Electronic circuits > Lights & L.E.Ds > Emergency Light
Emergency LightThe 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
Published on: 2004-08-21 (3067 reads)
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