1. I. NIGHTLIGHT ANALOG AND
DIGITAL CIRCUIT
Fig.1 Nightlight analog circuit Fig.2 Nightlight digital circuit
By Wagner Nunes

2. A. Light Depended Resistor (LDR)
Fig. 3 Example of LDR component
Fig. 4 Example of the LDR function
in the exponential decay graphic
Fig. 7 Example of an LDR when resistance
increases
Fig. 6 Example of an LDR when resistance
decreases
Fig.5 Example of the electrons in
the valence band jumping to the
conduction band
• The LDR is made of a semiconductor
material that is layered on an
insulating material such as ceramic.

3. B. The Comparator
• 1) The Logic:
– If (V-) > (V+) => Vout ≈ GND or “0” low (-saturation)
– If (V+) < (V-) => Vout ≈ +Vcc or “1” high (+saturation)
Fig. 8 Example of a comparator output “0” Fig. 9 Example of a comparator output “1”

4. C. Function of the NPN Bipolar Junction
Transistor (BJT) as a Switch
• NPN (N stands for negative charge and P
stands for positive charge or hole)
• BJT(bipolar junction transistor ) is because it
involves both of these types of charger.
Fig. 11 Example of a transistor tuning the LED onFig. 10 the three regions of a NPN transistor

5. 1) Cut - Off Region Conditions :
• Input base current IB= 0
• VBE < 0.7v
• Output collector current IC= 0
• Base – Collector junction BC is reversed biased
• Base – Emitter junction BE is reversed biased
• Maximum collector VCE= VCC
Fig. 12 Example of cut - off region conditions

6. 1) Saturation Region Conditions
• Input base current (IB) connected to VCC
• VBE > 0.7v
• Max collector current IC= VCC/RC
• Base – Collector junction BC is forward biased
• Base – Emitter junction BE is forward biased
• VCE= 0 (ideal saturation)
Fig. 13 Example of saturation region conditions

7. Calculation for the collector resistor ( ) , base
current ( ) , and base resistor( )
IC = 20mA(datasheet)
VCE (Sat ) = 0.2V(datasheet)
bDC (direct current gain) = 60(datasheet)
IC = bDc ´ IB
-VCC + IC RC +VLED +VCE(Sat) = 0
-6V + 20mA ´ RC + 3.4V + 0.2V = 0
-2.4V + 20mA ´ RC = 0
RC =
2.4V
20mA
=120W
IB =
IC
bDC
= 333mA
RB =
6V - 0.7V
333mA
=16KW
IB
RB
RC

8. D. LED (light emitting diode)
Fig. 12 Example of the barrier potential
Fig. 14 Example of current in forward direction
on a diode
Fig. 15 Energy released by the
electron of a single photon.
Fig.13 An LED will begin to emit light when the
on-voltage is exceeded.
Fig. 11 Example of a blue 5mm LED

9. Calculation for energy released by the
electron of a single photon.
h (Plank 's constant) = 6.626 ´10-34
J / s
C(Coulomb) = 6. 25´1018
e-
c (Speed of light) = 3´108
m / s
l (called lambda or wavelength)
u ( frequency)
eV (electron volts)
lu = c
u =
c
l
=
3´108
m / s
470 ´10-9
m
= 640 ´1012
s-1
E( photon) = hu = (6. 626 ´10-34
J / s)´ (640 ´1012
s-1
) = 423´110-21
J
E(eV ) = E( photon) ´1Coulomb = (423´10-21
J)´ (6.25´1018
e-
) = 2. 6eV

10. Analog Circuit Schematic

11. II. NIGHTLIGHT DIGITAL CIRCUIT VERSION –
BASIC OPERATION
• Arduino Function:
– There is no analog output on Arduino. Instead it is a
digital output (on or off).
– If the output is half of the time on and half of the time
off, it is said to be 50% duty cycle.
– Duty cycle is the percentage of time that the output is
on. The output can be 30%, 20%, 10% duty cycle, or
any percentage from 0 – 100%.
– The Arduino has a technic called pulse width
modulation (PMW) that makes its output work as
analog.

12. Arduino Function:
• In the nightlight device, it allows the LED to fade
in and out going from 0 to 255 (8 bits = as a
default resolution) value repeating in a loop.
• 0 correspond to 0 volts and 255 to 5 volts and any
value in between are proportional to those
voltages.
• Then theses values are sent to the PWM pin
(PWM Pins 3, 5, 6, 9, 10, 11) thus fading the LED
in and out.
28

13. Arduino
Function – Pulse Width Modulation
Fig.18 Example of pulse width modulation

14. 1) Formula for Duty Cycle:
DutyCycle=
TH
T
´100%
Voltage per step
(analogWrite( )) :
x=0 ® 255
xsteps
how many volts
xsteps
= x steps
5V
255 steps
=19.6mV per step
how many volts= x
5V
255
æ
è
ç
ö
ø
÷ =

15. E. Arduino
Function:
• This “function call” passes the read value
(argument) from the inputs (A0 to A5) to the
function in the library and its return an
integer, which is from 0 to 1023 (10 bits = )
as a default resolution).
• 0 corresponds to 0V and 1023 to 5V.
210

16. formula to calculate the voltages per
step on the input read
x = 0 ®1023
xsteps
how many volts
xsteps
= x steps
5V
1024 steps
= 4.88mV per step
how many volts= x
5V
1024
æ
è
ç
ö
ø
÷ =

17. Appendix C
Flowchart for the Digital Circuit

18. III. NIGHTLIGHT ANALOG CIRCUIT VERSION VS.
NIGHTLIGHT DIGITAL CIRCUIT VERSION
NIGHTLIGHT ANALOG CIRCUIT VERSION NIGHTLIGHT DIGITAL CIRCUIT VERSION
I spent more time with the digital circuit
than the analog one because of my
familiarity with it.
I improved my lack of skills with Arduino
the analog circuit required more space on
the board than the digital one due to
more parts used.

19. V. COST TO BUILD AND OPERATE
NIGHTLIGHT ANALOG CIRCUIT NIGHTLIGHT DIGITAL CIRCUIT
costs about $5.70 costs about $26.38, approximately five
times more (because of the Arduino )
+ Arduino can be used for other
applications because it is programmable.
Running on 6V (Maui Electric Company), 8
hours per day, the cost is $1.20 per
month.
Running on 6V battery, 8 hours per day,
the cost is $11 per month.

20. Conclusion
• As a result, this project spiked my interest
even more in electronics hardware, software,
and more about technology in general.
Consequently, after my associate degree in
the ECET, my next academic goal is to earn a
bachelor’s degree, become an electronic
computer engineer, and perhaps to earn a
master degree so I can teach electronics at a
university.