Pedal Power
Station!
Part 1
Words and design: Julian Edgar
Photos: Georgina Edgar
This project generates real power, keeps you fit and best of all, you can have
fun building it from scrap and salvaged parts at low cost!
H
ere’s a project that can develop lots of power
– just from pedalling. How much? Try 25A at 13V
(that’s 325W)! Use it to charge a 12V battery, or via a
normal car adaptor, any device that uses USB 5V charging.
Alternatively, you can easily configure the project as an
exercise bike, adding a low-cost display that shows precisely how many watts of power you are developing. To make
it even more exciting, you can add high-power lights that
glow brightly with your pedalling effort – so much better
than just dissipating your energy into a friction brake. Or, if
you live in a hot climate, you can even power a large fan to
keep you cool as you exercise!
There are three things that make this project better than
most pedal-based generators of the past:
1. An efficient two-stage gearing system
2. A large generator
3. Low-cost prebuilt electronic modules to condition the
output and display power.
This issue covers the bike mechanicals and the generator,
while next month we’ll explain the electronic instruments
you can fit, and how the electrical output can be conditioned.
However, note that the series is presented here more as an
‘ideas forum’ rather than a step-by-step project. Why? Because it depends on what you can scrounge for nearly nothing. To buy all the parts brand new would be quite expensive – but, by using other people’s discarded items it really
can be a cheap project.
First, let’s take a look at the main components.
Choosing a generator
The suggested generator is a discarded or low-cost DC lowvoltage motor. As many of you will know, when a DC brushed
motor has its shaft driven, it becomes a DC generator. And,
since motors are much more common than generators, looking for discarded motors is the first step in sourcing a generator suitable for pedal power.
Almost any low-voltage brushed DC motor can be used,
but when picking one look for a design that:
Is large and heavy, with an output shaft that is at least
8mm in diameter
Uses thick cables, indicative of a high-current design.
When you think you may have found a suitable generator,
it’s easy to test it. Connect a load to the generator (eg, a 50W
incandescent car headlight bulb) and then use your electric
power drill to spin the generator shaft. (To drive the generator, just lock the generator’s shaft in the drill’s chuck.)
Practical Electronics | October | 2020
Fig.1. The Pedal Power Station in its completed form. A
powerful DC generator is driven by two stages of step-up
gearing, giving a good output at a normal pedalling speed. The
prototype can develop a peak of 350W – and probably more
with a stronger rider than me!
Monitor the output voltage with a multimeter and ensure
you don’t spin the generator so fast that the bulb’s voltage
rating is exceeded. (If it looks like the voltage will rise too
high, add another parallel bulb – the greater the load, the
lower the output voltage.) If the 50W bulb does not light,
try a lower power one. If the 50W bulb glows brightly when
powered by the drill, it’s likely that the generator will be
suitable for pedal power.
Another test is to short the generator’s output (that is, connect its wires together) and then turn the generator by hand.
The resistance to turning should be much higher than when
the generator is open circuit.
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Use a stepper motor?
Instead of using a DC brushed motor as the generator, a
large salvaged stepper motor can be used instead. This
has two advantages: stepper motors used as generators
(alternators, really) typically have a high output at low
rotational speeds, and these motors are usually equipped
with ball bearings rather than the plain bushes used in
many DC brushed motors.
However, there are two downsides – the wiring is a
little more complicated, and large stepper motors are
more difficult to find as low-cost discards. (Of course,
you can use a small stepper motor, but the power output
will be much lower.)
As with DC motors, the easiest way of assessing the
suitability of a stepper motor is to drive it with an electric drill and see how well it powers a load. But how do
you do the wiring?
Most commonly found steppers are 6-wire designs,
with, as Fig.2a shows, two electrically separate centretapped windings. Use a multimeter to measure the resistances of the coils until you ascertain which wires
are which. Then place an incandescent light (eg, a 20W
12V bulb) across a winding (for example, connections
1 and 2 in Fig.2a) and spin the stepper motor with the
drill. The lamp should light brightly on the AC waveform generated by the one phase.
Next, short those two wires together. The stepper
should now be now much harder to turn, with a distinct
gritty ‘cogging’ action.
If the stepper motor performs well in these two tests,
it should be suitable for pedal-generator use.
To gain a DC output, wire diodes as shown in Fig.2b.
Specify the diodes based on the current you’re able to
generate from each phase – for example, you may need
four 6A10 (6A) diodes – cheaply available online.
The starting point for my Pedal Power Station came when
I found a discarded electric outboard motor. Often used on
lakes that prohibit internal combustion engines, these motors are usually conventional brushed DC designs. (However, one wrinkle is that they may include speed control
electronics within the motor housing. This prevents their
use as a generator (the electronics don’t work backwards!),
but the speed control electronics can be easily bypassed
just by connecting the output wires straight to the brushes.)
