Automatic Lower Power Emergency Light
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H ere is a white-LED-based emergency light that offers the following advantages: 1. It is highly bright due to the use of white LEDs. 2. The light turns on automatically when mains supply fails, and turns off when mains power resumes. 3. It has its own battery charger. When the battery is fully charged, charging stops automatically. The circuit comprises two sections: charger power supply and LED driver. 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 sec- tion, input AC mains is stepped down by transformer X1 to deliver 9V, 500 mA to the bridge rectifier, which com- prises diodes D1 through D4. Filter capacitor C1 eliminates ripples. Un- regulated DC voltage is fed to input pin 3 of IC1 and provides charging current through diode D5 and limit- ing resistor R16. By adjusting preset VR1, the output voltage can be ad- justed to deliver the required charg- ing 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. The LED driver section uses a to- tal 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 volt- age, produced at the cathode junc- tion of diodes D1 and D3, is fed to the base of transistor T2 through a 1- kilo-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 bat- tery and keeps the LEDs off as transistor T2 re- mains cut-off. During mains failure, the charging sec- tion stops working and the battery sup- ply makes the LEDs glow. Assemble the circuit on a general-pur- pose PCB and enclose in a cabinet with enough space for battery and switches. Mount the LEDs on the cabi- net such that they light up the room. A hole in the cabinet should be drilled to connect 230V AC input for the pri- mary of the transformer. circuit with twelve 10mm white LEDs. You can use more LEDs provided the total current consumption does not ex- ceed 1.5A. Driver transistor T2 can de- liver up to 1.5A with proper heat-sink arrangement. Fig. 1: Automatic high intensity LED-based emergency light Fig. 2: Pin configurations of LM317, BD140 and BC548 AUTOMATIC LOW-POWER EMERGENCY LIGHT Education On-Line v i no lab note. We have tested the
Transcript of Automatic Lower Power Emergency Light
H ere is a white-LED-basedemergency light that offersthe following advantages:
1. It is highly bright due to the useof white LEDs.
2. The light turns on automaticallywhen mains supply fails, and turns offwhen mains power resumes.
3. It has its own battery charger.When the battery is fully charged,charging stops automatically.
The circuit comprises two sections:charger power supply and LED driver.The charger power supply section is
built around 3-terminal adjustableregulator IC LM317 (IC1), while theLED driver section is built aroundtransistor BD140 (T2).
In the charger power supply sec-tion, input AC mains is stepped downby transformer X1 to deliver 9V, 500mA to the bridge rectifier, which com-prises diodes D1 through D4. Filtercapacitor C1 eliminates ripples. Un-regulated DC voltage is fed to inputpin 3 of IC1 and provides charging
current through diode D5 and limit-ing resistor R16. By adjusting presetVR1, the output voltage can be ad-justed to deliver the required charg-ing current.
When the battery gets charged to6.8V, zener diode ZD1 conducts andcharging current from regulator IC1finds a path through transistor T1 toground and it stops charging of thebattery.
The LED driver section uses a to-tal of twelve 10mm white LEDs. Allthe LEDs are connected in parallelwith a 100-ohm resistor in series witheach. The common-anode junction of
all the twelve LEDs is connected tothe collector of pnp transistor T2 andthe emitter of transistor T2 is directlyconnected to the positive terminal of6V battery. The unregulated DC volt-age, produced at the cathode junc-tion of diodes D1 and D3, is fed tothe base of transistor T2 through a 1-kilo-ohm resistor.
When mains power is available, thebase of transistor T2 remains high andT2 does not conduct. Thus LEDs are
off. On the other hand, when mainsfails, the base of transistor T2 becomeslow and it conducts. This makes allthe LEDs (LED1 through LED12) glow.
The mains power supply, whena v a i l a b l e ,charges the bat-tery and keepsthe LEDs off astransistor T2 re-mains cut-off.During mainsfailure, thecharging sec-tion stopsworking andthe battery sup-ply makes theLEDs glow.
A s s e m b l ethe circuit on ag e n e r a l - p u r -pose PCB andenclose in acabinet withenough spacefor battery and
switches. Mount the LEDs on the cabi-net such that they light up the room.A hole in the cabinet should be drilledto connect 230V AC input for the pri-mary of the transformer.
circuit with twelve 10mm white LEDs.You can use more LEDs provided thetotal current consumption does not ex-ceed 1.5A. Driver transistor T2 can de-liver up to 1.5A with proper heat-sinkarrangement. �
Fig. 1: Automatic high intensity LED-based emergency light
Fig. 2: Pin configurations of LM317, BD140and BC548
AUTOMATIC LOW-POWER EMERGENCY LIGHT
Education On-Line
v i no lab note. We have tested the
vinothkumar.J
vinoandvino
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