Design of a Hand-Powered Flashlight
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Design of a Hand-Powered Flashlight
Tan Yingyi
Abstract
This project aimed to come up with an alternative power source that could circumvent the problem of
battery leaks in hand-held flashlights. This was achieved through the use of a bicycle dynamo. It was
found that when the handle of the flashlight was wound at an average speed of about 1500 revolutions
per minute, the bulb would light up fully. Due to the high speeds involved, a gear train had to be
employed. Thus a special flashlight had to be designed. Though the finished product met the principal
objective, it was not the optimal working conditions. The compact space required for hand-held
flashlights, created friction between the shafts and gears, causing the turning of the handle to be
strenuous and preventing the clockwork spring from working. The findings demonstrated that though
a dynamo could replace the need for batteries in flashlights, batteries may still be the better
alternative.
Introduction
Almost all hand-held flashlights used today are powered by two or more alkaline batteries.
Often flashlights fail to operate when they are needed the most, such as during power-failures. This
may be due to leaks in the batteries or a result of the batteries decaying. When the potassium
hydroxide in the battery leaks, a gradual decrease in the amount of electrolyte means a depletion in the
amount of electrons available for electrolysis in the battery to take place. Eventually, a low degree of
electrolysis results in an insufficient amount of electrons travelling through the circuit to light the
bulb. Thus the battery can no longer power the torch. The continuous manufacture of batteries is also
an energy consuming process. The amount of energy required to manufacture one alkaline battery
alone is fifty times greater than the amount it can release. (Ministerial Sub-committee of Green
Ministers, 2001)
Therefore, in times of emergency, it is unreliable to depend on batteries to power the only
light source available. An alternative energy source, that is available anytime, would have to be found
to ensure that the flashlight works when needed. One may argue that rechargeable batteries could be
used instead. However, the battery may still run out while one is using the torch and the bulb will not
be able to light up, creating a situation similar to the problem at hand.
In 1831, Michael Faraday invented the generator – a device that could convert mechanical
energy into electrical energy when there is a change in a magnetic field. When the mechanical energy
input into the dynamo is used to turn an electrical coil between two magnets, the magnetic field
between the two magnets is disturbed and an electrical voltage is induced in the coil. The voltage
produced is directly proportional to the rate of change of the magnetic field:
∆Φ
voltage generated = − ,
∆t
∆Φ
where is the rate of change of magnetic flux with time. (Nasar and Boldea, 1990)
∆t
The dynamos used in bicycles to power headlights differ from ordinary electrical generators
in the way the magnets rotate instead of the electrical coil. The dynamo is attached to the front wheel
of the bicycle. As the bicycle wheel rotates, the dynamo wheel rotates along with it, turning the
magnet attached and disturbing the magnetic flux, thus inducing a current to flow to the bulb.
By the same principle, a dynamo when built into a flashlight could replace the need for
batteries. The mechanical energy generated through the winding of the handle of the flashlight could
be converted into electrical energy. However, to produce a constant stream of light, the handle would
have to be wound continuously. To solve the problem, a spiral spring would be used to store the
mechanical energy generated, releasing it slowly as it unwinds. Thus mechanical energy could be
supplied continuously to the dynamo and the handle would only have to be turned when the spring has
released all its stored energy.
1Materials, Methods and Results
Determining the Speed
Aim: To determine the dynamo wheel speed to light up a bulb
Apparatus: Bicycle dynamo
6V bulb
2.4V bulb
1.5V bulb
Lathe machine
Procedures:
1. A 6V bulb was inserted into the dynamo.
2. The wheel of the dynamo was clamped into the chuck of the lathe machine.
3. The lathe machine was set to the minimum speed of 91rpm
4. The level of brightness of the bulb was noted.
5. Steps 3 and 4 were repeated with the speed increased to 113, 136, 256, 494, 786, 1200,
1572, 3400 rpm
6. The degree of brightness of the light produced for each speed was noted.
7. Steps 1 to 6 were repeated with a 2.4V and 1.5V bulb separately.
Results:
When the lathe machine was turning at a particular speed, the chuck caused the wheel of the
dynamo to turn. Thus it was assumed that the dynamo wheel was turning at the same speed as the
lathe machine.
It was found that a minimum turning speed of 113rpm was required to light the 6V bulb. A
dim light was produced that could not be seen unless the surroundings were completely dark. As the
turning speed of the dynamo wheel increased, the light produced by the bulb grew stronger (Table 1).
At a turning speed of 494rpm the light produced by the bulb could be seen in normal daylight.
