Building A Battery-powered Motion Alarm

[Brad] was asked by his Sister to design a motion-based alarm that would help her catch her son sneaking out of the house at night. Obviously this didn’t need to be a long-term installation so he decided to throw something together that is only active at night and can be battery-powered. What he came up with is a light-sensitive motion sensor that uses very little power.

He knew that an Arduino would be overkill, and decided to try his hand at using the Arduino to develop code for an ATtiny85. It has an external interrupt pin connected to the output of the PIR module, which triggers action when motion is detected. The first thing it does is to check the photoresistor via the ADC. If light levels are low enough, the buzzer will be sounded. [Brad] measured the current consumption of his circuit and was not happy to find it draws about 2.5 mA at idle. He spent some time teaching himself about the sleep functions of the AVR chips and was able reduce that to about 500-600 uA when in sleep mode. Now all he has to do is find a nice place behind the house to mount the alarm and there’ll be no more sneaking around at night.

If you’re trying to keep a tight leash on your own kids you could always make them punch the time clock.

15 thoughts on “Building A Battery-powered Motion Alarm

  1. in b4 questioning moral integrity of this hack and proposing alternative parenting strategies

    I have a similar setup to catch when my landlord has been doing ‘repairs’ around the apartments. Connected with a Wall wart though, so I didn’t go that far with the power saving tips, but great cheat sheet for any power-conscious design!

      1. “Opinion and conjecture” that, once your kid’s grown to be a teenager, trying to lay down basic structures of respectful behavior like “don’t sneak out and raise hell at night against your parent’s express wishes” is a hopeless task without a Parris Island-level support structure behind it?

        You say so, I guess.

  2. The voice of experience, which remembers the sort of shenanigans it got up to in the cause of sneaking out to screw its also-then-teenaged girlfriend, tells me that won’t take a whole lot of circumventing. I’m willing to bet version 2 includes some means of detecting cut buzzer wires.

  3. You know my father did some stuff like this when I was growing up. I dont think it had the effect on me that he was hoping for I just figured out how to get around almost anything that he could think of and gave me the start in to my first line of careers which was home invasion. So you may be trying to use technology to do your parenting but you might just be recruiting and training the next generation thief, be careful nothing works as good as the old fashion ass wooping at least that would have worked for me.

  4. Just in time! I was thinking I should build one to know when the the kids in the apartment are running around my car/bike. Drives me nuts. I found all kinds of hand prints!!!

    Thanks!!! I will be building one soon and will have to run on off battery as well. Thanks for the head start.

  5. Hey, thanks! I know my project was overkill, please look beyond that and see the core behind the build. Also note, this is the first electronics project i’ve ever done in my life!

    The object was to implement with a microcontroller to help me learn the skills to build other things!

    Thanks Hackaday for publishing my first project ever!

  6. I couldn’t tell if the diode was there to give you a voltage drop (to make 3.3v parts happier) or just to make sure the batteries weren’t put in backwards. If the former, you could probably make it a bit louder by connecting the buzzer to the battery side of it instead of the far side.

    Meanwhile, it’ll detect stray dogs, and tell the kids to stay off your sister’s lawn…

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