Introduction: Generator Demonstration From Cordless Drill

About: I've built some weird stuff over the years, but most of that stuff has remained unseen by the world outside of me and a few friends. But then one day, one of these friends, he says to me, "Hey Jack, you shoul…

This instructable will show you how to make a crude, but sturdy, hand cranked generator, capable of supplying  just a few watts of unregulated DC, at a potential of a few volts or so.  This generator is suitable for classroom demonstrations, science projects, lending credence to the legend that a motor can be used as a generator, and amusing children of all ages who have not seen this trick before.   

By itself this generator is basically a toy.  The operator turns the crank, and he or she produces enough electricity to light up a old style, incandescent, flashlight bulb.

No doubt there are going to be questions like:

"How can I make/modify/improve this thing so it can power/recharge my cell phone/ mp3 player/ vibrating massage wand/ etc?" 

Such things may be possible and maybe even practical,  however the goal for now is simply to light up a flashlight bulb. Any designs more complicated than this will have to wait until a later instructable.

BTW, I apologize for using a blurry picture as the "main" image for this instructable, but this actually the best photo I've got that captures this generator in action.  I'm guessing this photo is clear enough to see what's going on, but if you need a few hints:  The big blue thing in the background is Jack's tee shirt. The bright pink-white blob is the light bulb, with current being driven through it.  The almost invisible blur on the left side of the picture, is Jack's hand turning the crank.

The second pic is a still shot of the generator on my workbench.

The third is another action shot, but this time with the generator clamped in a vise so it won't move around so much and make the picture blurry.

Step 1: Theory Part 1: Magnets and Wires

In very simple terms, a DC motor is coil of wire in close proximity to a permanent magnet. There is indeed some other stuff going on. For example there is a mechanical commutator that is actually switching different coils to the motor's (two) input terminals. Also there is more than one permanent magnet.

However at any particular instant in time, the system pretty much looks like a single coil (those windings which are connected at that moment in time ), and a single magnet(those magnets which are near those coils).

Motor action is usually explained in terms of the Lorentz Force Law: A current carrying wire, in a magnetic field, experiences a force, perpendicular to the direction of the current, and perpendicular to direction of the magnetic field. It is this force, which causes the rotor to move.  In this way, the interaction between current in wires and magnetic fields of permanent magnets, causes physical force, which in turn produces motion.

F = I*L x B
Generator action is usually explained in terms of Faraday's Law of Induction: The voltage induced in a coil of wire is proportional to the time rate of change of magnetic flux through the coil, multiplied by the number of turns in the coil.  This changing flux is caused by the relative motion of the rotor coils and the stator magnets. In this way, motion causes changing geometry, which causes changing magnetic flux through a coil, which causes a voltage to manifest across the coil.
V = N*(dΦ/dt)
A practical result of Faraday's law, one that can be directly applied to building homemade generators, is that the voltage across a unloaded generator (or motor) tends to be proportional to its speed.  The faster the generator turns, the greater (dΦ/dt), and the higher the voltage.

What this means for you, as a generator designer, is that you'd like your motor-as-generator to turn very quickly, at roughly the same as the speed it was running at when running as a motor.  Fortunately the cordless drill comes with a drive train which is geared favorably, to make the motor turn quickly at low torque, when the spindle is turned slowly at high torque. 

It seems fortunate that a cordless drill can be driven backwards this way.  It seems fortunate, but is a coincidence?  Or is it some sort of deeper law of nature? 

The reason I ask this question, is because it turns out the humble cordless drill is just one of many physical systems that don't seem to mind being "driven backwards".

For the sake of beating this topic to death, examples of these other physical systems are given in the next step.

Step 2: Theory Part 2: Transducers and Time Reversal

In the arcane world of physics and engineering there is the notion of a transducer,
defined simply as a device which converts one type of energy into another.

Some examples of transducers:
  • A solar cell converts light energy into electrical energy.
  • A speaker converts electrical energy into sound energy.
  • A motor converts electrical energy into mechanical rotational energy.
Interestingly, many transducers are capable of "working backwards". That is to say a transducer that normally coverts X energy into Y energy, is also capable of converting Y energy into X energy.

For example piezoelectric speakers can be driven as speakers, producing sound when driven by an electrical signal, or they can be used as a microphone, converting sound into an electrical signal.

Light emitting diodes (LEDs) produce small amounts of electric current when exposed to light. LEDs can actually be used as light sensors. Also solar cells are rumored to actually emit light (in the infrared) when current is driven through them backwards.

Motors being driven backwards as generators, or generators driven backwards as motors, are yet another example of this trend. Actually, I could have mentioned motors and generators first, and then LEDs, and piezoelectric crystals. In fact, motors and generators, probably should be explained first, because I think the underlying physics of motors and generators is easier to explain, and its best to start with the simplest explanations first.