In addition to electric outboard motors, car starter motors
that do not use reduction gears, wind generators and
even the DC motors from old computer tape drives and
similar can all be used. For a lower powered generator, a
brushed motor from an electric scooter is cheap and readily
available. A car alternator initially looks attractive, but
unless you use really high step-up gearing, the alternator
will usually have too low an output.
Incidentally, with second-hand motors, it’s a good idea
to open them up and give them an internal clean. (As the
brushes wear, carbon gets distributed through out the motor internals.) While the motor is apart, grease the bearings
(usually just plain bronze bushes) and, if the commutator is
scored, smooth it with some fine emery paper, using a long
strip wrapped around the commutator and a back and forth
sawing motion. When re-assembling the generator, be careful to ensure that the carbon brushes go back properly into
place – sometimes you’ll need to juggle them a little to get
them seated again on the commutator.
Fig.3. An old electric outboard motor makes an ideal pedal
generator. However, any heavy-duty, low-voltage, brushed DC
motor can be used. This electric outboard is a little smaller than
the final one used on the Pedal Power Station.
Fig.4. The armature of the electric outboard motor used in
the Pedal Power Station. When using a salvaged motor as a
generator, it’s a good idea to first disassemble it, lubricate the
bearings and clean the commutator.
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Common 1
Common
2
+
Load
Common
a)
Common
–
b)
Fig.2a. Most salvaged stepper motors are 6-wire designs
configured with two centre-tapped windings. Use your
multimeter to sort out which wires are which.
Fig.2b. To gain a DC output, wire four diodes as shown here.
Rate the diodes according to the current that you measured
when testing each phase.
Practical Electronics | October | 2020
Pedal machine
The best starting point for the ‘pedal’ end of things is an old
exercise bike. (Starting with a conventional bicycle frame
will mean a lot more work.) Try to find an unwanted exercise bike with these features:
Sturdy with a heavy-gauge steel frame
Easy access to a driven wheel so that it can be adapted to
drive the generator
High gearing (so that the driven wheel rotates fast)
As heavy a driven wheel as possible
A driven wheel with either an inflatable tyre or alternatively, a flat edge to the rim
Smooth operation – the bearings in good condition
Let’s take a look at these in turn. You want an exercise
bike with a strong steel frame because you will need to
either bolt or weld an additional frame to it, one that will
support the generator. Light-gauge material will not have
sufficient strength to support this attachment. In addition, the design of the bike needs to be such that there is
clearance (usually in front of the wheel) for the generator to be located.
The higher the pedal gearing, the faster the driven wheel
will rotate – and so, when you adapt a generator to it, the
faster the generator will turn. Typically, the generator will
have a higher output at higher speed – so faster equals better! And why a heavy driven wheel? A heavy wheel act as a
flywheel, helping to smooth your pedalling power inputs.
The exercise bike shown below was bought from the local recycling centre for £10.
If the exercise bike wheel has an inflatable tyre on it, the
easiest way to drive the generator is to install a small-diameter rubber-tyred wheel on the generator’s input shaft
and have this press against the tread of the bike’s tyre. This
Fig.5. Starting point for the Pedal Power Station was this old
exercise bike that was bought from a local recycling centre
for £10. Points to look for when selecting a suitable bike are a
sturdy steel frame, a heavy driven wheel, high step-up gearing
and space in front of the driven wheel to mount a generator.
Also check that the wheel and pedal bearings rotate freely.
Practical Electronics | October | 2020
Gearing
The Pedal Power Station pictured below uses 16.2:1
step-up gearing. The standard chain drive stage gives
a step-up of 2.8:1, and the second belt stage, 5.8:1. At a
cadence (pedalling speed) of 90 rpm, this gives a generator speed of about 1450 rpm, a speed which was
confirmed using a laser tachometer.
rubber-to-rubber contact will transmit a lot of power without slipping, and the inflated tyre on the bike will cushion the contact, giving good drive even if the two wheels
aren’t perfectly round.
If the exercise bike has a smooth and flat edge to the rim,
you can drive the generator via a belt. Two different types
of belt are commonly available: the classic ‘V’ belt and the
more modern multi-rib belt. (If you lift the bonnet of your
car, you will see one or other of these belt types.) While
not designed to do this, a large flat wheel on an exercise
bike will drive either of these belt designs without slipping. However, on the generator, it is important that you
install a pulley of the appropriate design – that is, either a
V-belt pulley or a multi-rib pulley.
Fitting the pulley
Of all the steps in making the Pedal Power Station, installing a pulley (or rubber-tyred wheel) on the generator shaft
is likely to be the trickiest – so let’s take a look at that.
Depending on how the wheel or pulley is designed, attachment to the shaft might be via a press-fit or slip-on with
a grub screw. In either case, the opening in the wheel or
pulley is likely to be larger than the diameter of the generator shaft, so requiring a reduction sleeve. It is important
that the sleeve is concentric and that its inner and outer
diameters are precisely the correct sizes. Unless you have
a metal-working lathe, this might be one job to take to a
Fig.6. Here the generator is driven via a V-belt. (Alternatives are to
use a direct friction drive or a multi-rib belt.) The V-belt runs on the
flat surface of the original bike flywheel and drives a new pulley
that has been fitted to the generator. The generator is mounted
on a welded steel tube frame that attaches to the exercise bike.
