31-03-2012, 04:15 PM
LIGHT SENSITIVE TRIGGER
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How it Works
The 555 is used in this kit as a monostable
multivibrator. Resistor R1 in the circuit diagram is
so adjusted that under normal conditions when light
is falling on LDR, the voltage across the LDR is
less than 1/3 of supply voltage. As the day light
fades or the light on the LDR is interrupted by an
intruder, the voltage across it rises above 2/3 rd of
Vcc – [supply voltage], tripping the IC flip flop. The
output goes low actuating the relay.
Adjustments
This kit does not need any adjustments, if you
follow the building instructions.
How it Works
The 555 is used in this kit as a monostable
multivibrator. Resistor R1 in the circuit diagram is
so adjusted that under normal conditions when light
is falling on LDR, the voltage across the LDR is
less than 1/3 of supply voltage. As the day light
fades or the light on the LDR is interrupted by an
intruder, the voltage across it rises above 2/3 rd of
Vcc – [supply voltage], tripping the IC flip flop. The
output goes low actuating the relay. When the light
is restored, the voltage falls below 1/3 Vcc, again
tripping the flip-flop causing the output go high &
the relay de-energises. R2 & C1 ensure a delay of
10mili-seconds before the relay is energised, so
that the circuit is rendered in sensitive to rapid
changes in light intensity.
The sensitivity of the circuit can be controlled if R1
is replaced with 1M pot or preset.
Construction
You shouldn’t face any special problems with this
project. The only unusual component is the LDR As
usual start building the circuit with the resistors
leaving the LDR for the final stage.