We will not be responsible for damage to equipment, your ego, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury or worse that may result from the use of this material.
What to do if you want to listen to music or the radio and you don't have
headphones or speakers. Right. Build one yourself.
Like the ultra-lowtech voltmeter, this is just another hack which was created
on a boring Sunday afernoon. As usually I tried to keep it as low-tech as possible.
You can see the front, top and back view here:
The diaphragm is made from a piece of plastic bag. Not the stiff, thick
ones, but the really really cheap thin ones. The diaphragm is tightened onto a
screw top taken from a jam jar into which I cut a big hole. The platic is
simply glued onto it. The problem is, that you can't glue it very thightly,
because unlike rubber the plastic (usually PE) is not as flexible. To correct
this, I stuffed a rubber band between the jar top and the diaphragm after the
glue had dried. (You can see it in the back-view.)
The voice coil is make out of very thin copper wire (it has the
thickness of about a hair) which was taken from an old 48V telecommunications
relay. I wound approximately 100 to 150 turns of it around the shaft if a
10mm drill. After slipping it off (which is only possible if you haven't
wound it directly onto the drill but placed a layer of paper between the
windings and the drill) i fixed it with some stiffer copper wire (otherwise
it would have fallen apart) and glued it onto the middle of the diaphragm.
The resistance of the coil is about 41Ohms - perfect for use with a
discman, because most headphones have 32Ohms.
The magnet Behind the voice coil is a wood block into which I mounted
a small (10mm x 1mm) NdFeB Magnet which itself is glued onto a metal plate.
I tried to use a electromagnet before, but it used lots of power and didn't
reach the field strength of the small permanent magnet. The volume with the
electromagnet was only a fraction of what could be achieved with the NdFeB
magnet.
Running from my CD player on full volume, the sound is well in a quiet room and the sound quality is surprisingly good but the lower frequencies are too loud compare to the higher ones. Hooking it up with my function generator reveals that the lowest resonance frequency is somwhere between 130Hz and 150Hz. Surprisingly, if you feed it with a 3kHz sine, the sound is still quite loud, the bandwidth seems to be quite respectable.
It might even be used as a microphone, because while speaking quite loud into it, by oscilloscope tells me that it produces approx. 30mVpp. :-)
If you build it somewhat more compact by leaving out the wooden board and using
a second jar top instead (to mount the magnet) you could get a nice homemade
earpiece for you fully homemade half-open headphones.
Note: While holding it near my ear I've noticed that the volume it produces
is more than sufficient for headphone use.
Of course, I'm not responsible for any ear-damage caused by this or any
other device!
Copyright (C) 2007 by Wiesner Thomas
Last change: April 1st 2007 (This is no April-joke!)
