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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
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