Note the lower pivot points and the upper bolts that allow the belt
tension to be adjusted. The generator’s steel frame could also
have been bolted together or made from wood.
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Fig.7. The Pedal Power Station is powerful enough to run almost
any 12V LED light source – from one as modest as this 10W LED…
…to a pair of 90W LED light bars. And yes, without the pedalpowered lights, this view is pitch black!
small machine shop and ask for an appropriate bush to
be made. (Don’t forget to take the generator and pulley or
wheel along with you.)
If you’re happy with a rough-and-ready approach, you
may be able to use a short length of rubber hose to make
the bush. The rubber can be stretched over the generator
shaft (ie, it doesn’t have to be precisely the right size) and
compressed within the wheel or pulley opening. The rubber is also easily sanded or filed to size. If taking this approach, ensure that the pulley or wheel cannot come loose
– the last thing you want is it flying off at high speed!
The cheapest way of obtaining either a V-belt or multi-rib
pulley is at a car dismantling yard or an auto electrician.
At the latter, you’ll normally find plenty of alternators being thrown away – complete with their pulleys. If you are
using a small rubber-tyred wheel, appropriate wheels are
used on a wide range of goods that are often discarded. (Or
of course you could also – gasp – buy a new one!) In both
cases, select the smallest available wheel or pulley for the
generator. This gives you the greatest step-up in gearing.
Once you have a pulley or wheel installed on the generator, you can work out how the generator is to be mounted.
Let’s take the ‘belt-drive’ approach first.
Position the generator in front of the exercise bike wheel
and, using a piece of wire looped around the pulley and
wheel, measure how long the required belt needs to be.
Take the piece of wire to an auto parts store and buy the
appropriately sized belt – both V and multi-rib belts are
available in an enormous range of lengths.
If you are using a rubber-tyred wheel, push the wheel
up against the exercise bike’s wheel and look at where the
generator will need to be positioned.
When positioning the generator, ensure that there is still
room for the rider to pedal – be especially careful to give
the required toe clearance.
Flywheels
When you pedal, you don’t develop a smooth torque
output. Instead, at the top and bottom of the pedal
stroke, the torque greatly decreases as the legs pass Top
and Bottom Dead Centres. This is not noticeable on a
bicycle being ridden on the road, because the inertia
of the bike smooths these variations. However, with a
stationary pedal generator the result can be a jerky development of power.
The exercise bike shown here came with a heavy
steel flywheel as standard, helping smooth the flow of
power. However, many exercise bikes don’t take this
approach. While I haven’t tried it, some on-line searches show people making flywheels by filling small bike
wheels with suitably reinforced concrete. Heavy steel
weights bolted symmetrically to the rim should also
work well. In all cases, ensure that the result is structurally strong enough to withstand the centrifugal forces
without failure. If there is any uncertainty as to this,
enclose the wheel in a guard.
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Practical Electronics | October | 2020
Fig.8. When locating the generator, be careful to retain sufficient
toe clearance during pedalling.
Frame
You will then need to make a frame that attaches to the exercise bike and holds the generator in its required position.
This frame should be strong and rigid, both because the generator is likely to be heavy and because the generator needs
to maintain its registration with the drive wheel.
This frame can be made in a wide variety of ways. For example, you could use square steel tube bolted together with
angle brackets, or even make a frame from wood, screwed
and glued together. The attachment to the exercise bike frame
can be via bolts passing through holes drilled in the frame,
or you can use U-bolts that pass around the frame members.
Having access to a MIG welder, I made the frame from
20mm square steel tube (1.6mm wall thickness), welded
together. I then welded tabs to the exercise bike frame to allow attachment of the generator frame.
The way that the generator is attached to its supporting
frame depends on the generator and the original design of
its mounts. In my case, where the outboard motor had only
a single mount that attached to a long pole, I cut the pole
off and used U-bolts to hold the generator to a flat plate on
the new frame.
Make the generator frame adjustable so that it can be moved
slightly to either tension the belt or vary the pressure with
which the rubber-tyred wheel pushes against the exercise bike
wheel. Note that this adjustment mechanism doesn’t need to
be elaborate – eg, some washers under the bolts can be used
to adjust clearances, or the generator frame can be hinged
and a spring used to give the required force. Having built alternator mounts for cars, where the belt tension needs to be
very high, I was surprised at how little tension was needed
to transmit pedal power to the generator without slippage.
Whichever drive approach you are taking, the frame must
position the generator drive directly in-line with the bike
wheel. If this is done, and the generator is held rigidly in
position, the belt will track truly on the flat wheel and the
driven rubber wheel will not ‘walk’ off the bike wheel.
Next month
That’s all for this month – next issue we will look at the different loads than can be driven by the Pedal Power Station
and how you can fit a low-cost digital instrument to measure your power output.
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