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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
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