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bone conduction pen
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bone conduction pen

by bxl on Wed May 01, 2019 11:14 am

I'm looking to put a bone conductor traducer with Bluetooth capabilities into a sharpie size (or slightly large) pen powered by batteries.

Is something like this possible? Could someone point me in the right direction, thanks is advanced.

(PS- This is all new to me...)

bxl
 
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Re: bone conduction pen

by adafruit_support_mike on Thu May 02, 2019 2:56 am

It's possible, but probably wouldn't run for more than a few minutes.

A bone conduction transducer is basically a speaker that uses the bones of the skull as the resonator. The transducer itself is just an iron slug inside an electromagnet.

All motors and speakers are inefficient.. only about 5% of the power that flows through them gets converted to motion. All of the rest turns into heat. To make matters worse, the strength of an electromagnet is proportional to the area inside the foop of wire. Reducing the diameter of a loop by half reduces its area to a quarter of its previous value, so making electromagnets smaller makes them much weaker.

To compensate for that weakness, you have to use more turns of wire. That works, but increases the end-to-end resistance of the wire, which in turn produces more heat when you run current through the coil.

The power to run the coil has to come from somewhere, and if you want a small, handheld device, it will probably be battery powered.

All batteries have what's known as an 'energy density', which tells you how much energy you can store per cubic centimeter. Lithium polymer batteries have about the same energy density as gunpowder, and are currently the highest energy density storage medium available on the consumer market. Obviously making the battery smaller means it can store less energy, which means a shorter battery life for any device connected to it.

A Sharpie marker is about the size of an AA cell, which holds 2000mAh of energy at 1.5V. The smallest standard LiPo is an 18650 cell that holds 2200mAh of energy at 3.7V:

https://www.adafruit.com/product/1781

An 18650 cell has a diameter of 18mm (0.71") and is 65.0mm long (2.6"). Most cells also come with a protection circuit that takes the length up to about 68mm (2.7").

A 2200mAh 18650 cell could run an 8-ohm transducer for maybe 3 to 4 hours. Reducing the whole thing to the size of a Sharpie would probably take the battery life down to 1 hour at best.

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Re: bone conduction pen

by bxl on Thu May 02, 2019 10:17 am

Thanks you for the incredibly comprehensive answer!

What pieces would be necessary for such a device? Perhaps I could use a King Size sharpie instead.

By the way, the sound only need to be faintly heard if that matters.

Thanks in advance!

bxl
 
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Re: bone conduction pen

by adafruit_support_mike on Fri May 03, 2019 12:48 am

Bluetooth audio devices need to use the protocol that's now called Bluetooth Classic, which isn't a hobbyist-friendly protocol. Among other things, every BT-C device has to transmit a Vendor ID number, and the only way to get one of those is to pay a licensing fee to the Bluetooth Working Group. In practice, you pretty much have to find an off-the-shelf BT audio device and scavenge the electronics out of that.

Once you have an audio signal, you'll need a power amplifier strong enough to drive a transducer, and the transducer itself. We don't have any transducers small enough to meet your requirements, so you'll have to search for suppliers to see what's available.

You'll also need a battery, and probably a charging circuit for that unless you want to swap batteries.

Once you have all the electronics, you'll need an enclosure to hold them all together. That's something you'll probably have to design and fabricate on your own, based on the physical sizes of the pieces that will need to fit inside it.

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Re: bone conduction pen

by daniel_abril13 on Mon May 27, 2019 11:46 pm

Good night
I would like to know what kind of physical principle bone conduction vibrator uses: is it dynamic or is it magnetostrictive?

Thank you

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Re: bone conduction pen

by adafruit_support_mike on Tue May 28, 2019 3:12 am

It's magnetostrictive. The magnetic field from the voice coil changes the shape of the vibrating element.

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Re: bone conduction pen

by daniel_abril13 on Wed Jul 10, 2019 12:43 am

Thanks for your answer. I have another question: The vibrating element is a permanent neodymium magnet or which?

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Re: bone conduction pen

by adafruit_support_mike on Thu Jul 11, 2019 11:50 am

It’s just a piece of steel. The coil is the magnetic element.

‘Magnetostriction’ means the metal changes shape when it’s in a magnetic field. Iron and steel are made up of microscopic crystals, and each crystal has its own magnetic field. The fields point in all directions in a normal piece of metal, so the effects of the fields tend to cancel each other out. Materials we call ‘magnets’ have a majority of crystals whose fields point the same direction and reinforce each other.

In magnetostriction, the randomly-oriented crystals in a regular piece of iron try to swing around so they’re in line with the external magnetic field. In the process, they push against each other, changing shape slightly. All of those small changes add up to a change in the overall shape of the bulk metal.

Magnets are less magnetostricitive than non-magnetized iron because the magnetic fields for most of the crystals point the same direction. Instead of trying to rotate individually, they all try to rotate together, making the bulk metal move as a unit.

Some alloys of steel change shape more than others in a magnetic field, and those tend to be used for magnetostrictive elements. I’m afraid we don’t know what alloy is used by the transducers in the shop.

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Re: bone conduction pen

by abartle on Thu Aug 08, 2019 8:32 pm

Question if I buy bone conduction transducers how do I connect them to music?

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Re: bone conduction pen

by adafruit_support_mike on Fri Aug 09, 2019 11:19 pm

They're basically another kind of speaker. You'd connect them to the output from a speaker amp like these:

https://www.adafruit.com/product/2130

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