Light Alarm Circuit with LDR

Posted on Feb 7, 2014

This musical light alarm circuit is very simple, uses only 7 components, a LDR and a 3. 6 V battery or 3 x 1. 2 volts rechargeable batteries. The well-known UM66 is used as the sound generator and will give a pleasent wake up alarm. As you probably know the LDR is a light dependent resistor. Normally the resistance of an LDR is very high, sometimes

Light Alarm Circuit with LDR
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as high as 1M ©, but when they are illuminated with light resistance drops dramatically. In the circuit adjust the 220K © preset to the desired sensitivity, meaning adjusting the threshold point where the alarm start singing. When there is light on the light dependent resistor the T1 transistor will start conducting and powers the UM66 musical integrated circuit. The produced musical note will be amplified by transistor T2 and fed into the 8 © speaker. On the UM66 IC are different numbers, each number giving a different musical note (in this example we use UM66T). You may use 2 x 1. 5V batteries but 3 x 1. 2V NiCad or NiMH are better because you can recharge them.

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