My car gets broken into quite frequently. The car is right outside of our bedroom window. The break ins happen at night, and we never hear them! They must be very sneaky.

I would like to design (or find a plug and play solution) for a motion detector that I will place inside the car that is either battery powered or something that can plug into the car adapter. When it senses motion, I want it to send a signal to our bedroom window so that we can call the police.

Here's an amazing diagram of what I want to happen.

enter image description here

Can you please suggest a design or a plug and play solution that I can use to do this? I know there are many motion detectors, but I would specifically need something that will work at only a 1 meter range because it should be inside of the car sensing any movement INSIDE of the car, and ignoring everything outside.

I don't know if this is relevant, but the windows are tinted.

  • \$\begingroup\$ Has Beverly Hills switched to Cyrillic?? \$\endgroup\$
    – stevenvh
    Commented Aug 8, 2012 at 16:00
  • \$\begingroup\$ Have you considered just installing a car alarm in the car. They have motion detectors which detect people inside the car only. When an intruder is detected, an acoustic signal is sent to all of the houses in the neighbourhood, causing adults and children alike to wake up and look out of their windows at the offending car. \$\endgroup\$ Commented Aug 8, 2012 at 17:14
  • \$\begingroup\$ @Rocket - "... alike to wake up", grumble something about unreliable electronic gadgets, turn around to go to sleep again. :-) \$\endgroup\$
    – stevenvh
    Commented Aug 8, 2012 at 17:24
  • \$\begingroup\$ @stevenvh - That's why I recommend the South African solution. \$\endgroup\$ Commented Aug 8, 2012 at 17:33
  • \$\begingroup\$ Curious to know if they enter the car and take it for a joy ride. If they do, how cool would would it be if you could rig the car to automatically lock when the thief is inside, and you can only open it from the outside? :D Haha that would give the thief a heart attack! \$\endgroup\$
    – capcom
    Commented Aug 8, 2012 at 18:18

3 Answers 3


Ultrasonic sensors seem to be the sensors of choice for many car alarms. You can often buy electronics kits which use ultrasonic sensors to detect motion. The outputs are often simply in the form of a relay, so you can connect it to almost any other device you can think of. Google around for 'ultrasonic motion detector electronics kit'. There's a bunch to choose from.

Ultrasonic motion detector

Simply mount one of these inside the car, and wire it up to the 'deterrent' transducer on the outside.

South African car security

Alternatively, you could simply wire it up to a wireless doorbell, and leave the chimer by your bed at night.

Wireless doorbell

Also consider wiring it up to a couple of cheap little video cameras, so that you can grass up the little urchin to the police.

Mini video camera

  • 1
    \$\begingroup\$ How about a PIR? \$\endgroup\$
    – stevenvh
    Commented Aug 8, 2012 at 17:25
  • \$\begingroup\$ I like the wireless doorbell idea to. KISS, but not nearly as fun as making something on your own! \$\endgroup\$
    – capcom
    Commented Aug 8, 2012 at 18:55
  • \$\begingroup\$ @stevenvh - You can't use a PIR in a car because PIRs can see through windows! You may notice that there are no PIR car alarms available. \$\endgroup\$ Commented Aug 8, 2012 at 19:05
  • \$\begingroup\$ @Rocketmagnet thank you very much for this awesome solution \$\endgroup\$ Commented Aug 8, 2012 at 19:12
  • 2
    \$\begingroup\$ Dismantle the button, and connect the switch to the relay outputs of the ultrasonic sensor. \$\endgroup\$ Commented Aug 8, 2012 at 19:15

Well, let's look at the problem you are trying to solve from a simple perspective. As I see it, you need two things to happen.

  1. You detect the motion
  2. You wirelessly notify yourself that motion has been detected

Simple enough. Now, it depends of course on what your requirements are. Things like:

  • How big you want the device to be
  • How much you want to spend on it
  • What kind of experience you have with electronics

You didn't provide any of this information, so I'll tell you what I would do if I were in your situation.

Detecting the Thief

Use an Ultrasonic Sensor

Grab the HC-SR04 ultrasonic sensor. It is super easy to interface with most microcontrollers, and very accurate. Very cheap too (~$4)

Use a PIR Sensor

They are also very cheap, and as feasible a solution as is the one above. This one would be a little more compact, and have a wider field of view though. Probably a little more expensive than the HC-SR04.

Transmitting the Information

Use an xBee WiFi Shield

You'll need just one of these for your car. Hook it up to your WiFi connection, and it can send data directly to your computer or even your mobile phone if you are willing to write a quick app which can alert you.

Use a pair of 2.4GHz Transceiver Modules

You can try the cheap-o NRF24L01 modules. You'll need two, one for your car, one for your room. The main advantage is that they are unbelievably cheap (~$2.50 a pop), and there is good sample code out there too.

A Microcontroller to Process the Data and Interface With Your Components

A typical recommendation would be to use the Arduino. All the components I listed above have lots of sample code available which should work flawlessly with the Arduino. You would likely need two in any of the scenarios, unless you are up to using the xBee shield to send the data directly over your WiFi connection. I have never tried it, but it is well documented and shouldn't be very hard to do.

Or you could go the cheaper route and use a standalone AVR or PIC microcontroller. Also not a bad solution, as you can keep things much more compact and portable. If you really want to use the Arduino code, you can install an Arduino bootloader on most AVRs, or you can just code them in C. The Arduino route is easier from a coding standpoint I suppose - but that's arguable.


Lastly, you'll need power. You have two options:

  1. Use a battery pack. Go for rechargeable NiMh packs if you choose this route.
  2. Use the cigarette adapter in your car.

Hope this helped!

  • \$\begingroup\$ Can't use PIR in a car because there are too many windows. It'll just detect people walking outside. \$\endgroup\$ Commented Aug 8, 2012 at 19:06
  • \$\begingroup\$ @Rocketmagnet what is the reason i can use the xBEE and not PIR? \$\endgroup\$ Commented Aug 8, 2012 at 19:11
  • \$\begingroup\$ does the fact that there are tinted windows have any effect on the motion detector? \$\endgroup\$ Commented Aug 8, 2012 at 19:13
  • \$\begingroup\$ I don't understand the question about xBEE and PIR. You can't use a PIR because it can't tell the difference between someone inside and someone outside the car. xBEE is not a motion detector and does not suffer from this problem. xBEE is a method to communicate from the car to your bedroom. \$\endgroup\$ Commented Aug 8, 2012 at 19:19
  • 2
    \$\begingroup\$ @Rocketmagnet Sure, but I think distance will be more accurate because it can't be disturbed by motion outside the car. If you point the HC-SR04 towards, say, the driver's seat, the one and only way it can be triggered is if someone obscures that part of the car. A motion sensor could be triggered by an animal outside, or pedestrians on the sidewalk if placed incorrectly. \$\endgroup\$
    – capcom
    Commented Aug 8, 2012 at 21:31


enter image description here

The sensitivity of these can be tuned via the trimpot on the back.

Connect to an Arduino running off a battery, or if you drive the car often, connect it to the car's battery. Then connect a pair of XBees for your Wireless communication


enter image description here

Have another Arduino set up as your 'base station' and it can light up, make noise, etc, whatever you want to do.

  • \$\begingroup\$ thank you very much! this look ssimple and awesome! can you recommend which arduino to use with this? \$\endgroup\$ Commented Aug 8, 2012 at 19:11

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