Build a Shake Flashlight

Introduction

Using simple electronic components, construct a flashlight that requires no batteries—merely a shake of your hand provides the power necessary to light it.

Concepts

• Basic circuitry
• Capacitance
• Electromagnetic induction
• Energy transfer

Background

The circuit components used to construct the shake flashlight are shown in Figure 1 and described.

• Capacitor: Capacitors are circuit components designed to store electrical energy. They function similarly to rechargeable batteries; once a voltage has been supplied, they will hold that voltage until some complete circuit loop is made, at which point the stored charge is released.
• Diodes: Diodes are devices with two leads that allow current to travel in only one direction.
• Light-Emitting Diode (LED): A specialized type of diode that lights up when it reaches its critical voltage. 
• Resistor: Resistors are common circuit components designed to limit current flow by producing a voltage drop in accordance with Ohm’s law:
  {12800_Background_Equation_1}
  where

  V is the voltage drop
  I is the current
  R is the resistance

• Breadboard: Breadboards are convenient, reusable devices used to build circuits without long and inconvenient cords, or the need to solder. The holes in each row (1, 2, 3...) are connected in series, meaning any two leads placed in the same row will be connected with one another (see Figure 2).
  {12800_Background_Figure_2_Breadboard schematic}
  Thus, a simple circuit diagram such as shown in Figure 3 could be attached to the breadboard as shown in Figure 4.
  {12800_Background_Figure_3_Circuit diagram}
  {12800_Background_Figure_4_Breadboard setup}

Experiment Overview

The purpose of this activity is to build a prototype of a “shake” flashlight that operates on the principle of electromagnetic induction, converting mechanical energy to electrical. A basic generator will be built using a magnet and wire core connected to an LED using a simple circuit consisting of diodes, a capacitor and a resistor.

Materials

Safety Precautions

The current generated from the flashlight is too low to be a danger to humans. Nevertheless, proper safety precautions are advised—do not touch bare wires while shaking the flashlight. Wear safety glasses when preparing the jumper wire. Use caution when handling scissors and wire strippers. Follow all laboratory safety guidelines.

Prelab Preparation

Preparing the Jumper Wire

1. Obtain the PVC-insulated wire, ruler, needle-nose pliers, and wire strippers. Using the wire strippers and ruler, measure and cut two 6-cm pieces, two 2.7-cm pieces, two 2.2-cm pieces and one 2.3-cm piece of wire.
2. Use wire cutters or wire strippers to remove 6–8 mm of the plastic sheath off each end of all the wires.

Procedure

Part A. Assembling the Generator

1. Obtain the magnet wire coil, sandpaper, plastic tube, scissors, cotton balls, electrical tape and neodymium magnets.
2. Use sandpaper to carefully remove the enameled coating on each loose end of the magnet wire coil. Sand carefully, as the thin wire may easily break. Note: The appearance of the wire ends should change from a bright bronze color to a dull brown when the enamel coating is removed.
3. Slide the magnet wire coil over the plastic tube, about 1½" from one end.
4. Slide three of the cotton balls into the plastic tube. The cotton balls will serve as a cushion to slow the magnets down before they switch directions while being shaken and should help preserve the life of the magnets.
5. Slide the three neodymium magnets into the tube. Note: If the magnets come with plastic separators, remove these and carefully attach the magnets directly to each other. The plastic separators may be discarded, or saved for future storage of the magnets.
6. Place the remaining three cotton balls in the plastic tube and cap it. Wrap electrical tape around the cap to secure it.
7. (Optional) Test the functionality by attaching the two coil leads to a multimeter. Set the multimeter to read alternating voltage, and test to ensure you can achieve at least 2 V (ideally 3 V), when rapidly shaking the plastic tube. When satisfied, use the hot glue gun to secure the wire coil in place.

