Please help with what ought to be a simple project - Tooth conduction radio

Discussion in 'General Electronics Chat' started by drabah, Apr 7, 2015.

  1. drabah

    Thread Starter New Member

    Apr 7, 2015
    Several years ago I attended a program for science teachers in which I made a simple device that consisted of a pencil on which was placed a rare earth magnet and around which were wrapped about 300 turns of fine copper wire. When the two wire ends were connected to a plug which fit into a transistor radio, and one placed the pencil in one's mouth, the radio could be heard playing in one's head! Also if someone else placed their ear against my ear, they could also hear the radio playing.

    I tried to make this again using a cheap transistor radio, but could not hear a thing. I have been told that the problem might be that newer radios have an output that is too weak to activate the pencil speaker. I bought a tiny audio output transformer from RadioShack today, hoping to somehow boost the signal, but I'm not sure this is the right device, or precisely how I should connect this. If anyone reading this has any suggestions or insights, I'd be most appreciative.

  2. DickCappels


    Aug 21, 2008
    The main point of bone conduction transducers is to accelerate a mass with the audio signal and couple what I guess I can call the "reactive acceleration" of the platform that supports the device that generates the magnetic field that causes the acceleration. In your example with a pencil, either the coil or the magnet (whichever has less mass) would be tightly coupled to the pencil and the other of the two would be loosely coupled as with a damped spring of some kind.

    This principle can be seen in the illustration below taken from US patent 8761416.

    In the illustration, a spring-mounted mass is accelerated by the electromagnet mounted in the frame that transmits the vibrational energy to the user's head.
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    put a pair of headphones on, you'll here it a lot better....
  4. drabah

    Thread Starter New Member

    Apr 7, 2015
    Thanks, Dave. I don't want to use headphones, because the whole point of this demonstration is to show children that sound which they can't hear through their ears is conducted through the teeth. I believe that the magnet and coil are working but the strength of the signal from the radio is too low for me to be able pick it up through the pencil. Hence, I thought that using a small transformer might help. In the original working model, no spring of any kind was used, just a pencil. a rare earth magnet and a coil of wire wrapped around the magnet (and the effect was truly astonshing).

  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    My guess is that your trouble owes to impedance mismatch (radio to coil) -- you may wish to experiment with the turns-count. Be advised that, due to its excessive transformation ratio, the (RS) audio transformer will not likely be of help...

    Best regards