Pedal Power
Station!
Part 2
Words and design: Julian Edgar
Photos: Georgina Edgar
This project generates real power, keeps you fit and yet you can build it
from scrap and salvaged parts at low cost. Last month, we developed the
mechanical section of the pedal-powered DC generator. Now it’s time to use
electronics to add instruments and condition the power output.
S
o, you’ve built your Pedal
Power Station – now, what loads
can you drive with it? The options are numerous, as we’ll explain
below. But before we get into the details, first a quick warning. Depending on the direction that you are driving the generator, the polarisation
of the connecting leads may have
changed over the original ‘motor’
orientation. For example, red might
now be negative and black, positive.
Therefore, before connecting any
loads, always check the generator’s
output polarity with a meter and then
label the leads accordingly.
Resistive loads
We’ll start with the simplest load; just
a resistor (and here of course, motor
output polarity doesn’t matter). The
power rating of the resistor will need
to be high (as in, hundreds of watts)
and the resistance very low. On the
machine shown opposite, best results
came from using a 0.9Ω resistor.
The easiest way of obtaining such
a resistor is to make one from galvanised steel wire of the type often used
in gardening and fence repair. You
will need to use thin wire, say 0.7mm
diameter. To start, select a length that
gives a measured resistance of about
1Ω checked via your DVM (do make
sure your DVM is genuinely accurate
in the single-ohm range). Form the resistor by wrapping the wire around a
small diameter tube to shape it into
a spiral, remove it from the tube and
then bend each end into a small loop.
Then use screws, washers, nuts and eye
terminals to make the electrical attachments to the looped ends (you cannot
easily solder directly to the steel wire).
Be careful when testing – the resistor I made could glow red-hot if I
Practical Electronics | November | 2020
If the Pedal Power Station is being used primarily as an exercise bike, a car radiator fan
makes an excellent load. The fan has been mounted on an adjustable stand made from
steel tube. In hot climates, the fan can cool you as you pedal!
pedalled hard enough. If the resistor
does get too hot, use a longer length
of thinner wire.
A simple way of varying the resistor rating is shown in Fig.1. Minimum
load occurs with both switches open,
medium load with switch 2 closed,
and maximum load with switch 1
closed. If the Pedal Power Station is
being used as an exercise bike, this circuit therefore gives a variety of different pedalling efforts. The ‘resistor’ can
be three resistors (or more) connected
in series, or you can make one long resistor and simply take tappings along
its length. Again, do remember that it
can get hot, so ensure it is mounted
safely away from you, small people’s
fingers and inquisitive pets.
PWM output control
If you want infinitely adjustable control of the load, you can use a DC variable PWM controller and the aforesaid
resistor (or the other loads covered in
a moment) – see Fig.2. These PWM
41
R e si st o r
D io d e
S w i t ch
1
S w i t ch
2
–
Fig.1. Using a multi-tap high-power
resistor (that you can easily make yourself)
and two switches allows adjustment of
load when the Pedal Power Station is
being used as an exercise bike. Here,
minimum load occurs with both switches
open, medium load with switch 2 closed,
and maximum load with switch 1 closed.
P e d a l
g e n e ra to r
–
P W M
m o d u le
L o a d
r e si st o r
Fig.2. A PWM module (often sold online
as a ‘DC motor speed controller’ or
similar) allows stepless adjustment
of load at the turn of a knob. Ensure
you buy a PWM module that has the
required current rating.
modules are often sold on eBay as ‘DC motor speed controllers’ or similar.
I tested the module I reviewed back in the May 2016 edition of PE/EPE on the pedal generator. This module has an
input voltage of 6-28V and a maximum current of 3A. As
you’d expect with this relatively low current rating, it was
easy to get the module rather hot! However, this module,
which is very cheap, would be quite suitable for a lowerpowered pedal generator. To my surprise this item (or one
very similar) is still available – at the time of writing eBay
item 233664457658. Higher current PWM controllers are
also readily available, for example eBay item 124329447989
has a claimed rating of 8A.
Hunt around on all the usual websites (eBay, Amazon,
Banggood, AliExpress…) for something along the lines of
‘Variable PWM power module’.
Lights
Much more exciting than driving a resistor is to illuminate
an appropriate number of LEDs or incandescent lights.
Depending on the power output of the generator, you
may be able to drive strings of literally hundreds of LEDs.
Be careful that even the hardest pedalling cannot exceed
the rated voltage of the LED strings (or alternatively, drive
the LEDs via a constant current converter).
Incandescent lamps are a really ‘rewarding’ load to drive.
For example, multiple 50W headlight bulbs are both very
bright and alter in level in a most satisfying way with pedalling effort. Unlike LEDs, you can also briefly over-drive
incandescent bulbs without damage. For a fun demonstration of the power available from pedalling, lights are best.