Part B. Assembling the Circuit

9. Obtain the breadboard, four diodes, the capacitor, the cut jumper wires and the resistor.
10. Assemble the diodes as shown in Figures 5 and 6. Note: Pay special attention to the polarity of each diode. The banded end marks the cathode. The “arrow” in a circuit diagram points towards the banded end of the diode.
    {12800_Procedure_Figure_5_Circuit diagram}
    {12800_Procedure_Figure_6_Breadboard components attached}
11. Attach the two 6-cm jumper wires in rows 1 and 2, as shown in Figures 5 and 6. These are the jumper wires that will connect the circuit to the magnet wire coil.
12. Take the two 2.2-cm wires and attach them across rows 9 and 11, respectively, in columns “E” and “F,” bridging the gap between the two halves of the breadboard (see Figure 6).
13. Connect the capacitor to the breadboard, ensuring that the two leads are in two separate rows—the negative lead should be in row 9, and the positive lead should be in row 11. Note: The capacitor will have the negative terminal indicated by a banded end containing the negative (“–”) symbol, as shown in Figure 6.
14. Attach one end of the 2.3-cm jumper wire to row 4, and the other to row 9, as shown in Figures 5 and 6.
15. Attach one end of one of the 2.7-cm jumper wires to row 5, and the other to row 11, as shown in Figures 5 and 6.
16. Attach one end of the other 2.7-cm jumper wire to row 11, and the other to row 17, as shown in Figures 5 and 6.
17. Obtain the resistor, and connect it to rows 9 and 14, as shown by Figures 5 and 6.
18. Obtain the LED, and connect the shorter, negative prong to row 14, and the longer, positive prong to row 18, as shown by Figures 5 and 6.
19. Obtain the switch. Attach it to the breadboard so the middle prong is in row 17, and the left prong is in row 16. Note: Make sure the switch in the “OFF” position (to the left).
20. Attach the assembled circuit to the prepared generator tube using the provided rubber bands. Note: Do not secure the rubber bands over any of the circuit components on the breadboard.
21. Attach the jumper leads in rows 1 and 2 (as prepared in step 11) to the magnet wire leads from the coil, by wrapping the thin magnet wire around the exposed jumper wire, and folding the jumper wire onto itself (see Figure 7).
    {12800_Procedure_Figure_7}

Part C. Demonstrate the Shake Flashlight

22. Give the assembled flashlight a gentle shake to ensure everything is secure and no components shake loose.
23. Flip the switch to the “ON” position, and give the flashlight a few more rapid shakes to ensure the LED flickers on. Note: If the LED does not light, increase the shaking speed. If it still does not light, recheck all the circuit connections to ensure a functional circuit.
24. Flip the switch back to the “OFF” position.
25. Begin shaking the flashlight once more. Slowly increase the shaking speed, once again ensuring that none of the components shake loose. Continue to increase until you reach a maximum speed, and maintain this speed for 20–30 seconds.
26. Once the capacitor has been charged, cease shaking and flip the switch on to demonstrate the sustained, steady light from the LED.

Student Worksheet PDF

12800_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration an indefinite number of times. All materials are reusable, and the breadboard circuit may be disassembled and reassembled as many times as needed.
• If the assembled circuit does not light the LED, isolate the problem by eliminating components. The following steps may be tested in any order:
  • Eliminate the wire coil: Replace the magnet wire coil with two 1.5-V batteries in series to charge the circuit, leaving them attached for 5–10 seconds. Flip the switch on. If the LED lights up, the coil itself is the problem. Test the coil again to ensure a complete circuit, and then try shaking faster. In addition, the resistor may be removed and replaced with a jumper wire.
  • Attach the batteries directly to the capacitor, leaving them connected for 5–10 seconds. Flip the switch on. If the LED lights up, the diode configuration is the problem. Recheck that they are placed in the correct rows, or check their continuity using the continuity tester function on a multimeter.
  • Attach the batteries in series with just the LED, switch, and resistor. Flip the switch on. If the LED fails to light, replace the LED with the extra one provided in this kit.
• The switch may be replaced with a strip of jumper wire. Using the extra jumper wire provided in the kit, cut a 2.2-cm piece of wire and strip 6–8 mm off each end. When the switch should be in the “off” position, leave the jumper wire off of the breadboard. To turn the flashlight on, connect the jumper wire into rows 17 and 18.
• This kit is a good introduction to the basics of electronics and circuitry, although an in-depth understanding of circuits is not required. It is also a great demonstration to show energy transfer from one form to another.

Sample Data

Observations

Draw a diagram of the shake flashlight, labeling the following major components: capacitor, diodes, LED, magnets, resistor, switch and wire coil.

Answers to Questions

1. Describe the difference in the LED when the flashlight is shaken slowly versus rapidly. Explain in terms of Faraday’s law.

   When the flashlight is shaken slowly, the LED will not light when the switch is closed. When it is shaken rapidly, it does. Shaking the light faster results in a greater change in the magnetic flux, which induces a higher emf (voltage).

