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Published on 21 Apr 2012 | over 4 years ago

A really easy and cool electronics project and lots of fun to play with once it finished.Its basically a laser tripwire but has a large application quotient in our everyday life's! so all you teenagers and mad scientists go out there and make your rooms and labs intruder proof :D

• Breadboard / Perf-board
• Transistor- BC 109 (or any equivalent general purpose NPN transistor)
link to the datasheet of BC109 : alltransistors.com/pdfview.php
• Wires for connecting
• 9 volt battery
• 9 volt battery clip
• 10k ohm resistor
• LDR- mine was 8k ohm

This electronic circuit uses a transistor to detect light falling on the LDR (light dependent resistor) to activate or deactivate an alarm (or any kind of electronic component). The two resistors (the resistor and the LDR) are connected in series and the common point between them is connected to the base of the transistor, making a potential divider. It uses the property of "threshold voltage" of the transistor to trigger the alarm. This means that we can set a minimal amount of light intensity to trigger the alarm. I have set it to the intensity of sunlight as the minimal. This means that, it won't trigger indoors in ambient light but only when there is a light of high intensity (like a laser). To set your own intensity or if you don't a LDR that is working with the current specifications then just replace the resistor with a potentiometer (or other kind of variable resistor) of something like 30k ohm. Then keep changing the value until you get the desired result.
A more scientific method will be to see the reading (resistance) of the LDR in the desired light. Check the threshold voltage from the datasheet and then used the potential divider formula to calculate the required resistance of the 2nd resistor.

Note: The relay is only used for simplicity. The output can also be controlled by power transistors, to control any other device.

Please feel free to pose any questions that you might have. And make sure that you comment, rate and subscribe.
Thank you :D

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