The notion of time reversal:

Still speaking in general about any kind of transducer, one way to sort of picture it "working backwards", is to ask, "What would it look like if time were moving backwards?" The answer to this question is, in general, is:

In forward time, energy flows from the driver, through the transducer, into the load.

In backwards time, energy flows from the load, through the transducer, back into driver.

For example, imagine a cordless drill, happily driving a screw into a piece of wood.

In forward time it looks like this:

Energy flows from the battery into the motor. The motor turns. Energy flows, throught the gears of the drive train, from the motor to the spindle.The motor end spins quickly at low torque, and the spindle end turns slowly at high torque. Freeeeeeooowww!!! The spindle turns the bit, and the bit drives the screw into the wood and most of the energy is dissipated as heat and sound.

In backwards time it looks like this:

Waves of heat and sound converge on the screw, causing it to turn backwards,  and causing the hole in the wood to mysteriously knit itself back together. Wwwoooeeeeeerf!!! The screw turns the bit, and the bit turns the spindle. Energy flows, through the gears of the drive train, from the spindle to the motor. The motor end spins quickly at low torque, and the spindle end turns slowly at high torque. The motor acts as a generator taking energy it receives from the drive train, and delivering it to the battery. The voltage across the battery is the same as it was in forward time, except now the current in the wires is flowing in the opposite direction.

Now, take a closer look at this picture... er movie,  of the drill in backwards time.  Some of the stuff that's happening is just crazy, like the hole in the wood knitting itself back together.

However, the rest of the movie doesn't seem all that crazy.  It seems like it really could happen.  Gear trains can, and do, move power in either direction.  Batteries really do recharge when electric current flows backwards doing work against the battery voltage.

Anyway, this section hasn't offered much in the way of proof  that motors, or any other transducer,  can be "driven backwards", but believe me:  the legends are true. I mean the truth comes out by way of experimental observations.  Many physical systems just "don't care that much" about whether time is flowing forward or backwards.

Step 3: Materials

For all the materials used here, additional specs are mentioned in parenthesis. You don't have to follow these specifications exactly, but there are certain principles at work here. Some of these principles that guide the selection of materials are discussed in the notes below.


old cordless drill (generic 9.6 V cordless drill, missing battery)
wire (18 AWG, stranded copper, insulated)
flashlight bulb  (incandescent style, for 2 cell flashlights, ~2.4 V nominal)
steel rod stock (5/16 inch diameter, 7+1/2 inches long)


An old cordless (battery powered) drill is the heart of this project.  The motor it contains should be a brushed DC motor, with permanent magnets in the stator.  Don't worry too much about this specification.  Pretty much all battery powered drills available at the time of this writing use a DC motor of this kind.

The wire can be anything capable of handing an ampere or so of electric current.

I picked a 2 cell flashlight bulb, 2.4 V nominal,  just because this seems pleasing to me.

The rod, out of which the crank handle will be made, should be sturdy.  It should fit the chuck of the drill. 

The rod stock should also be circular and smooth.  This is because the crank is  going to be sliding against your hand as you turn it.  That is to say using square, or threaded rod, might be kind of hard on your hands.

Step 4: Tools

Tools for connecting wires:

safety glasses
soldering gun ( or soldering iron)
solder  (60/40 Pb-Sn, rosin core)

Tools for making the crank handle:

safety glasses
leather gloves
old washcloth (rag)
MAPP gas torch
drill press
hack saw
sand paper
steel wool
tape measure
marking tool (pencil, sharpie, tubing cutter, etc)
cheater bar

Opening up the drill and connecting wires to the motor (Steps 5-8), is the easy part of this instructable.

Shaping the steel rod into a nice crank handle (Steps 9-12) is really the more difficult and tool-intensive part.

Step 5: Inside Your Cordless Drill

The cordless drill featured in this instructable has a plastic case consisting of two halves held together by screws.  Your cordless drill is probably similar.  Once you take out all the screws you should see something similar to the picture shown for this Step.

For those of you who have dissected such things before, the organs of this animal look very familiar:  DC motor, drive train, chuck, power MOSFET with heat sink,  trigger and power controller, battery connection terminals.

For everyone else, I've labeled these parts in the picture below

Step 6: Remove the Electronics.

Unsolder the wires from the motor. 

Remove the old trigger-power-controller, and also the power MOSFET and its heat sink, and also the battery connector terminals.  Take out the battery too, in the unlikely event that this old drill of yours still has one.

I mean the missing, or dead, battery, is probably the reason why this drill found its way into your junk pile.  If you found this cordless drill at a thrift store, or in a dumpster, or in someone else's junk pile, the story of how it got there probably involves a dead or missing battery.

Batteries can be weak, and fickle, and cruel, and it is not surprising that so many, initially happy, battery-drill marriages end in divorce.  It's usually the battery's fault.  I'm sorry to say such unkind things about batteries, but it's the truth. 

Anyway...  pull out the electronics, except for the motor. Also keep the drive train, the spindle, the chuck, both halves of the plastic case, and most if not all of the screws that hold it together.

