 230V in which LED2 and LED3 glows respectively. Now connect this circuit to AC mains, if the voltage drop below 175 volts no LEDs glow. First, a high voltage (more than 230V) is indicated by all three LEDs glows (LED1, LED2, LED3). Second, normally voltage (200V-230V) is indicated by two LEDs (LED1 and LED2). Third, a low voltage (175V-200V) is indicated by the glowing of LED1 only.
PARTS LIST
Resistors (all ¼-watt, ± 5% Carbon)
R1- R6= 1 KΩ
VR1 – VR3 = 10 KΩ
Capacitor
C1 = 220 µF/50V
Semiconductors
T1 – T3 = BC547
ZD1 – ZD3 = 9.1V zener diode
D1 = 1N4001
LED1 – LED3 = Simple LED
Miscellaneous
X1 = 230V AC primary to 9V-0-9V, 250mA secondary transformer
SW1 = On/off switch
