Thursday, 11 October 2012

Breath powered USB charger

Are you breathing? Do you have a gadget that can be charged via a USB port? Well if you answered yes to both, then you are in luck. This instructable shows how to make a device that will charge your USB-capable devices while you do what you do best. Breathe. Using some parts scavenged from an old CD-ROM drive, a simple electronic circuit, and a few rubber bands you will soon be huffing and puffing your way to fully-charged pseudo-useful electronic gadget nirvana.

Introduction and Step 1

This project requires a wide range of "maker" skills, such as PCB board manufacture, dismantling of electronics, cutting and drilling plastics, mixing epoxy, designing a gear train, kludging together a bunch of parts, bending paper clips, and risking the well being of your ever so expensive phone, camera, or PDA. All in all, good fun.

Since everyone will have a different collection of junk parts to build this from, I will just give you a detailed overview of how I went about it and you can apply these ramblings to your own project. Which will consist loosely of four steps.

1. Scrounge up some suitable parts for the generator
2. Build the charger circuit
3. Assemble the generator, thorax coupler, and mechanical return
4. Connect the charger circuit, and test

Step 1:

I had about four old CDROM drives hanging around and took apart a few of them to see what cool parts were inside. Turns out there are lots of cool motors, gears, and other parts inside that fully validate my insistence of keeping such crap laying around. Seeing the gear trains inside these units used for opening the tray gave me the idea for this project. The small low-torque, high-RPM motor is linked to the tray via a gear train that has a final ratio of about 20:1 Previously I had been using a parallel array of tiny pager motors to generate electricity from breathing (see below) but the linear travel from your chest expansion is not that great (around an inch) so in order to generate useful voltages you had to really huff and puff.

Anyway, tear into those CDROM drives, which you can find at pretty much any garage sale, thrift store, or landfill. The pic below shows the results. Lots of potential projects in there. For now, we are only interested in the plastic gears and the motors for opening the tray and/or moving the laser carriage.

Look over the various gears and drives and try to visualize a way to add additional gears to increase the gear ratio, or how to add another motor in series. You want to minimize the changes to the gear train. Alternatively you can just scavenge all the gears and build your gearbox up from scratch.

You are also going to need at least one motor with a small gear or pulley on it so that you can connect it to the gear train. The motors in the CDROM drive are typically simple permanent magnet DC motors designed to run on 5V, except for the spindle motor, which you don't want to use anyway.

At this point you also want to think about what you are going to use for a strap to go around your chest. An old belt, some webbing, an old shoelace, a name badge strap, or anything that will fit around you comfortably without any stretch to it. You want all the expansion to take place in your linear generator. Any stretch that occurs in your thorax coupler will be wasted energy.

Step 2 - Build the charger circuit

The charger circuit is pretty simple. It consists of:

1. A diode bridge to turn the AC voltage from the generator into rectified DC.

2. A rechargeable battery to level out the voltage and hold excess generated power when nothing is hooked to the USB port. You could use a big capacitor too, but batteries offer a more predictable voltage level.

3. A boost converter to bring the low voltage up to 5VDC for USB charging

4. A USB plug.

I've drawn up the circuit in EAGLE, a program that I highly recommend. You can download it for free from The schematic and single layer board layout are attached. The actual use of EAGLE and the board manufacture are beyond the scope of this instructable. Many great instructables are out there to cover these topics.

The parts list for the charger circuit (quantities in bold):

1x L6920 Adjustable output step up DC converter (1V minimum input, Datasheet here)
Digikey# 497-4593-1-ND

4x 1N4148 switching diodes (I used tiny SOD523 smds, but you can sub in what you have handy)
Digikey# 1N4148WTDICT-ND

2x 10uF ceramic or other low ESR capacitors (I used 1206 smds)
Digikey# 39901299-1-ND

2x 100k thin film resistors
Digikey# P100kFCT-ND

1x 10uH wirewound inductor
Digikey# 490-2519-1-ND

1x USB female Type A smd connector
Digikey# AE9924-ND

Above you can see the schematic and board files, and jpegs of them as well. The tough part is making a good PCB in your kitchen that has traces small enough for the TSSOP package of the L6920. As you can see in the pic, I made 4 boards at once since each is so small.

