Shaker Flashlight - University of Michigan



SHAKER FLASHLIGHT

EXPLORATION

Materials

Shaker Flashlight board

Hand generator

Alligator leads

Tape

The shaker flashlight set-up contains all the electronic components included in a commercially sold shaker flashlight. There are only 5 components in the set-up:

• 1 hand generator: converts mechanical energy into electrical energy

On the Board:

• 1 LED (light-emitting diode): generates light

• 1 capacitor: stores electrical energy

• 1 magnetic reed switch: switch to control current flow to LED

• 4 diodes

The electrical energy that powers the flashlight is not going to be converted from chemical energy (as it is in battery flashlights). It is instead going to be converted from the mechanical energy of shaking the flashlight. The advantage of this is it will work without batteries even after a long period in storage.

Magnetic Reed Switch

While you may not have heard of a magnetic reed switch, you are familiar with its essential function: it is a normally open relay. When a magnet is brought near the switch, it closes the switch, as shown in Fig. 1.

[pic]

Figure 1: Magnetic Reed Switch. Left: open switch.

Right: closed switch in presence of magnet.

Diodes

You already know about four of the five components: the diodes are a new component that has not yet been discussed. In this section you will experiment with how a diode behaves in a circuit and eventually use diodes to optimize the current generated by the hand generator.

1. Build the hand generator by mounting the coil on center of the magnet tube. Secure the coil with the tape.

Connect the hand generator’s alligator leads across the LED. Shake the flashlight and observe. Discuss with your group and record an explanation of the LED’s behavior.

2. Now connect the LED, one of the four plain diodes, and the hand generator in series as shown in the diagram below. Shake the hand generator and observe.

[pic]

Figure 2: LED and diode circuit, version 1.

NOTE: Direction of current is important!

Shake the flashlight. Compare what you observe to the original circuit.

3. Reverse the direction of the diode in the circuit (switch + to – and vice versa). Shake the hand generator again and observe what happens. Compare what you observe to the original circuit.

4. Describe the current produced by just the hand generator. Draw a current vs. time diagram.

Describe the behavior of the diodes (LED and diode) on the circuit.

[pic]Everyday Applications

• AC adapters for electronics (almost all include a full-wave bridge rectifier to convert AC to DC).

APPLICATION

Materials

Shaker Flashlight board

Hand generator components

Alligator leads

Rare earth magnet

Multimeter

We can optimize the flashlight to improve on the first circuit you built in the exploration phase. The original circuit has three main flaws: it strobes, it wastes energy, and the operator must be shaking it while it is operating. Flickering, shaking light would be aggravating to anyone trying to use the flashlight as a tool. Wasting energy happened because only using one diode (the LED) blocks half of the generated current (from the AC source). Wasted energy in this design will exhaust the flashlight user because of all the work the user must do to shake the flashlight.

1. Build a full-wave bridge rectifier with the 4 diodes that are provided. This will convert an AC signal into a DC signal. Present your rectifier to the GSI or instructor before moving forward. Tip: number the diagram and your board, and then match the numbers to get the correct connections.

[pic]

Figure 3: Full-wave bridge rectifier

2. Use the rectifier to optimize the current produced by the hand generator. Connect the hand generator across the LED directly. Note: current direction matters! Make sure the rectified output is correctly attached to the LED.

First predict which, if any, of the flaws using the rectifier will solve?

Shake the hand generator and observe: were any of the flaws solved? Explain.

Using the rectifier protected us from wasting energy; all of the energy we produced on the hand generator with our mechanical energy is going into the circuit. However, the flashlight still flickers and shakes while operating. We’ve only solved 1 of the 3 flaws.

Instead of using the rectified generator to directly power the LED, we will use it to charge the capacitor. The rectified generator is an unsteady source; it oscillates from 0 to a high voltage frequently, which leads to a strobe effect. A charged capacitor is a steady source once charged to a certain voltage. The capacitor discharges in a non-oscillating stream, even if it was charge by an oscillating source (like our hand generator). Capacitors will also store charge for a reasonable length of time.

3. Which flaws will be solved (and why) if we use a capacitor in the circuit?

4. Measure the voltage of the capacitor with the multimeter set to VDC. It should read nearly 0 volts. If not, short out the capacitor with an alligator lead until the voltage reads 0.

Build the circuit shown in figure 3. Charge the capacitor with the rectified hand generator to 3V (when the generator is at rest). Use the multimeter set to VDC across the capacitor. It should read 3V when the generator is at rest.

Note: Orientation matters! The capacitor must be charged in the correct current direction shown below. Take turns charging the capacitor, it takes time.

Figure 3: Capacitor charging circuit

5. Once the capacitor is charged, build the circuit shown below (leave the rectifier circuit intact). Notice the orientation of the capacitor and the LED. Bring the rare earth magnet near the reed switch. Recharge the flashlight as needed.

Figure 4: Flashlight circuit.

6. If you were engineering this flashlight for retail, describe other improvements you would make to the design and why.

Challenge Work:

1. Describe in your own words the type of current that the hand generator produces.

2. Describe in your own words what the diode rectifier does to the current produced by the hand generator.

3. The time you spend charging the capacitor is always less than the time the LED is lit with this arrangement. What components would you optimize to yield a more efficient charging to running ratio?

Summary

Final Clean-up

Please disconnect all alligator leads and reattach them to the clip card. Return all equipment to the carts.

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