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Musical Entrance Alarm
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Musical Entrance Alarm has a musical light operated burglar alarm. The circuit generates musical tone when light falls on LDR gets interrupted. This can be used to detect an unknown person when he tries to open the door. Working of the circuit is based on LDR and musical tone generator IC APM993D (IC1). LDR triggers IC 993D when light falls on it gets interrupted by a person. IC 993D is a programmed 1024 note ROM memory, which generates 16 musical tunes. It has inbuilt oscillator as well as preamplifier circuit.About the circuit : In the presence of light, the resistance of LDR is very low. The voltage developed at pin4 of IC APM993D due to preset VR1 and LDR is insufficient to trigger IC1. When light falling on LDR is obstructed, it's resistance increases to a very high value. Now voltage at pin4 becomes sufficient to trigger the bell. If it does not, then reduce the resistance of the preset by adjusting its wiper till the circuit functions properly. With the preset at a low resistance, the bell rings continuously. On the contrary, when it has a high resistance, the bell will not ring even by blanking LDR. Transistor T1 is used as an amplifier driver connected to on chip preamplifier output at pin14 to drive 8W loudspeakers directly. Speed calibration : Resistor R3 controls the speed at which tune is played. By changing value of R3, speed can be varied.
