Light Alarm Circuit

Posted on Feb 5, 2014

This is a circuit that will go off once it detects a certain amount of light. Once the light the circuit is exposed to increases to a certain level, a loud buzzer will go off, alerting us to this. So the alarm will not go off when the circuit is exposed to a darkly lit area but if taken out and exposed to bright light like of sunlight or the light

Light Alarm Circuit
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of a room will go off. An example of where we could use this circuit is in the following real-world example. We could place this device in a very dark area such as the drawer of a cabinet. Let`s say we have a lot of valuables which we want to protect in a chester drawer such as jewelry. And we regularly keep this drawer closed. Being closed, no light enters and the circuit is in enclosed in a dark area with no light. Under these conditions, the alarm does not go off because it is not exposed to light. However, say, if a thief breaks into the house and opens this drawer either during the day or at a night, shining a flashlight into the drawer. Now exposed to light, the circuit will trigger and buzz an alarm. So the housekeeper can know if someone has broken in by hearing this device. This one possible application of this light alarm circuit we will build. What we will use for this circuit is a 200KG photoresistor. Photoresistors are also called light-dependent resistors (LDRs). This can be obtained at digikey at the following link: Digikey- 200KG Photoresistor. If you don`t have a 200KG photoresistor, just make sure to use one that has a dark resistance of 100KG or greater. A photoresistor is a resistor whose resistance changes according to the amount of light that it is exposed to. When exposed to total darkness, the photoresistor`s resistance is very high. This is called its rated dark resistance. For example, in our...

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