Two screw terminals have been mounted on a box to provide
the power output connections.
42
+
+
+
+
P e d a l
g e n e ra to r
P e d a l
g e n e ra to r
–
+
B o o st
m o d u le
+
B a tte ry
Fig.3. A constant voltage boost module can
be used to electronically ‘gear-up’ the Pedal
Power Station. For example, here the boost
module has been used to increase output
voltage to allow the charging of a 12V
lead-acid battery. Note the required diode
– boost modules don’t like having battery
voltage applied to their output!
Fan
Another really rewarding load to drive is a 12V car radiator
fan. These are available quite cheaply from car dismantlers
and people selling car parts.
These fans will usually start turning at a quite low voltage (eg, 3V) and the load they provide goes up rapidly with
speed. Therefore, this load is – just like incandescent lights
– effective for a variety of people. Even a child pedalling
gently will get the fan to spin, while a strong person pedalling hard will generate a gale!
In hot weather, using a fan as the load also has the benefit of cooling the rider – ideal when the machine is being
used as an exercise bike.
USB car chargers
Commonly available and cheap, 12-24V USB 5V chargers
can be used with even a small DC generator. These chargers are specified for a maximum of 24V, so it is less likely
you will exceed their rated input voltage (but of course
always check the maximum no-load voltage output of the
generator). The small current these chargers draw means
that charging even multiple phones will take little pedalling effort – another great educational demonstration of
pedal power.
Battery charging
The Pedal Power Station can be used to charge standalone
batteries. Best results will usually come from using a dedicated charger module designed for the specific battery type.
For example, an 18650 li-ion car charger with a 12-24V input is suitable for most pedal generators.
For lower-power generators, a PWM module like this one allows
you to adjust the output power (and hence pedalling load).
Similar higher-current PWM modules are readily available.
Practical Electronics | November | 2020
To overcome this, the output
can be electrically smoothed by
D io d e
using a supercapacitor pack.
+
+
+
P e d a l
+
+
Feed the pack through a diode
+
S u p e r ca p a ci t o r
P e d a l
g e n e ra to r
L o a d
S u p e r ca p a ci t o r
p a ck ( e g , 2 0 F )
g e n e ra to r
V o ltm e te r
L o a d
(otherwise the supercaps will
–
p a ck ( e g , 1 F )
–
drive the generator as a motor
when you stop pedalling!) and
then feed the load from the supercap pack. Remember to obFig.4. An ‘electronic flywheel’ effect is
Fig.5. A 2-wire voltmeter is a simple way of
serve correct polarity and enachieved by using a supercapacitor pack
monitoring output. However, you may find that
sure that the voltage rating of
to smooth the generator’s output. Here
the display ‘dances around’ a lot with the slight
the pack cannot be exceeded –
a large 20F unit has been used, but
variation in voltage that occurs as you pedal.
depending on the load, you may be able
Here, a 1F supercapacitor pack has been used to refer to Fig.4.
I used a 20F (yes, twenty
to use a smaller (and cheaper) 1F pack.
give the voltmeter a long time constant, stabilising
farads!), 16V supercapacitor
The diode prevents the supercapacitor
the display.
module, one that incorporates
pack from driving the generator as a
charge-equalisation circuitry.
motor when you stop pedalling.
This worked well in smoothBoosting the voltage output
ing the output, with the minor downside that when first
Constant voltage boost converters are an ideal match with starting to pedal, the effort was higher while the supercaps
the Pedal Power Station. Taking this approach allows you initially charged. To find similar modules, search eBay unto electronically ‘gear’ the generator higher than achieved der ‘16V 20F Ultracapacitor Engine Battery Starter Booster
through mechanical means.
Car Super Capacitor’ or similar.
For example, at a moderate pedalling speed, the Pedal
Power Station shown on these pages had a no-load output Instrumentation
of about 13V. That sounds like enough to charge a 12V lead There is a wide range of instruments you can fit to the
acid battery, but by the time you add the voltage drop of Pedal Power Station.
a series diode, the resulting voltage is just a little low for
A digital 2-wire DC voltmeter is cheap and easy. These
charging. However, if you add the boost converter and set are available from eBay from only a few pounds and some
it to say 14.5V (that takes into account the diode voltage are available that will work from 5-120V DC – just connect
drop) and you’ll have a practical charging machine.
the voltmeter directly to the generator’s output.
To find these boost converters, search eBay under ‘DCAlternatively, an LM3914-based bar-graph display can be
DC Boost Converter Power Module’.
used – these are also available on eBay and have adjustNote that if using a boost converter to charge a battery, ment pots to allow the high and low voltages to be indiyou must use the series diode between the converter and vidually set. (Search under ‘LM3914 Battery Capacity Inbattery – some of these converters will be destroyed if the dicator Module LED Power Level Tester Display Board’ or
battery voltage is applied directly to their output. Fig.3 similar) These display boards are suitable for voltages up
shows the essentials of the set-up, but do undertake fur- to 20V and cost only a little more than the LED voltmeter.
ther research if you are new to this topic.
