Wal-Mart Shopping Cart Locker

For the last few years at I have heard my friends at school and Heat Sync Labs talk about how they wanted to make a device that can lock the wheels of shopping carts at different stores. This last month I decided to try my hand in it and see where I can go with it.

The first step in the process was to find the signal that they use to lock up the shopping carts. To do this i decided to make a coil antenna so that I could find the signal. The transmitter wire is not very hard to find, it is usually a back line around the edge of the parking lot. I have to admit you get lots of weird looks sitting at the edge of a parking lot at 3 am with a laptop in you hand and sitting on the ground behind your car..

Sitting on the ground behind the car getting the signal

The Signal Wire at the edge of the parking lot

The process to get the signal is rather simple, all you have to do is hook up the loop antenna into the audio input of your computer and use an audio recording program, i recommend Audacity (http://audacity.sourceforge.net/) After recording you should get a signal similar to the following: Wal-Mart unfiltered (Screen Shots Below) after doing a little signal analysis you should be able to get something more like this: Wal-Mart Filtered

Wal-Mart Unfiltered

Wal-Mart Filtered

After getting the clean signal the only thing that was left to do was to export that as an mp3 file onto the ipod. This file alone combined with the antenna that you used to pick up the signal works to lock up the shopping carts from a short distance. The next thing that I did in order to increase the signal strength was to build an amplifier to boost the signal.

Amplifier

After all of this was done all I did was put the ipod on repeat on the filtered signal and walk around the store, unfortunately i am only able to get about a foot range with this setup but I am planning on revising this to see if I can get more . I will keep updated if I do so. I am hoping to post a video sometime soon.

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