rwep final4 14 projectreportsolution v5

8/13/2019 RWEP Final4 14 ProjectReportSolution v5

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Sample Student Project Report

Unit 1: “Let There Be Light”

Problem Definition: The purpose of this unit is to explore the effects Ohm’s Law. This law is

qualitatively verified by observing the brightness of an LED used in the test circuit.

Methods: To verify that Ohm’s Law holds, we connected the circuit from the lab assignment,varying the resistance from 100-500 in 100 steps. We used two AA cells and a red LED

(Lite-On LTL-2R3VRKNT; Digikey 160-1498-ND): 631nm, clear lens, 30 degree viewingangle, 1.9V forward voltage drop, 990 mcd of light, and 50 mA max current.

Results: The following table outlines the data gathered. R nom is the expected value of theresistance and R actual is the actual resistance measured using a DMM. Vresistor is the voltage

across the resistor, Vled is the voltage across the LED, VR+L is the sum of the resistor and LEDvoltages, and V bat is the voltage across the battery. I is the calculated current through the circuit

and is determined using Ohm’s law: I = Vresistor /R actual.

Rnom

()

Ractual

()

Vresistor

(V)

Vled

(V)

VR+L

(V)

Vbat (V) I

(mA)

Brightness

100 98.4 1.30 1.81 3.11 3.13 13.2 Brightest200 196.7 1.36 1.78 3.14 3.15 6.91 Quite Bright

300 294.9 1.39 1.76 3.15 3.15 4.71 Medium400 392 1.40 1.75 3.15 3.16 3.57 Quite Dim

500 491 1.42 1.74 3.16 3.16 2.89 Dimmest

Conclusions: An interesting thing to note here is that the labelled values on components

(batteries and resistors) are not exact. The nominal 100 resistor, for instance, measures 98.4.

There may be some situations where deviation from nominal values will affect performance.

Vled is mostly constant for all circuit currents. However, as the current increases, the voltage

increases. This is evidence that the “corner” model of operation, although useful for quickcalculations, is not as accurate as the “squares” model where the LED does not turn on sharply.

The Vled value is relatively close to the manufacturer’s specification of Vfwd . The sum of thevoltage across the LED and the resistor is approximately the battery voltage. This indicates that

the sum of the voltage drops in this circuit equals the battery voltage. A decrease in the current inthe circuit results in a dimmer LED. This observed dimming supports the calculation that the

circuit current decreases as the resistance increases. To remain below the maximum ratedcurrent of 50mA specified for this LED device, a minimum circuit resistance of

==

= 22

050.09.10.3

max

min

I V V R fwd bat

is required when the battery voltage is 3.0V. For a battery voltage of 6.0V, the minimumresistance is 82. Note that these minimum values are derived from ideal (nominal) values,

including the battery voltage, and the LED forward voltage drop using the “corner” model. Assuch, these minimum resistance values are to be used only as a guideline: actual circuits should

exceed these values in order to accommodate more accurate models and measures.



Unit 2: “How Bright”

Problem Definition: The purpose of this unit is to explore how we can use a photo-sensitiveresistor (photoresistor) to measure the amount of light that an LED emits. This allows us to

numerically compare different LEDs. We also confirm the “Inverse Squares Law” which states

that the intensity of light drops to

when the distance of measurement (perception) is doubled.

Methods: To make the measurements in this experiment, we mount a CdS photocell on the end

of a cardboard tube, minimizing ambient light. A circuit identical to that outlined in Unit 1 isconstructed. We “sweep” an LED’s current using different resistor values, as in Unit 1. At each

step we determine the current flowing to the LED, and measure the photoresistor’s resistance.Using the manufacturer’s information about the response of the CdS cell (mounted at 10cm from

the base of the tube), we can convert this resistance into illuminance (Lux). To verify the“Inverse Square Law” the trials are repeated for two LEDs (white and red) and a 28 cm tube.

Results: Data is collected for all 18 LEDs with the sensor mounted in the 10cm tube. A summary

plot shows how Lux varies with LED current

1

. The legend names the Digikey part numbers.Experimentally-Determined Illuminance vs. LED Current

(Using sensitivity = -0.7 Ohm/Lux, Rcds@10Lux=15K, 10 cm sensor distance)

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35 40 45

LED Current [m A]

I l l u m i n a n c e [ L u x ]

160-1772-ND

67-1696-ND

160-1728-5-

ND160-1498-ND

160-1713-ND

160-1087-ND

160-1669-ND

160-1130-ND

160-1089-ND

160-1702-ND

160-1677-ND

67-1097-ND

160-1503-ND

160-1496-ND

P600-ND

R,990,30,C

R,1700,15,C

W,4200,35,C

A,1300,30,C

A,7800,8,C

W,13000,15,C

W,680,34,D

B,520,30,C

B,1900,8,D

B,300,60,D

Color [R=red,G=green,B=blue,A=amber],power

[mcd],

lense angle [degrees], lense opacity

[C=clear,D=diffused]

Data was then collected for two of the same LEDs, but with the photoresistor mounted in the28cm tube. The following plot summarizes the Illuminance vs. Current data gathered and showstrials at both 10cm and 28cm sensing distances. It should be noted that it was necessary to

increase the circuit’s battery voltage for, in particular, the blue and white LEDs due to theirhigher forward voltage drop. This was achieved by placing two AA battery packs in series.

