In this instructable, I explain how I have transformed my bike helmet to incorporate a bone conduction device in order to listen to music in a safe way while riding my bike. I have spent a lot of time on this project and it is fully functional, but it is not completely optimized (see conclusion), therefore I consider that this is a prototype. So I will probably write other Instructables for the next versions of this bone conduction bike helmet.

About this project:

To go to work I have to ride my bike for about 40 minutes. Twice a day, so 1 hour and 20 minutes. This is a quite long trip, and being able to listen to some music would be perfect. But I live in Marseille, the most crowded city in France, and riding a bike with a headphone is illegal, and extremely dangerous because you are not aware of the environment.

So I have decided to transform my bike helmet and add a bone conduction device to listen to some music in the background while riding my bike to go to work.

Below is a footage of me riding my bike in Marseille, facing the traffic jam while going to work...

This gif I made is extracted from the French movie "Taxi 3". This scene actually takes place in Marseille's streets, that I am used to take by bike. But this is not me in the video...

Challenges:

For this project I had many challenges:

Make a bike helmet displaying music, in a safe way for riding my bike (=bone conduction).
Make it quite simple, and easy to command.
Being able to use my sunglasses with the helmet (most bone conduction devices are placed on top of the ears, which is not very practical if you wear glasses).
Easy to assemble/disassemble.

Step 1: Review & Questions

What are bone conduction devices?

Before to start making the helmet, I have looked for information concerning bone conduction devices. I found this article, which is a review concerning new developments in bone conduction hearing implants. I have considered that it would be a good start for this project.

According to this paper, the bone conduction devices (or BCD) are separated into three different types:

Direct drive devices, that directly stimulate the bone.
In-the-mouth devices, that stimulate the bones through the teeth.
Skin-drive devices, that vibrate the bone via the skin.

Of course, it was not possible for me to use direct drive devices. I have considered making an in-the-mouth device (see this instructable for example, or this video), but riding my bike with a metallic piece in the mouth is probably not the best choice... So I have decided to go with a conventional skin-drive bone conduction device.

Is it dangerous for the ears?

This kind of device by-pass the eardrum (the sound vibrations travel through the bones), so there is no risk to damage it. However, the hair cells found into the cochlea still transduce a mechanical signal into an electric signal, and they can still be damaged if the mechanical signal (in other words the vibrations from bone conduction) is too high. So the risks are similar than with regular air conduction devices: if it is too loud, it can damage the hair cells and cause deafness.

Bellow is an image of how the hearing works, found on this webpage.

Is it legal to ride a bike with this device?

According to this article , in France, it is forbidden for any driver to use headphones. I could not find any information concerning bone conducting devices, so I have decided to give it a try.

Step 2: Find the Best Transducer

The main part of the project is to find a suitable transducer so that electrical signals coming from the music player are converted into vibrations. I have selected the following:

DC motors harvested from old devices.
Cell phone vibrators harvested from old cell phones. They are actually DC motors, but they also are also unbalanced which make the phones vibrate.
Piezoelectric speakers. By applying a voltage to the plate, the speakers get deformed and creates sound, so I thought they would be good candidates for my project.

Below you can see a motor connected to an audio source. When the music plays, the motor vibrates and move.

I have also tested how these transducers help to propagate sound vibrations. First I have used the piezoelectric speakers, and I have run twice the same music. The first time (on the left below), the piezoelectric was not touching the recording device (with the microphone), and it was really close (less than 5mm). And then I have placed the piezoelectric in contact with the recording device (on the right below). You can clearly see that the amplitude of the signal is amplified when the piezoelectric is touching the recording device.

Then I have tried the same experiment with a DC motor. The results are a lot less impressive, but this is also explained by the fact that the surface of the piezoelectric speaker touching the recording device is a lot bigger than the surface of the DC motor (because it is cylindrical).

Step 3: Where to Place the Transducer?

Testing the different transducers, I have found that we do not hear the same things according to where it is placed on the face. The following results are empiric.

According to the tissue:

Skin and cartilage tend to propagate a wide range of frequencies. If just placed on the surface of the skin, near the ear, the low frequencies are quite loud. And if it is pressed against the skin, high tones are getting louder.
Bone conduct well, rather high frequencies.
Similarly, teeth are like bones and conduct high frequencies.

