When there is no light, the light dependent resistor or LDR have a very high resistance, several Mega-ohms. When there is illumination in the surrounding, the LDR resistance drops to several kilo-ohms, this time it will activate transistor Q1. When Q1 is on, the musical UM66T IC will turn on and will generate sounds to the speaker via transistor Q2. You can use 3V to 3.6V battery as a power supply.
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| light activated alarm schematic |
Part List:
R1 - 1 kilo ohms 1/4W
R2 - 120 kilo ohms 1/4W
R3 - 470 ohms 1/4W
POT - 100 kilo ohms potentiometer
ldr - any light dependent resistor
Q1, Q2 - 9013, 2N2222 or similar NPN transistor
SPK - 8 to 16 ohms speaker
UM66T - any Melody generator IC
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| UM66T pinout |
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| 9013, 2N2222 pinout |
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