1 Information used to create these plots is at a much higher resolution than the students could be expected to achieve

in the amount of time allocated for this Unit.



Experimentally-Determined Illuminance vs. LED Current for d = 10 and 28 cm

(Using sensit ivi ty = -0.7 Ohm/Lux, Rcds@10Lux=15K)

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35 40 45

LED Current [m A]

I l l u m i n a n c e [ L u x ]

160-1728-5-ND

(10cm)

160-1728-5-ND(28cm)

160-1498-ND(10cm)

160-1498-ND

(28cm)

White, 28cm

White, 10cm

Red, 10cm

Red, 28cm

Conclusions: In all trials, an increase in a circuit’s current increases the Illuminance and the perceived brightness. This means that as current increases, more photons are emitted from an

LED. Indeed, as the brightness increases, the photocell’s resistance decreases resulting in alarger Lux measurement (according the manufacturer’s specifications).

Drawing comparison between the curves is a little difficult. On first impression, from the data

we gathered it appears that the red LEDs emit more light than the others. This could, however, be related to the wavelength sensitivity of the CdS cell. For the most part, LEDs with higher

millicandela ratings are higher on the plots, indicating that more light is emitted by these devicesat a given current. However, there are some discrepancies. Viewing angle has some effect on

the measurements. For narrow angles, the light might not be focused on the photoresistor,yielding skewed results. It is important to note, therefore, that the mounting of both the sensorand the LED is important for direct LED comparison, requiring a more advanced setup.

However, with the same LED the illuminance-current relationship is clearly visible.

Regarding readings for LEDs at two distances (10 and 28cm), illuminances are much lowerwhen the sensor is mounted further away. The “Inverse Square Law” states:

2

22

2

11 d Ld L =

where L1 is the illuminance at distance 1, d 1. Using sample data gathered in our experiment at adistance of 10cm and a current of approximately 40 mA, we expect illuminances at d 2 (28 cm):

Red LED White LED

using L1 Lux @ 10cm, 40mA ( )2

2

2

28.0

1.0512= L

2

2

2

28.0

1.0166= L

calculated L2 Lux @ 10cm, 40mA 65.3 21.2measured L2 Lux @ 10cm, 40mA 110 76

Error (Lux) (measured-calculated) 44.7 54.8% Error 68% 258%



These errors indicate that the “Inverse Square Law” does not hold very well. One reason is thatthe “Inverse Square Law” requires a “point” source. This is not the case with the LEDs. The

viewing of the Red and White LEDs is 30° and 15°, respectively. Since an LED with a widerviewing angle is closer to a point source, this explains why the lower Red LED error. Secondly,

the Law assumes no reflections, which is not the case with the cardboard tubes.

In conclusion, we used a photoresistor to to measure the LED illuminance. These measurementsare sensitive to wavelength differences and mounting method, but the yield good relative

measurements. We have confirmed that increasing LED current yields more light. However, photoresistors do not indicate reading light quality. Personal observations indicate that while the

white and blue LEDs seem to provide what we consider the “brightest” light, it seems cold andharsh. This is consistent with how human’s perceive red as a warm color, and blue as cold.

Unit 3: “The Flame that Burns Twice as Bright…”

Problem Definition: The purpose of this unit is to demonstrate that the expected useful life of a

charged battery is directly related to the amount of current drawn from the battery.

Methods: In this unit, we constructed two circuits: one that draws a relatively large amount ofcurrent, and another that draws substantially less. We recorded the battery voltage as it decays

with respect to time for both circuits. Additionally, we obtained information about the amount oflight being emitted by the LEDs connected in the circuit that draws less current. These tests

were conducted for a two-hour period. The Loaded circuit: this circuit was constructed with 9red Lite-On LTL-2R3VRKNT (Digikey 160-1498-ND) and 6 amber Lite-On LTL-2R3VYKNT

(Digikey160-1503-ND) LEDs connected in parallel to a 3V battery source. Each LED has itsown 50 current-limiting resistor. The nominal current drawn totals 312 mA, as calculated

using the forward LED voltages for each of the devices. The Lightly Loaded circuit: this circuituses one of each of the devices used in the loaded circuit. The total nominal current drawn from

the battery is 42 mA, approximately 1/10th

of that drawn from the loaded battery. It is from thesetwo LEDs that the light-level is read.

Results: The gathered data is plotted in the following figure.

