Build an Electronic Night Light
Electrical and Computer Engineering, Northeastern University
What Do Electrical Engineers
Do?
• Design computers, cell phones, MP3
players, plasma TVs, electrical power
systems, satellite communications,
radar, GPS…
About
how many electrons
are in a short wire?
• We make electrons do useful things.
>1020!
Making electrons move
-
-
+
-
- -
1.5 volts
-
-
-
- -
-
-
-
-
-
-
-
Voltage: determines how hard
the electrons are pushed.
120 volts
170 volts peak!
9 volts
Parts List
• Resistors
• 1,000 ohms:
• 10,000 ohms:
• 100,000 ohms:
• Light emitting diode (LED)
• Photoresistor
• Transistor
2N
3904
The Power Supply and Protoboard
set to 10 volts
Horizontal rows of 5
are connected
V-
V+
All holes in each
vertical column are
connected
switch the
meter to
measure
current
V
Vresistor
(ohms, W)
1,000 W
10,000 W
100,000 W
LED
brightness
mA
Circuit #1
V+
current
(mA)
The Results:
resistor
1,000W
LED
brightness
current
(mA)
bright
8 mA
The Schematic
- +
10 volts
10,000W
dim
0.8 mA
Resistor
100,000W
almost
out
0.08 mA
Current: tells us how many electrons pass
through the circuit every second.
Resistor: resists the flow of current…like
putting a kink in a garden hose.
LED
Our night light needs
a device that
can “see” light
Photoresistor
photons
more light 
more electrons 
lower resistance
Circuit #2
more light…
lower resistance…
brighter LED…
backwards!
Tricking the electrons
Circuit #3 
How does circuit #3 work?
electrons take the path of least resistance
- +
- +
-
photoresistor: low R
LED
photoresistor: high R
10,000 ohms
LED
10,000 ohms
Actually, the current is in the direction of positive charges
…thank you, Ben Franklin!
- +
- +
+
+
photoresistor: low R
LED
photoresistor: high R
10,000 ohms
LED
10,000 ohms
We need amplification!
…the LED is too weak…
The Transistor
V+
large
current
small
current
large
current
Circuit #4: the night light
1. the transistor has a flat face
2. the 1,000 ohm resistor is on top
3. the 100,000 ohm resistor is on the bottom
4. don’t forget the wire!
100k
wire!
How does this circuit work?
remember, electrons take the path of least resistance
- +
LIGHTS ON
photoresistor: low R
+
100,000 ohms
Transistor
1,000 ohms
LED
How does this circuit work?
remember, electrons take the path of least resistance
- +
LIGHTS OFF
photoresistor: high R
+
100,000 ohms
+
Transistor
1,000 ohms
LED
QUESTIONS?
Supplemental
Materials
Electronic Night Light Parts List
1000W RESISTOR (5%)
(brown-black-red-gold)
10,000 W RESISTOR (5%)
(brown-black-orange-gold)
100,000 W RESISTOR (5%)
(brown-black-yellow-gold)
CdS Photoresistor (note: the 100k resistor may need to be replaced with a larger or smaller value depending
on the characteristics of the CdS photo resistor available.)
Light emitting diodes
(red, yellow, and/or green)
Transistor
(Part no. 2N3904)
1 – Protoboard; 1-Power supply or 9v battery; hook-up wire and wire cutters
Written by J. Hopwood
ECE Department,
Northeastern University
Title page photo credit:
Prof. N. McGruer
This lab is intended for
college freshmen or high
school students considering a
career in engineering. It was
developed for the “Building
Bridges” program of
Northeastern University,
Boston, MA with support from
the National Science
Foundation.