DIY Knee Energy Harvester - .2W at 2 Mph!




Posted in TechnologyWearables

Introduction: DIY Knee Energy Harvester - .2W at 2 Mph!

Are you ready to make your own DIY knee energy harvester? For a total cost of ONLY 29 USD?

This harvester can generate an average of .20 W of power when you walk at 2 mph (0.9 m/s) on flat ground. .234W for an 8% grade decline.

Build it and use it on your next hike so you don't have to worry about your phone charge while you're up in the clouds!

Guess what? That's enough to power most modern portable devices, including your phone when it's in standby mode.

Get ready to generate some usable power without even trying - you just have to walk!

Are you a couch potato? Don't worry, because this device will definitely get you excited about moving! :)

Step 1: Gather All Your Parts.

Parts you will need:

(1) .25" (6 mm) acrylic sheet, approximately 3" x 13" should do the trick.

(2) Laser cut (I used a CO2 laser and adjusted settings based on the laser cutter's specifications. You should use the recommended settings your laser cutter suggests for this thickness of acrylic) these parts:

(3) LewanSoul 20MG High Torque Metal Gear Servo (You can use any high torque full metal gear servo that can be rotated a full 360 degrees when not powered. I know this sounds very niche, but these are often used for quadcopters and robotic arms so they are pretty common online and in hobby shops):

(4) Hose clamp (I believe I used a 1.5" diameter one, approximately. You can use any size that can tighten the servo very firmly onto the upper leg acrylic piece)

(5) 4 schottky diodes (1W rating to be safe)

(6) 4 strips of 1.5" double sided velcro (I used 36", 25", 24", 19". These sizes seem to be universal, but of course you can adjust the lengths to whatever you're comfortable with)

(7) 4 1.5" plastic buckles such as these:

(8) Wire (18 awg or 22 awg, preferrably stranded, NOT hook-up wire)

(9) [Optional] Silicone Tape (Try this or similar:


(1) Small phillips screwdriver that can fit the head of the screw that is meant to attach servo horns onto your servo.

(2) Soldering iron + solder (Stick to either Pb-free or leaded. Do not mix the two! Or else your connections will break apart when heated and cooled - this is not so uncommon during the summer)

(3) Flathead screwdriver for tightening hose clamp

(3) Wire cutters

(4) Wire strippers

(5) Sewing machine or needle & thread

Step 2: Time to Destroy Your Servo! :)

Take apart the servo by first removing the screws from the underside with your screwdriver.

You should be able to see a motor and a small PCB inside.

Step 3: Now Rip Out That PCB!

I think this step is pretty self explanatory.

Just kidding. Use some wire cutters to cut out the PCB. You won't need it when you're using the servo in reverse like we will be doing.

Step 4: Solder on New Wires to the Servo.

Now cut two wires, each at least the length of your upper leg + half the length of your torso. This is to ensure that you can hold the output of the harvester in your hand once you've put it on.

Solder one wire onto each of the two motor terminals. You don't have to keep track of + and - terminals, because at the output, if you're getting negative voltages [post-rectifier], you can just reverse the order of the wires as you plugged them in. Then you'll see only positive voltages post-rectifier.

What is this rectifier I speak of? You'll see in a few steps!

Step 5: Hot Glue

Hot glue that set of two wires (braid them first!) together, and hot glue it to the underside of the servo as well so it's more out of your way while you walk.

Step 6: Solder Together Your Bridge Rectifier!

What is this?

Well, it's a full-bridge rectifier made with schottky diodes.

Full bridge rectifier (takes a voltage that has positive and negative components and essentially takes the absolute value, minus some lost voltage V_f):

Schottky diodes (minimizes the lost voltage, V_f, and therefore minimizes the power you lose in the rectifier):

Solder your two motor wires to two opposite corners of the rectifier.

To each of the remaining two corners, attach a few inches of wire. These will be our output terminals.

You can also add a smoothing capacitor if you want, but choose wisely! One with low leakage current (See #4: is preferrable, or else you'll be losing your generated power to the capacitor!

If you want to be even more advanced, make a circuit with a boost converter that can take the output voltage of the rectifier-capacitor combo and convert the voltage to a higher voltage. Add in a battery charging circuit and BAM, you can start charging some stuff! Since this last part is personal to what you would like to do, I've only described the rectifier component. Enjoy finding cool applications for this!