Decay of Alkali ne Battery Voltage and Emitted Light

with respect to Time (120 minutes)

2.4

2.5

2.6

2.7

2.8

2.9

3

3.1

3.2

3.3

0 20 40 60 80 100 120

Time (minutes)

B a t t e r y V o l t a

g e ( V )

0

50

100

150

200

250

I l l u m i n o s i t y

( L u x )

Loaded Vbat (I = 312mA)Lightly Loaded Vbat (I = 42mA)

Illuminosity (Lightly Loaded)



Conclusions: It can be seen that the all the waveforms are decaying exponentially with respect totime. The illuminosity decreases as the voltage drops since the current flowing through the

LEDs is decreased. This is as expected. The time constant for each of the two circuits (Loadedand Lightly Loaded) is determined from the data gathered, recalling that

( ) ( ) dc

t

dc V eV V t V +=

0

and solving for :

( )

=

dc

dc

V t V

V V t

0ln .

Parameter Loaded Battery Lightly Loaded Battery

V0 2.953 V 3.065 V

Vdc (highest of LEDvoltages)

2.0 V 2.0 V

V @ t = 120 minutes 2.493 V 2.881 V 182 minutes 633 minutes

The battery voltages at 120 minutes, as shown in the table, are used in the calculation of the time

constant. Since the battery voltage decay is exponential, it will theoretically never reach a lowenough value that an LED will not be turned on, at least to a degree. The duration of light useful

for reading, however can be determined by observing the minimum amount of current requiredfor adequate reading light, calculating the voltage required for this current to flow, and using the

time constant of the lightly loaded battery circuit (assumed to be similar to the final design) todetermine the time at which this voltage occurs. This approach is not outlined here.

A discussion about the calculated time constants as they relate to the current drawn from the

circuit could be carried out. One would expect to see the time constant of the lightly-loaded

battery higher than shown: with approximately 1/10

th

of current flow, the time constant isexpected to be roughly 10 times that of the loaded battery voltage waveform. It is expected thatthe ratio of approximately 3.5 experienced here arises from using slightly discharged batteries

from a different manufacturer in the lightly loaded circuit.

It is truly startling how much light can be generated by the LEDs when connected to a small battery pack. This qualitative observation, coupled with the time constant calculations, confirm

that the batteries used in this experiment are more than sufficient for two hours of light

generation for a reading task. However, there are other practical limits to the amount of light

that can be generated: cost, the nature of how batteries are recharged (although these tests usednon-rechargeable alkaline batteries), etc.

Unit 4: Final Project: “It Keeps Going, and Going, and Going…”

This section describes the design of the system that we have selected as the final “product”. We

chose a two-LED system: one diffused red LED (Lite-On LTL-10223W, Digikey 160-187-ND)and one clear blue LED (Lite-On LTL-353TBK, Digikey 160-1716-ND). Our rationale for this

selection is that the closely-focused (8 degree) harsher light of the blue led is important fortypical reading work, but the red led provides a wider (60 degree) focus that provides some



warmth to the color. This wider focus also allows the user to better see objects outside the areaof higher-intensity light, allowing them to move the unit to refocus the higher-intensity light as

necessary.

For packaging, we used the battery casing removed from a dollar-store toy. This is an

inexpensive solution that allows the batteries to be easily removed for recharging. Removing the batteries is also how we propose the user turns the device off. The LEDs and resistors weresoldered directly to the battery holder terminals and then taped down to hold them in place. A

clear plastic cover, also found from an inexpensive toy, was placed over top of the LEDs,allowing them to shine through. The cover is held in place with electrical tape, but silicon

sealing could be used to improve water resistance. Photographs of our packaging solution andthe resulting light are shown here.

To test the product, we should issue it to an actual user for them to “put it through the paces”.This feedback could then be used to improve our design. However, we instead used the product

ourselves. Our approach is non-destructive, so we did not drop the device or put it throughdestructive tests. We expect, however, that in mild climates the plastic case and circuitry will

perform well – the plastic remains somewhat pliable preventing cracking from moderate drops.Recharging should be as simple as removing the batteries from the housing and inserting into the

charger. Repairs in the event of breakage can be achieved by the use of tape, a soldering iron,and solder, assuming the problem is not the failure of a component itself.

There are some weaknesses with the product, however. One weakness we noted is that the

plastic door on the battery housing uses a plastic hinge. We expect that after a relatively lownumber of uses, the hinge will need to be repaired. This warrants further analysis, particularly in

light of the fact that removal of the batteries is how we propose the user turns the device off.

In the end, we feel that we have designed a reasonable prototype for a system to help with the problems of night-time reading in underdeveloped countries. The light, although not as nice as

reading with other light sources, is sufficient. We are pleased that the lifetime of the unit

between charging dramatically exceeds the stated objective of 2 hours.

rwep final4 14 projectreportsolution v5

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