How to Sweep for Nails in a Garden Using Neodymium Magnets

This is my first Instructables, so please bear with me and be kind.  Thank you.

I just had the old ceramic roof tiles of my home removed and installed new concrete roof tiles.  As part of the process, rotten wood had to be replaced and lots of other wood pieces ("ripas" and "contra-caibros") had to removed and then hammered back since I also installed DuPont Tyvek Reflex sheet covering (http://www2.dupont.com/Tyvek_Construction/pt_BR/products/Subcobertura_residencial/residencial/instalacao_tyvek_reflex.html ). Needless to say, hundreds of old nails were removed from the rooftop and they were all over the place around the house.  With 3 kids in the house, they were prohibited from going outside until the job was completed.  After that, I needed to make sure there were no nails in the garden but how to be 100% sure?

I built a simple "Nail Detector" to sweep for nails in the garden.

You will need:

1. a "rodo" (in Portuguese), or a simple rake or squeegee (in the USA)
2. eight (8) Quarter-sized neodymium magnets (bought mine at www.kjmagnetics.com for other projects)
3. tape
4. cones, or anything else to mark a path in the garden

Steps:

1. using tape, attach the Quarter-sized neodymium magnets onto the rake at regular intervals
2. that's it! :-)

PS. If you don't own neodymium magnets, you can get 2 of them for FREE inside an old discarded harddrive.

This is the Garden that needs to be swept by the "Nail Detector"

After spending about 1.5 hours sweeping the garden slowly, 35 nails were found by the "Nail Detector".

The kids are still prohibited from going outside until it's fully swept.  Actually, just to be sure, I'm thinking of building a new "Nail Detector" using a 110Volt Electromagnet used for doors: http://www.dmpbrasil.com.br/site/images/Produtos/trava-eletroima-1.jpg.  I just cannot imagine having one of my kids or their friends stepping on one of these abandoned nails.

Thank you!