However, the degree of brightness was still faint and insufficient for the intended purpose of the
flashlight, which was to provide sufficient light during power failures or for use in the outdoors. Only
when the turning speed of the dynamo was raised to 1572rpm, the light produced by the bulb was of
sufficient brightness. The degree of brightness was equivalent to that produced by a 3V bulb powered
by four AA dry-cell batteries.
When a 2.4V bulb was used, the turning speed required to just light the bulb was increased to
256rpm. While the speed required to produce a faint light was almost the same as that required for a
6V bulb: 494rpm. To ensure that the bulb produced sufficient light, a speed of 1527rpm was required.
Thus there was no significant difference in the rotational speeds required for a 6V and 2.4V bulb.
For a 1.5V bulb, the minimum rotational speed necessary to produce light was 256rpm.
While that required to produce a faint light was similar to the 2.4V bulb: 494rpm. The optimum level
of brightness produced by the bulb was only reached when the rotational speed was increased to
1527rpm.
As a speed of 1500rpm was too high to be attained by just turning a handle, a series of gears
had to be used to step up the speed. The first gear attached to the shaft of the handle could then be
turned at a comfortable speed, but would still be able to light the bulb.
Table 1 Degree of Brightness of Light at Different Rotational Speeds for Different Bulbs
Rotational Speed of Degree of Brightness of Light produced
Dynamo Wheel / rpm 6V Bulb 2.4V Bulb 1.5V Bulb
91 No light produced No light produced No light produced
Very dim – cannot be
113 No light produced No light produced
seen in daylight
Very dim – cannot be
136 No light produced No light produced
seen in daylight
256 Dim light Very dim Very dim
Faint – can be seen in
494 Faint Faint
daylight
786 Moderate Moderate Moderate
21200 Bright Bright Bright
1572 Very bright Very bright Very bright
3400 Very bright Very bright Very bright
Calculating the Number of Gears required
Aim: To calculate the number of gears and gear ratio required to ensure that the handle is turned
with a comfortable speed for the bulb to light up.
Procedures:
1. The speed at which the dynamo wheel had to be turned to light the bulb with sufficient
brightness was taken to be 1572rpm.
N 1 V2
2. Equation = was used to calculate the speed which a gear, interlocked with
N 2 V1
another turning at speed V1 rpm, was turning. ( N 1 and N 2 are the number of teeth on
gears one and two respectively. V1 and V2 are the speeds in revolutions per min at which
gears one and two are turning respectively.)
3. A trial and error process was employed to arrive at a reasonable speed of 50rpm on the nth
gear. (n is the number of gears used in the gear train.)
Results:
N 1 V2
From the equation = , it can be seen that an increase in the number of teeth from a
N 2 V1
gear to the next would decrease the speed by the same ratio. (Ballaney, 1980) Thus, in order to fully
reduce the speed required to turn the handle of the flashlight, the gear attached to the shaft of the
handle would have to have a maximum number of teeth while that attached to the dynamo would have
to have the least number of teeth possible. As the maximum number of teeth of a plastic gear available
was 80, while the minimum was 15, if only two gears were used, the speed would only decrease by 80
÷ 15 ≈ 5.33 times. Thus the handle would have to be turned at a speed of 1527 ÷ 5.33 ≈ 286rpm.
This speed was still too high to be reached by normal human power. Thus another set of gears would
be required. This time, the smaller gear would be attached to the larger gear of the previous gear
train, allowing the smaller gear to rotate at the same speed of 281rpm as the larger gear (Photograph
1a). The 4th gear would then rotate at a speed of 286 ÷ 5.33 ≈ 53.7 rpm, which is fairly comfortable.
Therefore, the total number of gears required in the gear train would be 4, 2 with 15 teeth and another
2, 80 teeth.
Designing and Building the Flashlight
A simple design that could accommodate the dynamo and the gears in the smallest possible
manner was created. The flashlight would be L-shaped with the overall size dependent on the size of
the dynamo used (Drawing 2a). In this way, the flashlight could be kept as small as possible. The
constraint in size is due to its portable nature. The flashlight was intended to be hand-held to provide
a light source during power failures or used in the outdoors such as for the exploration of caves.
However, the standard bicycle dynamo used was fitted with a supporting rack that could not be
removed. Thus the flashlight had to be modified as the rack was not included in the original design
and would take up additional space if housed in the casing of the torch. Instead, a new rectangular
model was devised with the dynamo secured to the outside of the casing (Photograph 1b).
