Morning Alarm. Sensor Circuit.1

This LDR based alarm can be used as wake up alarm. The Alarm turns on in the morning automatically and turns off only when manually switched off ,so you will have to wake up. It is a simple LDR based light sensor circuit.

LDR or Light Dependent Resistor is a kind of variable resistor and its resistance depends on the intensity of light falling on it. In complete dark, its resistance is as high as 1 Meg Ohm, but it reduces to 100 ohms or less in full light. So it is an ideal device to use as light switch. Sensitivity of LDR depends on its size also. Commonly available sizes are 3mm, 5mm, 10mm etc. But costly types with greater sensitivity are also available.

When the circuit is place in dark, near the window, LDR fails to conduct and T1 will not get base bias. So it remains off and buzzer will be silent. When the sunlight falls on LDR, IN the morning, LDR conducts and provides base bias to T1. T1 then conducts to activate the buzzer. Capacitor C1 is provided for the clean switching of T1. T1 will conduct only when C1 is charged fully. This prevents chirping sound during light transition.You can use an easily available Music buzzer to get melody tone.