"faraday" pen light

Thread Starter


Joined Jun 21, 2006
My friend came up with the idea of making one of those induction powered lights using an LED mounted in the top of a pen. Eventhough the chances of this working seem small - not to mention its limited usefulness - we decided to think about how we might design it in an attempt to learn something.

Basically we were thinking of getting one of those larger plastic pens with retractable ballpoint. The idea is to drain the inner cylinder which stores the ink and wrap it tightly with electromagnet wire and place a cynlindrical neodymium magnet in it (like this one: http://www.kjmagnetics.com/proddetail.asp?prod=D18). We would use button the end as a pressure switch to turn on the LED. Shake the thing a few times and we're good to go.

Of course, it's obviously not that simple. It seems pretty like we would want some sort of capacitor. Would you need a diode between the coil and the capacitor to ensure that the cap doesn't drain back through the coil? Also, on the subject of diodes, would you need to set up like a full wave bridge rectifier in this so it charges in both directions of magnet motion, you think?

Who knows if we'll ever actually try to make this thing but I was hoping someone could nudge me in the right direction in terms of design and calculating the output of the coil. Thanks for reading.


Joined Jun 30, 2006
I think you'll need a bridge diode, four 1N4148 diodes should work. You can then check the output voltage and get a capacitor that can handle it. The LED can only take a certain current and I'd recommend a constant current source or at minimum a resistor.