The original wheel of the dynamo had teeth that were of a different size from the teeth on the
gears, thus had to be replaced by a 15-teeth gear. However, the internal diameter of the gear was too
large for the axle of the dynamo. Instead, the axle was connected to the shaft of the gear. A hole was
drilled into each of the gears and set screws were used to attach them to brass shafts, which were held
in place by 2 aluminium plates. The other 4 sides of the flashlight were covered in perspex sheets for
easy viewing of the interior. Bearings of internal diameter 6mm and 10mm, thickness 3mm and 5mm
were used to reduce friction between the shafts and aluminium walls. 4 additional brass shafts were
placed at the top and bottom of the flashlight to secure the aluminium plates.
3The dynamo wheel had to be turned continuously in order to produce electricity. To
overcome this constraint, two spiral springs were attached to the bottom two shafts (Photograph 1c).
As the handle turns, the spring would unwind and the torch would light. When the spring unwound
completely, the tension built up would cause it to wind up by itself. Thus releasing energy gradually
and allow the bulb to continue lighting up without having to turn the handle.
The final design of the flashlight is shown in Photographs 1a-1g and Drawings 2c-2e.
Discussion
After the flashlight had been fabricated, it was discovered that the springs were not strong
enough to work. After unwinding, the spiral springs would curl up and become entangled, instead of
winding back (Photograph 1f). Thus the springs could not store energy and the handle had to be
wound constantly in order to produce light continuously.
The main reason for the malfunction was due to large friction created in the gear train of the
flashlight. Though the bearings helped reduce some of the friction, the contact between gears and
between the shafts and aluminium plates were still rough, thus a lot of energy was lost. This made the
system stiff and hence the strength of the springs was insufficient. It also caused the winding of the
handle to be difficult. The second reason is the lack of precision in the design and drawings of the
flashlight. No leeway was allowed for the various parts of the flashlight. Thus the gears were too
tightly fitted together, generating additional friction and heat.
Another drawback of the flashlight is its size. The flashlight has dimensions of 11.5cm ×
18cm × 9.5cm, which is too big and bulky to be carried around. This was due to a difficulty in
obtaining suitable materials. Firstly, there was only one type and size of dynamo available. If the
supporting rack of the dynamo could have been removed and the overall size of the dynamo made
smaller, the flashlight would have been more portable. Secondly, the plastic gears used were of a
different internal diameter from the dynamo wheel. Thus an additional shaft had to be used to
connect the gear to the dynamo, taking up space. The two different sized gears were also of different
internal diameters. Shafts were needed to connect them (Photograph 1g). Thirdly, the plastic gears
used were also very large. There was a lot of unnecessary space between the centre of the gear and
the teeth, making the gears very bulky.
The lack of suitable materials also contributed to the spring’s failure. The strongest spiral
spring available (30N) would not have been able to turn the gears on its own, thus preventing the
system from storing energy.
Conclusion
Batteries are not the only form of energy source available for flashlights. With the right
materials and design, kinetic energy can also be used to provide electricity to light a bulb. However,
the dynamos used on bicycles are not suitable for use in a flashlight. Their size and structure was
built only for use in bicycles. For flashlights, special dynamos would have to be built, which would
be quite costly. Thus batteries are still the better energy source for flashlights.
4Photograph 1a: Gear Train Used in Flashlight Photograph 1c: Bottom View
Showing Enclosed Spring
Photograph 1b: Side Elevation of Flashlight with Photograph 1d: Isometric View of
Dynamo Fixed on Exterior Flashlight
5Photograph 1e: Side View of Flashlight Photograph 1f: Entanglement of Spring
Photograph 1g: Shaft between Gears Taking up Unnecessary Space
67
8
9
10
Drawing 2e: Isometric View
¾ Size
11Acknowledgements
Associate Professor Lai Man On
Mr Thomas Tan
Mr Abdul Khalim Bin Abdul
Mr Juraimi Bin Madon
Mrs Tan Sooi Peng
References
(1) Ministerial Sub-committee of Green Ministers, Green Guide for Buyers, Action Sheet:
Batteries. Online. Internet. 9 Sep. 2003. Available: http://www.sustainable-
development.gov.uk/sdig/improving/partf/greenbuy/13.htm
(2) S.A. Nasar and I. Boldea. 1990. Electric Machines Steady-State Operation (Series in
Electrical Engineering). Hemisphere Publishing Corporation, Moscow.
(3) P.L. Ballaney. 1980. Theory of Machines. Khanna Publishers, Delhi. p937-947
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