OBJECTIVE

The objective is to build a fully automatic emergency light that remains off in the normal circumstances and switching on automatically in the event of mains power failure. Such gadgets are very useful when used in places such as electricity meter box or the place where the various cut outs are located. The other useful places for these gadgets are cupboards and other dark corners where a little bit of light in case of power failure is of great utility. Another gadget that had similar application but was not automatic was discussed earlier in one of the projects. In that case, one needed to switch on the light manually at the location where the gadget is located. It causes a little bit of inconvenience when reaching that place in complete darkness is itself a big problem.

Circuit Description

The circuit operates as follows: In case mains power is available, it is rectified, filtered and regulated to produce 9VDC T-1 is the step-down transformer, diodes D1 and D2 constitute the full wave rectifier along with the center tapped secondary winding, C2 is the filter capacitor and VR_1 is a 9V three terminal regulator. C1 and C3  are decoupling capacitors. This 9V is used to provided constant current charging for the 6V battery (5, 1.2V, Ni-cd cells in series). Diode D3 is forward biased during charging. The forward biased diode D3 ensures that emitter potential of transistor is less than that of its base potential. As a result, transistor (Q1) remains in cut-off.(R1 ) is so chosen that the charging current for the Ni-cd battery pack is about 100mA. To sum up, in the presence of mains power, transistor (Q1) is OFF and the battery is charged from the DC supply produced from mains.

In case of mains failure, the anode voltage of diode (D3) falls below the cathode voltage of (D3). D3 becomes reverse biased, the base emitter junction of (Q1) is forward biased and it conducts. The lamp is lighted. Thus as long as mains power is absent, the lamp gets its power from the battery. The time period for which the lamp Could be maintained in the lighted state depends upon the capacity of the battery. The lamp extinguishes automatically when the mains power is restored. The charge drained out of the battery during mains power OFF condition is now replenished again.

CONSTRUCTION GUIDELINES

Figs. 15.2 and 15.3 respectively show the PCB layout and the components layout.

In case, the three terminal regulator type 7809 is not available, as it is not a commonly used value, the same could be simulated from a 5V regulator (type 7805) as pestle circuit configuration shown in Fig. 15.4. That is, what you have to do is to connect a 3.9V zener diode from common terminal

of the regulator to ground instead of grounding the common terminal.:

PARTS LIST

Resistors & Capacitors

R1 : 27Ω, 1/2W

R2 :  470 Ω, 1/4W

C1, C3 : 0.1µF (Geramic disc)

C2 :  1000 µF, 16V (electrolytic)

Semiconductor Devices

D1, D2,  :    1N4001 or equivalent

Q1 : 2N2907

VR-1 : Three terminal regulator, Type No. 7809

Other Components

S1 : Mains power ON/OFF switch

F1 : Fuse (0.5A rating) with holder

T-1 :  Mains transformer Primary : 230VAC Secondary : 12-0-12, 250mA

Lamp : 6 VDC Lamp

Battery : 5×1.2V, 2Ah Ni-cd Cells

Miscellaneous

Multistrand wires, suitable mounting cabinet, solder metal, mains lead etc.

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