OVER LIGHT SENSOR ALARM CIRCUIT

  
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The LDR is a variable resistor whose resistance decreases with increase in light intensity. When the light falling on an LDR has normal intensity, its Resistance is large enough and the voltage across R is insufficient to trigger the SCR. However, when light falling on LDR is of large intensity, the Resistance of LDR falls and voltage drop across
OVER LIGHT SENSOR ALARM CIRCUIT - schematic

R becames large enough to trigger the SCR. Consequently the buzzer sounds the alarm. It may be noted that if the strong light disappears the buzzer continues to sound the alarm. It is because once the SCR is triggered the gate loses its control.



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