Morning Alarm With Night Lamp Senses darkness and light in the room and gives melodious alarm when sun rises. The morning alarm with night lamp circuit utilizes light-dependant resistors (LDRs) for sensing darkness and light in the room. The resistance of LDR is very high in darkness, which reduces to minimum when LDR is fully illuminated. LDR1 detects darkness, while LDR2 detects light in the morning.
About The Circuit :
When LDR1 is illuminated with ambient light in the room, its resistance remains low, which keeps trigger pin 2 of IC2 at a positive potential. As a result, output pin 3 of IC2 goes low and the white LED remains off. As the illumination of LDR1’s sensitive window reduces, the resistance of the device increases. In total darkness, the specified LDR has a resistance in excess of 300 kilo ohms. When the resistance of LDR1 increases, a short pulse is applied to trigger pin 2 of IC1 via resistor R1 & VR1. This activates the mono-stable and its output goes high, causing the white LED to glow. LDR2 and associated components generate the morning alarm at dawn. LDR2 detects the ambient light in the room at sunrise and its resistance gradually falls and transistor T1 starts conducting. When T1 conducts, melody-generator IC2 (UM66) gets supply voltage from the emitter of T1 and it starts producing the melody. The musical tone generated by IC2 is amplified by single-transistor amplifier T2. Resistor R8 limits the current to IC2 and zener diode D3 limits the voltage to a safer level of 3.3 volts. The preset VR1 & VR2 is used to adjust the sensitivity of LDR. D1 & D2 are 5mm white LED used for ambient light.

Morning Alarm With Night Lamp Senses darkness and light in the room and gives melodious alarm when sun rises. The morning alarm with night lamp circuit utilizes light-dependant resistors (LDRs) for sensing darkness and light in the room. The resistance of LDR is very high in darkness, which reduces to minimum when LDR is fully illuminated. LDR1 detects darkness, while LDR2 detects light in the morning.
About The Circuit :
When LDR1 is illuminated with ambient light in the room, its resistance remains low, which keeps trigger pin 2 of IC2 at a positive potential. As a result, output pin 3 of IC2 goes low and the white LED remains off. As the illumination of LDR1’s sensitive window reduces, the resistance of the device increases. In total darkness, the specified LDR has a resistance in excess of 300 kilo ohms. When the resistance of LDR1 increases, a short pulse is applied to trigger pin 2 of IC1 via resistor R1 & VR1. This activates the mono-stable and its output goes high, causing the white LED to glow. LDR2 and associated components generate the morning alarm at dawn. LDR2 detects the ambient light in the room at sunrise and its resistance gradually falls and transistor T1 starts conducting. When T1 conducts, melody-generator IC2 (UM66) gets supply voltage from the emitter of T1 and it starts producing the melody. The musical tone generated by IC2 is amplified by single-transistor amplifier T2. Resistor R8 limits the current to IC2 and zener diode D3 limits the voltage to a safer level of 3.3 volts. The preset VR1 & VR2 is used to adjust the sensitivity of LDR. D1 & D2 are 5mm white LED used for ambient light.