According to the position on the face:

Placing the transducers behind the ears is the best option to my point of view. This way the transducer is in contact with the cartilage, the skin, and the bones, and conduct a wide range of frequencies.
On the tragus, the sound conduction is also quite good, for similar reasons.

Step 4: Materials and Circuit

This step concerns the electronics of the project. This is a quite easy step, the material being cheap and simple to assemble if you know how to handle a soldering iron.

Here are the materials I have used:

An audio cable with an audio jack harvested from an old headphone.
A class D audio amplifier TPA2012. To amplify the audio signal which is too low otherwise.
A 18650 battery and the battery holder. This battery is used to power the amplifier.
A charger module TP4056 to charge the 18650 cell.
2 switches. One is used to switch OFF the amplifier when the battery is charging. The other is used to disconnect the battery from the circuit.
Finally the transducers: 2 DC motors.

Here you might wonder why I have used an audio cable an not a Bluetooth module. In fact, because I had no idea about what would be the final result of this project, I have preferred using a simple audio cable. Plus, I am using an old smartphone which is sometimes a bit difficult to use with Bluetooth.

This is what the circuit looks like once soldered.

Step 5: 3D Printed Pieces (part 1/2: Motors Holders)

In this step you can see the pieces I have designed and 3D printed to hold the DC motors. I have made these pieces so that the position of the motor can be changed, in order to make a better contact between the motors and the back of the ears.

Each piece is made out of 4 parts (from left to right on the picture below):

A kind of clamp, to attach the piece to the helmet.
A rack with a slot for the DC motor.
A gear.
A pin to fix the gear to the clamp.

This is also visible on the images below, viewed from 4 different angles.

Because of the shape of my bike helmet containing many curves, I had to make many tries before to obtain a correct piece. Below a some of the pieces I made:

And this is how the pieces work. Rotating the wheel makes the motor to move forward/backward, and it is possible to adjust its position so it touches the ear.

And finally, below are the motors and the racks. I have designed each slot so the motors fit perfectly and stay there, and with an offset to protect the "pins" of the motors.

Step 6: 3D Printed Pieces (part 2/2: Electronics and Battery Holder)

Then I have designed and 3D printed a piece to hold the electronic circuit and the battery. It contains slots for every of electronic parts as you can see below. There are 2 holes for the switches, and also 2 holes for the charger module (one to see the LED when it is charging, the second to plug it to charge the battery). Again, the shape of the helmet made the task quite difficult.

Below is an image of the piece with the electronics. It is a bit messy but it does the job. Maybe one day I'll learn how (or I'll take time?) to make enclosures that perfectly fit the electronic pieces?

Step 7: Conclusion

And here are some pictures of the final result. As said in the introduction it is finished and functional, but I consider it as a prototype, mainly because it can be improved.

So first this is why I am happy with this project:

I learned about bone conduction devices, and I know it is possible to make one easily with suitable transducers.
The electronics is really simple, cheap and easy to assemble.
I managed to make a first version of the helmet.

And this is what I want to improve on the next version:

The way I have attached the 3D printed pieces to the helmet. Because I did not want to damage the helmet, I have designed some kind of clamps. They work fine but they do not exactly fit the helmet. So I would like to change their shape and probably use some screws to attach the pieces to the helmet.
I would like to modify the electronics/battery holder (the part on the back of the helmet). It can be greatly reduced.
I would like also to replace the cable with a Bluetooth module.
And change the 18650 cell battery with a smaller Li-ion battery.

Finally, I found out that the sound vibrations are well transmitted by the PLA used to print the pieces, and by the polystyrene used for my helmet. As a result, the full helmet behave as a kind of giant speaker, and the music played is transmitted through the air (not very loud but you can still ear the weak music when standing beside the helmet). On a first hand, it is quite fun to hear the music coming from the full helmet. But on the other hand, I don't want to disturb people with this music. So I will probably try to isolate the motors from the helmet so that sound vibrations are not transmitted through the PLA.

This was a fun project. I hope some of you are going to try to make their own bone conduction bike helmet, so we can share about this.

Feel free to comment, and share what you think about this project!