This is LED emergency light that will turned on automatically when mains supply fails, and turned off when mains power resumes. This circuit produces highly bright because the use of ultra bright white LEDs. This circuit also has its own battery charger. When the battery is fully charged, the charging process will stop automatically.

Components List:

R1           = 180R - 1/2W
R2           = 1.2K
R3 - R14     = 100R - 1/2W
R15          = 1K - 1/2W
R16          = 16R/5W
VR1          = 2.2K
C1           = 1000uF/25V
D1-D5        = 1N4007
D6           = 6.8V - 0.5W Zener
LED1 - LED12 = Ultrabright White LEDs
T1           = BC548
T2           = BD140
B1           = 6V - 4.5Ah Rechargable Battery
IC1          = LM317
T1           = 9VAC-Transformer

The circuit contains two important blocks: charger power supply section and LED driver section.

Charger Section

The charger power supply section is built around 3-terminal adjustable regulator IC LM317 (IC1), while the LED driver section is built around transistor BD140 (T2).

In the charger power supply section, input AC mains is stepped down by transformer X1 to deliver 9V, 500 mA to the bridge rectifier, which comprises diodes D1 through D4. Filter capacitor C1 eliminates ripples. Unregulated DC voltage is fed to input pin 3 of IC1 and provides charging current through diode D5 and limiting resistor R16. By adjusting preset VR1, the output voltage can be adjusted to deliver the required charging current.

When the battery gets charged to 6.8V, zener diode ZD1 conducts and charging current from regulator IC1 finds a path through transistor T1 to ground and it stops charging of the battery.

LED Driver

In this LED emergency light circuit, the LED driver section uses a total of twelve 10mm white LEDs. All the LEDs are connected in parallel with a 100-ohm resistor in series with each. The common-anode junction of all the twelve LEDs is connected to the collector of pnp transistor T2 and the emitter of transistor T2 is directly connected to the positive terminal of 6V battery. The unregulated DC voltage, produced at the cathode junction of diodes D1 and D3, is fed to the base of transistor T2 through a 1K ohm resistor.

When mains power is available, the base of transistor T2 remains high and T2 does not conduct. Thus LEDs are off. On the other hand, when mains fails, the base of transistor T2 becomes low and it conducts. This makes all the LEDs (LED1 through LED12) glow.

The mains power supply, when available, charges the battery and keeps the LEDs off as transistor T2 remains cut-off. During mains failure, the charging section stops working and the battery supply makes the LEDs glow.

You can construct this LED emergency light circuit on a general-purpose PCB and mount the circuit in a plastic with enough space for battery and switches. Mount the LEDs on the cabinet such that they light up the room. A hole in the cabinet should be drilled to connect 230V AC input for the primary of the transformer.

Note: Since the transistor T2 can deliver up to 1.5A with proper heatsink, you may use more LEDs provided the total current consumption does not exceed 1.5A. Standar LED current consumption is about 20mA only.

The schematic diagram shown right here is the automatic switching-on emergency light circuit which is controlled using IC. The most important capabilities of this circuit are: automatic switching-on of the light on main power failure and battery charger with overcharge protection.

When mains electrical power is absent, relay RL2 is in deenergised state, feeding DC source from battery to inverter section via its N/C contacts and switch S1. The inverter section comprises IC2 (NE555) that is applied in stable mode to generate sharp pulses / wave with frequency of 50 Hz to drive the power MOSFETs. The output of IC3 is fed to gate of MOSFET (T4) directly while it is applied to MOSFET (T3) gate just after inversion by transistor T2. Therefore the power amplifier designed close to MOSFETs T3 and T4 functions in push-pull mode.

The output across secondary of transformer X2 can simply drive a 230-volt, 20-watt fluorescent tube. In event light isn’t needed to become on during mains power failure, then just flip switch S1 to off position.

Battery overcharge preventer circuit is designed close to IC1 (LM308). Its non-inverting pin is held at a reference voltage of about 6.9 volts that is obtained implementing diode D5 (1N4148) and 6.2-volt zener D6. The inverting pin of IC1 is connected to the positive terminal of battery. Thus when mains electric supply is present, IC1 comparator output is high, unless battery voltage exceeds 6.9 volts. So transistor T1 is normally forward biased, which energises relay RL1. Within this state the battery stays on charge via N/O contacts of relay RL1 and current limiting resistor R2. When battery voltage exceeds 6.9 volts (overcharged condition), IC1 output goes low and relay RL1 gets deenergised, and thus stops more charging of battery.

MOSFETs T3 and T4 may be mounted on appropriate heat sinks to prevent overheating on the MOSFETs and keep the MOSFETs in good performance.

This automatic switching-on emergency light circuit taken from EFY magazine. The circuit is already tested and should be working properly. This circuit idea available in PDF document, download from the following link:

Automatic Emergency Light Project

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