This circuit serves both as an emergency light (using a 6W fluorescent tube) and an ultraviolet light (using a 6W ultraviolet tube).
Step-down transformer X1, diodes D1 through D4, capacitor C1 and 6V regulator IC1 form the regulated power supply. The series combination of diodes D5 through D7 connected to common pin of IC1 increases the output voltage to 7.8 volts. This regulated voltage is applied through resistor R1 and diode D8 to charge the battery.
When mains power is available, the battery starts charging and LED2 glows. However, glowing of tubes depends on the switch positions as follows:
1. If both switches S2 and S3 are closed, transistor T2 conducts and provides ground path to the base of transistor T1. Thus the inverter does not oscillate and L1 (for emergency light) and L2 (for ultraviolet light) tubes don’t glow.
2. If switch S2 is closed and switch S3 is open, transistor T2 does not conduct and hence transistor T1 oscillates. Thus the inverter oscillates and L1 and L2 tubes glow.
When mains supply fails and inverter switch S2 is closed, irrespective of the position of switch S3, transistor T1 starts oscillating and inverter transformer X2 provides output voltage at its secondary, which causes glowing of tube L1 or L2 depending on the position of switch S1.
Switch S2 is used to switch off the emergency light or ultraviolet tube when it is not in use. Switch S1 is used to connect L1 or L2 tube according to your requirement. Glowing of LED1 indicates that the inverter is ‘on,’ while glowing of LED2 indicates that mains is available.
Ultraviolet light can be used for counterfeit currency detection. The fluorescent fibres of the proper currency notes clearly reflect when the currency note is illuminated by ultraviolet light.
Warning: UV light is very dangerous to the eyes and causes damage to the skin cells. So avoid direct exposure and looking directly into a UV light source.