low cost emergency light project class 12 physics
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low cost emergency light project class 12 physics
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[Image: LOW-COST-AUTOMATIC-EMERGENCY-LIGHT.gif]

 Here is an emergency light based on a white LED that offers the following advantages:
o It is very bright due to the use of white LEDs.
o The light turns on automatically when the mains supply fails and turns off when the mains supply is resumed.
o Has its own battery charger. When the battery is fully charged, charging stops automatically.

The circuit consists of two sections: charger power supply and LED driver. The power supply section of the charger is built around the 3-way adjustable regulator (IC1) LM317, while the LED controller section is built around the transistor BD140 (T2). In the power supply section of the charger, the input of the AC network is reduced by a transformer to deliver 9 V, 500 mA to the bridge rectifier, which comprises diodes (IN4007x4). The filter capacitor (25v / 1000uf) eliminates undulations. The unregulated DC voltage is fed to the input pin 3 of IC1 and provides charging current through the diode IN4007 (D5) and the limiting resistor (16ohm) R16. By adjusting 2.2K preset (VR1), the output voltage can be adjusted to deliver the required load current. When the battery is charged at 6.8V, the zener diode conducts and the regulator charge current (IC1) finds a path through transistor BC547 (T1) to ground and stops charging the battery. The LED driver section uses a total of twelve white 10 mm LEDs.

All LEDs are connected in parallel with a 100 ohm resistor in series with each. The common anode junction of the twelve LEDs is connected to the collector of the pnp transistor T2 and the emitter of the transistor T2 is connected directly to the positive terminal of the 6 V battery. The unregulated DC voltage, produced at the junction of the cathode of the bridge (diodes), is fed to the base of transistor T2 through a resistance of 1k. When the network power is available, the base of transistor T2 remains high and T2 does not drive. Therefore, the LEDs are off. On the other hand, when the network fails, the base of transistor T2 becomes low and leads. This makes all the LEDs (LED1 to LED12) shine. The mains power supply, when available, charges the battery and keeps the LEDs off when transistor T2 remains cut off.

During network failure, the charging section stops working and the battery supply causes the LEDs to glow. Assemble the circuit to a general-purpose PCB and enclose it in a cabinet with enough space for the battery and switches. Mount the LEDs in the cabinet to illuminate the room. A hole must be drilled in the cabinet to connect the 230 VAC input to the primary of the transformer. I tested the circuit with twelve white LEDs of 10 mm. You can use more LEDs as long as the total current consumption does not exceed 1.5A. The conductive transistor T2 can deliver up to 1.5 A with the proper disposition of the heat sink.
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