2. Why are diodes used in this circuit?

   If no diodes were used, only alternating current would be produced, which cannot be stored in a capacitor. The LED would not light. Diodes are used to ensure the current is traveling in the proper direction.

3. What would happen if the capacitor were removed from the circuit and the diodes were connected directly to the LED and the switch?

   The LED would light only when it is being shaken and the switch is on. Since the current produced by shaking is not steady, the LED would flicker.

4. Shaking the flashlight produces mechanical/kinetic energy, which is converted into electrical energy in the coils as the magnet falls through.

   The energy of the current flowing through the wire coils is store as potential energy in the capacitor.

   When the switch is closed, the stored energy in the capacitor is converted to light energy in the LED.

5. Explain how each of the following changes would affect the flashlight performance.
   1. Adding more coils to the wire: Adding more coils of wire would result in more voltage generated, meaning the light would shine brighter and last longer.
   2. Using a stronger magnet: A stronger magnet would cause a greater change in the magnetic field, which creates a greater magnetic flux, again inducing a higher voltage, which allows the LED to shine brighter and last longer.
   3. Shaking the flashlight faster: Shaking the flashlight faster would also cause a quicker change in the magnetic field, which creates a greater magnetic flux, inducing a higher voltage and causing the LED to last longer and shine brighter.

Discussion

A shake flashlight, sold commercially under trade names such as Forever Flashlight^® and Eternity Flashlight^®, is a flashlight powered not by batteries, but by mechanical shaking. Other forms include crank and squeeze flashlights. All take advantage of the same principle—converting mechanical energy to electrical energy.

Three main parts of shake flashlights are required to allow them to function. The first is the “generator,” the power source, created by shaking the magnet through the wire coils. In 1831, Michael Faraday (1791–1867) discovered through careful experimentation that “any change in the magnetic environment of a coil of wire will cause a voltage to be ‘induced’ in the coil.” This is known as Faraday’s Law—a changing magnetic field produces a voltage called electromotive force, or emf (see Figure 8).

A mathematical description of Faraday’s Law is shown in Equation 2: where

emf = “Electromotive Force” or voltage (V)
N is the number of turns in a coil of wire
ΔΦ is the change in magnetic flux (webers)
Δt is the change in time

Magnetic flux (Φ) is defined as the average magnetic field multiplied by the perpendicular area it penetrates (Equation 3). where

B is the magnetic field
A is the perpendicular area

This is the essence behind how the generator used in this demonstration works. A changing magnetic environment—the magnet being shaken in the coil of wire—induces a voltage in the coils. When voltage is induced, electrons flow through the wire. The amount of voltage or emf induced depends on the strength of the magnetic field—which effects how much the field actually changes—the number of wire coils, and the speed at which the wire or magnets are moving.

The basic generator design has two drawbacks which are resolved in the circuit of the flashlight. First, it is impractical to have to continually shake a flashlight while using it. A way to store the charge and release it when desired is needed. This is the function of a capacitor. Capacitors receive and store charge, similar to batteries.

The second problem is both the capacitor and the LED used to light the flashlight accept only a uni-directional current—in other words, direct current (DC). Moreover, alternating current (AC), produced by the generator, is difficult to store.

The series of diodes neatly rectifies this. Diodes are devices that will only pass current in one direction. A single diode would ensure that only current going the desired direction would be allowed to pass (see Figure 9), but at the expense of losing all the energy of the current travelling in the opposite direction. The diode configuration however retains the entire signal (see Figure 9). With sufficient shaking, the capacitor will store a good deal of charge. When a complete circuit is made by flipping the switch, the capacitor will release that charge in a controlled manner. A shows the electricity provided from the generator as AC. B shows how the current can be changed to positive using a single diode which eliminates the negative. C shows using the diode configuration in this kit causes the entire signal to be positive.

The third part to the flashlight is the LED circuit. When the switch is closed, the LED and resistor are connected in series with the capacitor. This controls the rate of current flow from the capacitor, and prevents the LED from burning out. LEDs generally have a very small amount of maximum current they can handle and are usually attached in series with a resistor to ensure a longer life. Commercial flashlights usually include a reflector and a lens as well; the reflector ensures all the light from the LED is pointed forward, as with a flashlight, and the lens controls the size of the light beam.

References

Alden, E.; Kennedy, M; Lorenzon, W; Smith, W. “An Electromagnetic Induction Flashlight Experiment,” The Physics Teacher; American Association of Physics Teachers: College Park, MD; 2007; Vol. 45, pp 492–5.