Step 7: Attach the Bottom Leg Acrylic Piece.

This one should have the 25-tooth fit to the head of the servo.

Use a screw and washer to secure the acrylic attachment.

Beware: This piece might break after 24-48 hours of use. You can print another one or: you can CNC/waterjet a 1 - 2 mm aluminum/steel piece and 2 3 mm acrylic pieces all with the same file. And then sandwich the three together with the aluminum piece in the middle. That should make the lower leg attachment piece last a lot longer. Test it out and let me know how long it lasts you!

Step 8: Attach the Upper Leg Acrylic Attachment With the Hose Clamp.

Attach your upper leg acrylic attachment to the servo as pictured here. Tighten with flathead screwdriver.

Make sure your harvester has 90+ degrees of rotation afterwards. This is so you can still sit down, squat, etc. while wearing this device.

Step 9: Attach the Buckles + Straps!

For each strap, you'll have to sew on one of the buckle sides (I sewed on the female side to the velcro strap as you can see here. I used two layers of a zig-zag stitch using a Singer machine. You can also do it by hand.)

Take each strap with the sewed-on buckle and wind it through two adjacent slots as pictured above. Make sure the two open sides of the strap are coming out of the acrylic attachment on the side OPPOSITE to the side that the servo is mounted onto.

Do this for all four straps. By the way, you should choose slots that fit well with your leg. The end result should fit snugly onto your leg without slipping, and tightening location matters!

Step 10: Now Weave Through the Other Side of the Buckle.

These buckles are really cool because they will not come apart when you weave the velcro through it. The friction force of the velcro weaved through the buckle and back onto itself is high enough to prevent slippage.

This is hard to describe so reference the pictures to see what orientation it should be in. Once you've got it correct, you should be able to fasten the buckle in the correct orientation (no twists and turns).

Another cool feature of this: Since we're using double-sided velcro, if you happen to be skinnier than the length of the velcro, then you can wrap the excess over the buckle and onto itself and it'll stay! See the third picture.

Step 11: You Now Have a Working Energy Harvester!


Here's a picture of what the final harvester should look like, assembled as well as on two different users.

We recommend putting the harvester on while standing and bending down at the hips - minimal bending at the knee.

Most important thing! MAKE SURE THAT THE AXIS OF ROTATION OF YOUR HARVESTER IS ALIGNED WITH THE AXIS OF ROTATION OF YOUR KNEE. You'll know it's misaligned if you feel some lag between your movements and the harvester's movements. You should not feel this. Move the harvester up/down or slide it horizontally along your leg.

If you still feel the harvester is slipping after you had it adjusted, totally add some sillicone slips between the straps and your leg and you should be all set!

Tighten the buckles, fold over the excess straps, and start walkin'! Let me know what you think!



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    This is awesome! I wonder if it would work for roller skating?

    Nice. It also works when the sun is down, and in caves or other dark areas, unlike solar panels.

    1 reply

    For about $2.00 one can find a 6cm x 8cm solar cell on eBay that produces 6 times the power. Simply glue it to the brim of your hat.

    3 replies

    Thanks for the comment :) Yes of course you are right. There are certainly much more efficient ways to harvest "wearable" energy, such as the idea you stated. But I suppose this knee energy harvester falls into something like basic science - exploring possibilities in a certain area of research because you can and it's an interesting problem to try to solve. Perhaps it's important to figure out how you can make a biomechanical energy harvester in the DIY space so others can have some fun making their own usable energy harvesters. That is what I'm exploring here :)

    Also, see the note above about my original calculation error in average power. The correct power is, quoted from above:

    "This harvester can generate an average of 2.0 W of power when you walk at 2 mph (0.9 m/s) on flat ground. 2.84W for a 15% decline. (Note: Originally, I wrote 100 mW/140 mW - apologies for that. My data analysis had an error in it. If you ever make power calculations, make sure you calculate your total power as the integral of [instantaneous power = instantaneous voltage squared / resistance], all divided by the total time interval, NOT the average voltage squared / resistance. :))"

    Gotta correct myself again here. So the correct average power is 0.2W at 2 mph on flat ground and 0.234W at 8% grade. This is less than your solar panel cap idea you said, but hey it's research!

    I agree! In fact that would be one of the applications of this - it's a portable exercise machine! :)