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Neither I, nor brighthub.com, can be held responsible for any consequences of building or attempting to build these. Proceed at your own risk.
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Your very own ECG machine
So how about starting off with an ECG machine that you can set up for a dollar? All you need is a few resistors that cost next to nothing, a microphone cable, an OP-AMP (don't worry, that's just a small IC) and 3 coins.
Putting these things together on a breadboard is simple enough, and the coins act as electrodes that are attached to the body so that the output ECG signal is obtained on the PC. Though not as accurate as a full-on ECG machine, it is fun to play around with.
Full construction details can be found here.
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A very basic motor
Collect one ferromagnetic screw (a drywall screw or nail will do), one battery cell, a few inches of copper wire, and a neodymium disc magnet. Set the sharp tip of the screw on the positive end of the battery, and the disc magnet on the other end of the nail/screw. Now let one end of the wire contact the negative side of the battery. The other end of the copper wire is allowed to touch the side of the disc magnet. This causes the screw to rotate on its point on the battery. A more detailed how-to can be found here.
This contraption, something that Michael Faraday cooked up way back in the 1800s, depends on Lorentz force to work. For a theoretical explanation, check this page out.
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A paper transformer
The word 'transformer' conjures up an image of either Michael Bay's movies or an old animated series, depending on when you were born. But, what I'm going to show you here is related to neither of those, this is something you can make with paper and perseverance.
You'll need to watch this video for the exact steps. All you need is paper (preferably the kind that greeting cards are made of), tape and scissors. A ruler would be useful too.
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A roll-up keyboard
Next on our list of fun gadgets to build at home is a roll-up keyboard. If you've always had your eye on a friend's fancy keyboard - one that just rolls up and can be throws in a bag - you can make one too if you have an old unused keyboard lying around. Inside it is a transparent membrane composed of three very thin layers. All you have to do is rip it apart and reassemble the membrane alone. Complete instructions can be found here.
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DIY fisheye lens
A fisheye lens for your DSLR camera can help you get more of what's happening in front of the camera into a photograph, but professionally made ones will set you back a pretty penny. For this DIY venture, we'll be needing just an old pair of glasses (should be ones prescribed for far-sighted people) and electrical tape. Basically, you'll be taking those thick lenses and affixing them in front of your camera lens. A detailed how-to can be found here. You might also be interested in reading How to Make a Homemade Peephole Fisheye Lens for Your Camera.
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A multi-touch pad
For this fun gadget you can build at home, gather an old cardboard box, glass or acrylic pane (with tracing paper on one side), a PC with Windows on it, and a USB webcam. Put the camera at the bottom of the box (ensure no light leaks through any holes), glass pane across the top, and hook the camera up to the PC. You'll need some software too - follow this link for a step-by-step guide with video.