3D printed shake-a-gen with rectifier and capacitor storage


two views of shake a gen

Shown above is a 1000 turn (34 SWG wire) shake-a-gen (see full article link below) made with 3D printed parts. The generator shows how electricity can be produced when a coil of wire experences a changing magnetic field - you simply shake the device and the moving magnet creates the electricity. The two photos show both sides of the shake-a-gen so you can see the components (see below for a bit more info.). Inside the tube is a strong rare earth magnet (18mm diameter, 10mm height). In this version I have incorporated a rectifier and capacitor to store the voltage from the generator (see 'storage device' link below). When you shake the device the magnet inside the tube moves in and out of the coil generating electricity in the wire. This AC voltage is rectified by the four diodes (BAT85 - low voltage drop) and stored on the 1000uF capacitor. A hyper bright LED is connected via a 1k ohm resistor to the DC voltage on the capacitor. A brighter light can be obtained using a lower value resistor (e.g. 100 ohms) and a longer 'charge' can be obtained using a larger value capacitor (e.g. 10,000 uF or a 1F 'gold cap' see 'modifications' section below).

shake a gen
article 		shake a gen
storage 		3D .stl file
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The magnet I used had a hole through the middle and so I have delibrately made the base of the shake-a-gen tube with a ca. 8mm hole in it. So you could fit a piston / arm to the magnet and feed it out through the bottom of the shake-a-gen so that the magnet can be moved by a lever or other mechanical device. This way you could link up the shake-a-gen to a moving object or machine and make electricity.


shake a gen lights LED

Photo of the four parts printed on the 3D printer (ca. 2 hour print time. Note: the camera flash makes the parts look rougher than they really are): A magnet tube (longest, back middle), two smaller tubes with coil former discs and the LED / electronics support 'plug' (left). First the smaller of the two coil formers is pushed down the long tube till it abuts with the tube lip / base. This will mean that the coil former lip will be about 1/3rd way along from the tube end. The longer coil former goes on next (its reversed to the first) so that it is on the last 1/3rd of the tube length. The coil is wound between the coil formers. The magnet is then put in the tube. The final part to go on is the 'plug' that goes on the end of the tube (actually it goes into the coil former and stops when it comes up against the end of the inner tube). The plug has a number of small holes that can be used to mount the electronic components and coil wires prior to soldering.


shake a gen lights LED

A photo of the red LED alight after shaking for a few seconds.
With the 1000uF capacitor and 1k series resistor the LED lights for only a few seconds but larger
capacitor values can be used to give a longer ON time and a lower resistance will create a brighter light
(but it will discharge faster of course). See 'modifications' section below for more info about capacitors.

shake a gen lights LED

photo showing the four 3D printed parts (right) next to the assembled shake-a-gen (left) before coil winding and components have been fitted.
I found the the parts fitted together snuggly but a little super glue could be used at the final stage to secure everything.


Making and modifications
 The 3D printer prints out four parts for the shake-a-gen (see photo). The longest part is a tube whose base (printing bed end) has a 8mm hole in it so that you can feed a bar through to move the magnet by some other means than shaking if you want to. The smallest part is a short section of tube that has a disc attached. This slides onto the long tube, all the way to the end in such as way that the disc is positioned about a third the way up the tube (this forms one side of the coil former). The third printed part is a little longer this slides onto the long tube so that its disc forms the other half of the coil former. You can adjust the position of this third part to change the size of the coil former. Once you are happy with the size of the coil a bit of super glue on the inside of the third part will glue it securely in place onto the main tube. Once the coil has been wound (remember there should be two wires coming from the coil, dont embed the starting wire!) you can put the magnet in and slide in the fourth part into the long tube to form a stopper. The electronics can then be assembled on this fourth part and the two wires from the coil attached to the circuit.

My first experiments used a 1k resistor and a 1000uF capacitor. This worked but the light only lasted a few seconds after each shake. I then tried a 4700uF cap and lowered the LED resistor to 390R this gave similar performance but a brighter light of course. However the capacitor was a little large. Then I tried a 0.22F super-cap which are physicaly much smaller (only about 10 mm in diameter and 4mm wide). The super caps have quite a large internal resistance so it takes a while to charge them up - 4 or 5 mins of shaking will do the trick. The LED will flash with each shake but it only starts to stay ON after a few mins of shaking. After 5 mins or so the LED is alight all the time and will stay on for a few mins with no shaking. You can of course use a longer magnet and I have tried a pair of the magnets which greatly improves the response (only need about 1 min of shaking). With the super-cap you have a simple low intensity torch that might come in handy in an emergency or if you find your batteries in your main torch are dead :-)


THE CREATIVE SCIENCE CENTRE

Dr Jonathan Hare University of Sussex, Brighton.
e-mail: j.p.hare@sussex.ac.uk

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