The circuit of automatic emergency
light presented here has the following features: 1. When the mains supply (230V
AC) is available, it charges a 12V battery up to 13.5V and then the battery
is disconnected from the charging section. 2. When the battery discharges up
to 10.2V, it is disconnected from the load and the charging process is resumed.
3. If the mains voltage is available and there is darkness in the room, load
(bulb or tube) is turned on by taking power from the mains; otherwise the battery
is connected to the load. 4. When the battery discharges up to 10.2V and if
the mains is not yet available, the battery is completely disconnected from
the circuit to avoid its further discharge. The mains supply of 230V AC is stepped
down to 18V AC (RMS) using a 230V AC primary to 0-18V AC, 2A secondary transformer
(X1), generally used in 36cm B&W TVs. Diodes D1 through D4 form bridge rectifier
and capacitor C5 filters the voltage, providing about 25V DC at the output.
Charging section includes 33-ohm, 10-watt resistor R2 which limits the charging
current to about 425 mA when battery voltage is about 10.2V, or to 325 mA when
battery voltage is about 13.5V. When the battery charges to 13.5V (as set by
VR2), zener diode D17 goes into breakdown region, thereby triggering triac TR1.
Now, since DC is passing through the triac, it remains continuously ?on? even
if the gate current is reduced to zero (by disconnecting the gate terminal).
Once the battery is fully charged, charging section is cut-off from the battery
due to energisation of relay RL2. This relay remains ?on? even if the power
fails because of connection to the battery via diode D10. S4, a normally closed
switch, is included to manually restart the charging process if required. Battery
disconnect and charging restart section comprises an NE555 timer (IC2) wired
in monostable mode. When the battery voltage is above 10.2V (as indicated by
red LED D15), zener diode (D16) remains in the breakdown region, making the
trigger pin 2 of IC2 high, thereby maintaining output pin 3 in low voltage state.
Thus, relay RL3 is ?on? and relay RL4 is ?off.? But as soon as the battery voltage
falls to about 10.2V (as set by preset VR1), zener diode D16 comes out of conduction,
making pin 2 low and pin 3 high to turn ?on? relay RL4 and orange LED D13. This
also switches off relay RL3 and LED D15. Now, if the mains is available, charging
restarts due to de-energisation of relay RL2 because when relay RL4 is ?on,?
it breaks the circuit of relay RL2 and triac TR1. But if the mains supply is
not present, both relays RL3 and RL1 de-energise, disconnecting the battery
from the remaining circuit. Thus when battery voltage falls to 10.2 volts, its
further discharge is eliminated. But as soon as the mains supply resumes, it
energises relay RL1, thereby connecting the battery again to the circuit. Light
sensor section also makes use of a 555 timer IC in the monostable mode. As long
as normal light is falling on LDR1, its resistance is comparatively low. As
a result pin 2 of IC3 is held near Vcc and its output at pin 3 is at low level.
In darkness, LDR resistance is very high, which causes pin 2 of IC3 to fall
to near ground potential and thus trigger it. As a consequence, output pin 3
goes high during the monostable pulse period, forward biasing transistor T3
which goes into saturation, energising relay RL5. With auto/bypass switch S2
off (in auto mode), the load gets connected to supply via switch S3. If desired,
the load may be switched during the day-time by flipping switch S2 to ?on? position
(manual). Preset VR3 is the sensitivity control used for setting threshold light
level at which the load is to be automatically switched on/off. Capacitors with
the relays ensure that there is no chattering of the relays. When the mains
is present, diode D8 couples the input voltage to regulator IC1 whereas diode
D10 feeds the input voltage to it (from battery) in absense of mains supply.
Diode D5 connects the load to the power supply section via resistor R5 when
mains is available (diode D18 does not conduct). However, when mains power fails,
the situation reverses and diode D18 conducts while diode D5 does not conduct.
. The load can be any bulb of 12 volts with a maximum current rating of 2 amperes
(24 watts). Resistor R5 is supposed to drop approximately 12 volts when the
load current flows through it during mains availability . Hence power dissipated
in it would almost be equal to the load power. It is therefore desirable to
replace R5 with a bulb of similar voltage and wattage as the load so that during
mains availability we have more (double) light than when the load is fed from
the battery. For setting presets VR1 and VR2, just take out (desolder one end)
diodes D7, D10 and D18. Connect a variable source of power supply in place of
battery. Set preset VR1 so that battery-high LED D15 is just off at 10.2V of
the variable source. Increase the potential of the variable source and observe
the shift from LO BAT LED D13 to D15. Now make the voltage of the source 13.5V
and set preset VR2 so that relay RL2 just energises. Then decrease the voltage
slowly and observe that relay RL2 does not de-energise above 10.2V. At 10.2V,
LED D15 should be off and relay RL2 should de-energise while LED D13 should
light up. Preset VR3 can be adjusted during evening hours so that the load is
?on? during the desired light conditions