  The MOSFET, power controller, etc, put all that stuff in your junk box.  You won't be needing those things  for this instructable.

Step 7: Connect the Load Directly to the Motor Terminals.

The load, in this case, is the flashlight bulb.  Solder two wires to it.  Then solder those wires to the terminals of the motor. 

This wiring is really too easy to justify a diagram, but, just to be clear, I'll include one, in the second picture for this Step.

Step 8: Put the Drill Back Together.

Put the two plastic "clam-shell" pieces of the case back together. 

Make sure everything fits back the way it was before.  I.e. the motor and the drive train are nestled snugly into their little spaces, and all the little slots and tabs line up.  Then put all the screws back in.

Also try not to squish the new pair of wires you've added.  With the power controller electronics gone there should be plenty of room.  Snake the wires and the flashlight bulb out through any hole that the wires will fit through, e.g. the hole where the trigger used to go, or the hole where the battery used to go.

If you've got a nice looking metal crank, then insert that into the chuck.  Tighten the jaws of the chuck... and you're done!

If you don't have a nice metal crank, you can just twist the spindle with your hands,  and that will sort of work.  You won't be able to turn it as fast as you could with a crank though. 

Turning it with a crank is definitely more effective, and more fun too!

Steps 9 through 12 will show how to bend a steel rod into a crank.

Step 9: In Rod We Trust.

First I cut a piece of steel rod (5/16 inch diameter) 7+1/2 inches long using ye olde hacksaw.

Then I load the rough piece of rod into the drill press, where I'll smooth down the rough edges from the hacksaw cut, and any other rough edges that happen to be on the piece. 

This smoothing action is accomplished via a metal file, sand paper, and steel wool.  The technique is the same as shown in my instructable on how to make homemade soldering iron tips..  If its not clear what I'm doing with in the pictures below, reading that instructable might help.

Step 10: Crank Design.

Basically I want a piece of steel rod with two right angle bends in it.  The other part of the design is choosing where to make these two bends.

Calling one edge of the rod, x=0, make marks at:  x=1+1/2 inches, and x=4 inches.

The rod is 7+1/2 long.  So these marks divide it into segments of length, 1+1/2, 2+1/2, and 3+1/2 inches.

Originally I made these marks with a Sharpie(r) pen (see 2nd picture in stack), but then after realizing that the torch just burned away the ink marks, I decided to go over the marks with a tubing cutter, actually scoring a thin line into the metal.

Step 11: Fire Makes Steel Easier to Bend.

This Step might be a little bit dangerous. It involves heating steel until it is red hot using a MAPP gas torch. The whole rod doesn't get red hot, rather it is just one spot on the rod. 

The idea is to heat up a spot on the rod until it glows red, then quickly clamp it into the vise, and make the bend at that spot. Heating the steel to redness makes it much easier to bend, and to get nice tight corners.

One of the important assumptions of this trick, is that the steel is not red hot at the location where you're holding it.  

However, I found the steel was getting really uncomfortably hot where I was holding it and starting to burn the leather gloves too!  Hence, I decided I needed more insulation than just a leather glove, and that's what the washcloth/rag is there for.  BTW, this is a dry washcloth, not a moistened one, and it kinda got burned and blackened a little bit on the parts that were touching the steel.

The pictures attached to this Step show this process of heating a spot, then moving the piece to the vise, and making a sharp bend at that spot.

Step 12: Let It Cool. Make Small Corrections With the Cheater-bar.

After both bends are finished, let the metal cool.

At this stage, it may be tempting (no pun intended) to dunk the piece of steel into a bucket of water, just because of all the fun steam, and hissing and phfwisssshing noises it would make, and it would cool the piece down really fast.  Instead, find the patience to let the steel cool slowly.  You want this crank to be tough, not hard and brittle.

If you didn't get the angles exactly square, that's OK.  You can make small adjustments to the piece while it's cold.  Using a cheater bar helps.

The finished crank is shown in the second picture for this Step.

Step 13: Chuck the Crank, and Crank It Up!

Put the crank in the chuck.  Tighten the jaws of the chuck so the crank is clamped firmly in place.

Now turning the crank should make the flashlight bulb light up. 

It may be possible to burn out the flashlight bulb, if you get really crazy with der cranking, so you know, go easy on the darn thing! 

Or supposing that going easy on things just isn't your style, perhaps you should just get a large quantity of flashlight bulbs, and/or also use a bulb-socket of some kind, so that you don't have to do any soldering each time a bulb needs replacing.

The second picture shows the working drill generator clamped into the vise, so as to take a photo that is not so blurry.  Note that on all the action shots of this toy I'm intentionally not using the camera flash, because it tends to drown out the light from the flashlight bulb.

Third picture is just the drill generator toy, at rest on the workbench.

Step 14: Troubleshooting

If you followed all these steps kinda closely, and your drill generator toy still isn't working...

Well, I'm not sure why that is, but there's probably a logical explanation for it.