# Generating electricity by moving a door

I'm entirely new to electrical engineering. This is for a hardware/software project I have in mind, where I need electricity without having to plug the device into a power source.

My intention is to make a bluetooth "controlled" door lock. For that I need to be able to charge a tiny battery to at least a level of 2V (the minimum voltage the device needs to run) to power a small bluetooth 4.0 device.

The device I intend to power is the ConnectBlue OLP425.

Is there any way you can generate electricity by moving something up/down or forward/backward?

• I have a flashlight that you can shake and it causes a magnet to move back and forth inside a coil. This generates enough energy to light the flashlight.
– Eric
May 28, 2013 at 19:03
• How much current do you need? With a transformer, it is no problem to get 2V from induction, but the current may suffer... ;) May 28, 2013 at 19:09
• @anorton I'm not quite sure, but i can supply i link for the device i intend to power: connectblue.com/products/bluetooth-low-energy-products/… May 28, 2013 at 19:22

Yes. Any magnetic field moving around any conductive material will create a current in that conductor, called electromagnetic induction.

Electric motors and generators operate on this principle, motors and generators being essentially the same machine, but perhaps slightly optimized or tested for different applications. Of course, these machines usually are designed to convert between rotational mechanical torque and electrical power.

The equivalent machine for linear motion is a solenoid. Drop a magnet inside a coil and apply a current to the coil, and the magnet will shoot out one way or the other, depending on the orientation of the magnet's poles and the polarity of the applied voltage. For a common example, take a look at the internals of a doorbell (the kind with a mechanical bell). When the doorbell is pressed, it applies 12V to a solenoid, which shoots a metal rod into the bell.

This also works in reverse: if you move the magnet in the coil, some of that kinetic energy you put into the magnet is converted to electrical energy in the coil.

If you make either the magnet or the coil very light so that sound waves can move it, you get a microphone. Or run it in the other direction, and you get a speaker, which is designed to handle vibrations of a much larger magnitude, but is essentially the same machine.

So, if you wanted to generate electricity from a doorknob, the easiest method would be to attach a generator to the knob. You can get any voltage you desire by mechanically gearing the generator or by selecting the appropriate generator. What you can't do is generate more power out of nowhere. Electrical power is the product of current and voltage:

$$P = IE$$

Mechanical power is the product of angular velocity and torque:

$$P = \omega \tau$$

If current is in amperes, voltage in volts, angular velocity in radians per second, and torque in newton$\cdot$meters, then power is in watts in both cases. The generator will convert mechanical power to electrical power with an efficiency of maybe 80%, and you will also have friction losses in the knob and any mechanical transmission. So, how much electrical power do you require? How much harder are you willing to make the doorknob to turn, and how many times are you willing to turn it?

Let's guess that a reasonable torque for a doorknob is $0.5Nm$ (about $5 in-lbs$) and we turn it 90 degrees over half a second. Our angular velocity is $1rad/s$. The mechanical power is then:

$$P = 0.5Nm \cdot 1rad/s = 0.5W$$

We can further multiply that by time to convert power to energy:

$$0.5W \cdot 0.5s = 0.25 J$$

This is how much energy you have to work with in a single doorknob turn. But you won't be able to convert it with 100% efficiency; let's assume that after losses you have 50% of that energy, or one fourth of a joule. To put that into perspective, wolfram alpha tells me this is about equivalent to the acoustic energy in 25 whispers. Wikipedia describes a joule as approximately the energy required to lift an apple one meter, so you can lift it 25 centimeters. For comparison, a AAA alkaline battery can hold on the order of 5000J of energy.

Or we can think of this electrically. A capacitor stores energy like a battery. You said you need two volts: what value capacitor has stored 0.25J when charged to 2V? The energy $W$ in a capacitor is given by:

$$W = \frac{1}{2} C V^2$$

$$0.25J = \frac{1}{2} C (2V)^2$$

$$\dfrac{2\cdot 0.25J}{(2V)^2} = C = 125mF$$

So to get some idea if your idea is feasible, charge a 125mF capacitor up to 2V and see if it can power your device.

• I'm aware of the theory of electromagnetic induction, however i fail to see how you convert such a system to fit into either a doorhandle or the lock of a door... The problem is that i need a voltage generation of at least 2V somehow.. The problem with putting such a system in the doorhandle is the horizontal design of the handle and the limited space for a moving magnet in either .. ? But thanks for the answer! :) May 28, 2013 at 18:59
• @EsbenKlindt you should have put that in the question, but I've expanded my answer. Better now? May 28, 2013 at 19:07
• Well somewhat, i'm not quite sure on how to convert those two formulas into a formula that gives me the angle(in degrees) to generate those minimum 2V, nor do i see how to apply it to the horizontal motion of the door. Furthermore another problem is 'attaching' the device to the doorhandle, the entire idea is that both the powergeneration and OLP425/lock should be inside a custom made doorhandle/lock (by 3D printing to start with) Assuming your advice i could in theory make the doorhandle as a thin shell holding a coil with a magnet inside? May 28, 2013 at 19:20
• @EsbenKlindt this is electrical engineering, not mechanical engineering. I don't know how you put the generator in the door. But, answer expanded, with math shown. May 28, 2013 at 19:43
• Much appreciated. As a node i have yet to purchase any electrics (I however have a OLP425) I am aware that it is not a mechanical engineering site, however i believe it to be both a mechanical and electrical problem. I posted the problem since i was corious to see if there was any actual technology that could solve my problem. I will only start the actual project if i find some feasible solution the the power problem. The mechanical problems i will deal with as they rise, the main problem is getting power so it won't run flat. May 28, 2013 at 19:53

Besides the magnetic energy sources mentioned in other answers, also have a look at the many piezoelectric energy harvesting modules available on sites like Mouser and Digikey.

For instance, one such product range of interest is the MIDE Volture product family.

While these energy harvesting devices are not ideal for the doorknob use case, they are ideal for vibration harvesting, at typical vibration frequencies of 25 Hertz and up.

It would be worth investigating whether the door itself, or the surrounding ground, can source such low frequency vibrations. While energy harvested per unit time may be small for these devices, they have access to the energy source constantly, whenever people are moving around or air movement occurs. Such devices have been successfully used for various energy-autonomous sensors and other electronic devices in recent times.

In short: The device would harvest energy not for brief periods when someone rotates a doorknob, but all the time. If the target devices needs power only for short bursts with extended quiescent periods, this approach might be the way to go.

• Thanks, that sounds very nice, however i dont believe that there is much motion within a door when it isnt being opened/closed. But combined with the other technologies it could possibly work.. May 29, 2013 at 14:37
• @EsbenKlindt If you look up the piezo harvesting products I linked to, you will see that they have been applied in situations where the tiny vibrations on the floor due to people walking by, or the slightly larger vibrations on doors due to conversation and air movement, are used to charge batteries through the harvester. If there is any human activity around at all, even other doors being open and closed, that will produce usable vibration in your door of interest. May 29, 2013 at 14:41
• Thanks, atm. i'm writing my bachelor thesis so i havent got the time to dig into the theory and possible application of the device, but i will definitely look into it afterwards! May 29, 2013 at 16:10

Yes. Moving a magnetic field along a piece of wire creates electricity. This is how things like wind turbines work. Usually they involve many loops of that wire rotating past many magnets. I doubt you'd get enough electricity out of one push on a door handle but you haven't given any specs so it is hard to tell.

• My bad, added specs to the question. May 28, 2013 at 19:03