The trick to putting it together is to start in the middle and move your way out, begin with the L6920, and add the SMD discretes as you go. A pair of tweezers is essential, along with good eyes or a magnifying glass, bright light, and a steady hand. Don't worry about getting too much solder in there, use your solder wick to clean up any accidents, and check your work with a multimeter after every step. Practice makes perfect.

Step 3 - Build the generator

Now you need to make the generator. You should play around with the gears and motors until you get a satisfactory arrangement. You will want to use a multimeter on the motor while turning the gears to see how much voltage you are getting. You want to get in the 2-3 volt range while moving the linear gear slowly about an inch in travel. When setting up the gears, you want to use the ones that have a large gear molded with a smaller gear. Stacked in series these will give you a good gear ratio as shown in the drawing. (ignore the fact that the teeth are the wrong size in the drawing, I was too lazy to redraw with matching tooth pitch) You should shoot for somewhere in the 25-50:1 range. More is better but eventually the losses in the gear train pile up and it get too hard to turn the motor and the gears will strip.

One of the keys is to find a way to use the linear gears on the CD tray or other piece to turn your breathing motion into rotation of the DC motor. I included a pic of another prototype version of the CD drive generator where you can see the linear tray gear clearly. Also visible are the cut marks in the plastic. This prototype was also capable of lighting the LED array pictured. Don't be afraid to chop this thing up to suit your needs.

In the other pic the DC motor is mounted in place in the plastic of the drive I cannibalized. Near this was a linear slider that I used to couple the breathing motion to the gear train. I also added another gear (see pic) to the drive train in order to increase the ratio and to allow mounting another motor in the future to increase output. The main challenge is to effectively get the breathing effort translated into rotation of the motor efficiently.

Step 4 - Put it all together and test it out

Once you have a satisfactory generator setup, then you want to connect the generator to the charge circuit, insert the battery, and use your multimeter to test the output voltage at the USB port. If you don't see 5V then there is a problem. Fix it before plugging your pricey gadget into the USB port.

Below you can see my assembled breath powered USB generator in all its glory, top and bottom. You can see the rubber band used for return, along with the linear gear carriage, the strap and the paper clip I used to connect the linear gear to the strap. The key here is to have all motion transferred to the linear gear so you want the strap and connection method to be stiff with no give. The strength of the rubber band or spring return is up to you. My half-assed experiments indicate that you can pretty much handle a 1N force without feeling too labored in your breathing. Ideally you want as small a rubber band as will return the linear gear to the starting position when you exhale. If you get enough generating capacity either through high gear ratio, extra motors, or a bigger motor, then you will need a bigger spring return. Essentially you are storing mechanical energy during your inhalation that is used to turn the generator on the exhalation so that you can generate on both push and pull. You need the diode bridge to successfully take advantage.

So I strapped on this monstrosity and hooked it up to my trusty data acquisition box from DataQ. Attached is the voltage plot output of the generator before step-up conversion to the 5V USB. Basically the battery runs the step up converter and the breath generator charges the battery. In the plot you can see the leveling effect of the battery, with the voltage spikes when I was breathing. Actually I was approaching hyperventilation, but in the name of science. The results can be seen in the photo of the phone charging. One thing to mention is that I had to modify a USB cable to get the RAZR to charge as detailed on this website. I don't have any solid numbers on the power I was generating, I haven't come up with a good way to measure that yet.

Typical resting metabolism is on the order of 50-75W of which a substantial portion is due to breathing effort (I have seen north of 50%). So if we assume 25W continuous energy used for breathing, it seems reasonable that we could increase that 4% to harvest 1W for charging a cell phone. Based on my cell phone, and these assumptions it would take about 3 hours to charge the 3.7V 800mAh battery. Assuming 100% efficiency.

Sadly, based on the few measurements I was able to make, the breathing generator I built is putting out more like 50mW. Way to breathe no breath. It would charge the phone, but the NiMH battery would be doing most of the work until it was drained. Then you would have to breath for a day or so to recharge the NiMH battery. You were planning on doing it anyway right? So there is room for improvement. One area I am looking into is using carbon nanotubes and polyurethane to make an electroactive polymer generator. This is the type of technology that is being used to make boot-strike generators for the military.

Future improvements could get this device into the 1W range. Specifically, using a better DC motor (higher voltage per rev) and custom building the drivetrain to be more comfortable and better coupling to breathing motion.

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