D io d e
+
+
Smoothing the output
If the pedal machine has only a light flywheel, the electrical output of the generator is likely to noticeably vary
with each pedal stroke. If you are driving lights, for example, this can be annoying as the lights alternately brighten
and then dim.
Charging of phones and other equipment that uses USB 5V
charging is easily achieved with a charging module like this.
Ensure the module is rated for the maximum voltage the Pedal
Power Station can produce off-load.
Practical Electronics | November | 2020
A supercapacitor pack can be used to smooth the power output.
This is a 20F 16V module that incorporates charge equalisation.
Feed the supercapacitor pack from the generator via a diode, and
be careful to observe correct polarity.
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A 20A boost-voltage converter like this is highly effective with the
Pedal Power Station. For example, it allows constant voltage
charging of a 12V battery, even if the generator’s ‘raw’ output is a
little too low in voltage to charge the battery.
You may find that a voltmeter display ‘dances around’
as your pedalling output varies. You can use a capacitor
across the voltmeter input to settle its reading. For example, Fig.5 shows an LED two-wire voltmeter with a 16V
1F supercapacitor pack – you don’t have to use the (more
expensive) 20F supercapacitor pack mentioned above. The
1F pack gives a long time constant for the display – and as
a bonus, the LEDs stay alight for about five minutes after
you stop pedalling. (If the load you are driving has a very
low resistance, you may need to add a series resistor after
the diode – this will slow both the charge and discharge
rates of the supercapacitor pack).
How much power?
More sophisticated than a voltmeter – but also more expensive – is an LCD instrument that displays measured
voltage and current, and from these calculates power.
Here an LCD power meter has been fitted. This meter is
powered by the bike itself and displays a variety of parameters,
including peak and average power. Its mount (made from
folded aluminium sheet) also provides a rest for a smart phone
below the meter, allowing the rider to be entertained as they
ride. Add a USB charger and you can even charge your phone
while you exercise.
44
After testing a few different modules, the best I found is
a device that is low in cost, uses a two-line back-lit LCD
and can measure a wide variety of parameters. You can
find it by searching online under ‘150A Digital LCD Watt
Meter Power Analyser Solar Caravan Anderson Plug Tool’
or similar. At the time of witing, for example, eBay item
402438984452. It costs around £10 delivered – check the
pictures below to ensure you select the correct module.
If the exercise bike develops an output of more than 8V
under load, this meter can be powered directly from the
bike generator. (You can apparently also power the meter
from a separate battery, but I didn’t do so.) Connections are
very simple – the heavy cables on the ‘source’ side of the
module connect to the generator, and those on the ‘load’
side connect to, yes, the load! Maintain the correct polarity when making both these connections. The device is
supplied with Anderson-type plugs, but I found it easiest
to cut these off and connect directly to the flying leads.
So, what is displayed? The LCD shows two types of data –
some shown continuously, and some cycling through different parameters. Shown continuously are amps (maximum
of 150A, with a resolution of 0.01A), volts (maximum of
60V with 0.01V resolution) and watts (maximum of about
1600W with a resolution of 0.1W). The bottom-left display
cycles through the following in turn: Ah (amp-hours), Wh
(watt-hours), Ap (peak amps), Wp (peak watts).
This line-up of data allows you to measure almost every
parameter you’re interested in – whether you’re charging
a battery or having a competition as to who can generate
the most pedal power! Note that unless you use an extra
battery to power the display, all peak data is lost as soon
as you stop pedalling.
Conclusion
I’m very happy with how this project turned put. The V-belt
drive is as smooth as silk, and everyone who has seen the
Pedal Power Station in action has wanted to ride it. The use
of low-cost, pre-built electronics also allows the charging of
batteries or powering of lights in ways that previously were
difficult and/or expensive to achieve with pedal generators.
Used as an exercise bike, a PWM module allows stepless
load changes – or alternatively, the machine can be electrically simple, with multiple load pre-sets achieved just by
switching in different home-made resistances.
Instrumentation can be as simple or complex as you want,
with even a sophisticated digital instrumentation display
that is low in cost.
All you need to do now is keep an eye out for discarded
exercise bikes and heavy-duty DC motors!
The power meter display. Here the display is showing about
6A, 10V, a peak current of 7A and a power of 63W. The
display costs around £10 delivered and is powered by the
bike’s generator. (The odd-looking digits are an artefact of the
camera shutter speed.)
Practical Electronics | November | 2020
