Now you need not fear dark nights when power breaks down. Here’s a white LEDbased Emergency light that automatically turns on when the mains power supply fails.
Things which you can learn:
*Application of NE555 inastable mode.
*Application of LDR in light and dark.
*Application of bridge rectifier.
Build an Emergency light which will automatically switch on when the power fails, thus obviating the need to search the light and switch it on manually!!
The switching circuit is built around an NE555 timer (IC1), which is wired in monostable mode. When a low voltage is applied at trigger pin 2 of IC1, the timer activates and its output pin 3 goes high. It remains in that state until IC1 is triggered again at its pin 2. Light-dependant resistor LDR1 is connected between the positive supply of the battery and trigger pin 2 of IC1. Resistor R3 is connected between pin 2 of IC1 and ground. The resistance value of LDR1 remains high in the dark (at night) and low in ambient light (in daytime). This phenomenon is utilised to control the switching circuit.
The working of the circuit is simple. In daytime, when ambient light falls on LDR1, its resistance decreases to make trigger pin 2 of IC1 high. As a result, output pin 3 goes low and the LEDs(LED1 through LED7) remain off. At night (in the dark), the resistance of LDR1 increases and a low voltage is applied to trigger pin 2 of IC1. This activates the monostable and its output goes high to make all the LEDs glow.