What goes Bump in the Night??
Nothing...after we build our own night light!
(We will set up power supply with banana plugged jumpers coming out of the power supply – they will
need to adjust and connect)
Materials &
Equipment
LDR – Light Detecting Resistor Red LEDs, Jumpers, Wire, Wire Cutters, Breadboard
Op-Amp – LM741 – Operational Amplifier, resistors.
Digital Multimeter (Metex Universal System MS-9150)
Dual DC Power Supply (Shenzhen Mastech DC Power Supply HY3003-3)
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Lyles College of Engineering, California State University, Fresno
Introduction
Sensors allow us to interact with the world around us and use the measured physical quantity to our
benefit (i.e., digital thermometers, touch screen, voice activated calls, etc.).
Today, we will use a sensor that detects light intensity to build a circuit that mimics the night light (i.e.,
once it gets dark in the room the light will automatically turn on and if the room is well lit the light will
go off. The complete circuit that we are going to build is shown in Figure 1 below.
Figure 1. Circuit diagram that uses light dependant resistor (sensor) to control the light in the room.
Before we attempt to construct the circuit in Figure 1, we should first learn about the basic building
blocks that allow us to sense the light and turn on a lamp (or in our case a light emitting diode).
Circuit Components:
1.
Resistor –
a. The night light circuit contains in total five resistors. The symbol for the resistor and the
actual component are shown below. Your kit should contain two resistors.
Symbol
2
Component
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b. The purpose of a resistor is to control the current flow. It turns out that larger the resistance
less current will flow and vice versa.
c. If you take a look at two resistors that came in your kit, you will notice that they have four
distinct color bands. The first three color bands determine the value of the resistor and the
fourth band (which can be either silver or gold) determines the tolerance of the resistor.
d. In your case, the two resistors will have the same value because both have the same color
code. The YELLOW-PURPLE-BROWN color band corresponds to a 470 Ohm resistor and the
gold band corresponds to a 5% tolerance. This means that given a box full of 470 Ohm
resistors, if you randomly pick one the resistor can range anywhere from 446.5 Ohms to
493.5 Ohms.
e. Another way to find a resistor value is to use a Digital Multimeter with the settings set to
Ohms (Ω).
2.
Light Dependent Resistor (LDR) –
has the ability to change the resistance as the light
intensity changes (i.e., it is sensing the light). Once part of a complete circuit, the change in the
resistance will cause the change in the current flow through the circuit. The change in current flow
will allow us to make an action of turning the LIGHT ON/OFF.
a. The circuit symbol for the LDR and the actual component are shown below. (Locate the LDR
in your kit.)
LDR symbol
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Component
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3.
Operational Amplifier (OPAMP) –
is the device that can do mathematical operations (add,
subtract, multiply, etc.) with electrical signals (currents and voltages). Besides math operations, the
amplifier can be also used as a comparator. The circuit symbol and the actual component are shown
below. (Locate the OPAMP in you kit.)
Circuit symbol
Component
a. The OPAMP that we will use is LM741. The LM741 amplifier is contained in an 8-pin
package. The dot in the left hand corner designates pin 1. (Locate pin 1 on your LM741
amplifier.)
b. Pins that we are going to use are pins 2 and 3 (input pins), pins 4 and 7 (power supply
pins), and pin 6 (the output pin).
c. IMPORTANT! – Pins 4 and 7 are important pins. The amplifier is useless unless it is
powered up. Pin 4 connects to a negative DC power supply and Pin 7 connects to a
positive DC power supply.
4.
Light Emitting Diode (LED) – is going to serve as a light source for our night light circuit. In
our circuit we will use two LEDs, yellow and red. The yellow LED will be on during the day time (light
intensity is high) and the red LED will turn on when the light intensity drops.
a. The circuit symbol for an LED and the actual LED are shown below.
+
-
Circuit symbol
4
+
-
Component
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b. Locate the two LEDs in your kit. You will notice that on lead is longer than the other. This
means the placement of an LED in your circuit is important.
c. The +/- symbol placements establish the connection between the symbol and the
component. This will help you correctly place an LED in your circuit.
Finally we are done with the introduction…Let’s build some circuits!!!
Test the LDR’s
1. Set Digital Multimeter to read  (Ohms).
2. Use banana clip wires to connect one side of the LDR to the black lead of the multimeter
and the other side of the LDR to the red lead of the multimeter. There is no directional
preference here.
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Units
3. Record the resistance (pay attention to the units… ohms (), kilo-ohms (k), or megaohms(M) ) on your work sheet with the LDR uncovered.
4. Cover the LDR with your hand. Record the resistance (again – pay attention to the
units!) on your worksheet.
What happened to the resistance? How did it change? Did it go up or
down as you covered the LDR with your hand?
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The Breadboard
1. In order to build our night light we need a blank canvas. Our blank canvas is a
breadboard – this is where we create circuits.
2. Figure X below shows a breadboard. The white plastic board has a number of holes or
sockets in it.
Bus Strips
3. In this laboratory we will be plugging components (resistors, Op AMP, LDR) and wires
into those sockets. The components will be connected electrically.
4. For the electrical connections to occur, we have to follow some rules when using a
breadboard.
5. The breadboard has numerous rows of sockets separated into two columns by a trough.
The trough splits each row of the sockets into two rows of X. Each row of X sockets is
electrically connected inside the breadboard. You can use these rows of sockets to
connect components together as shown in circuit schematics.
6. The rows are not electrically connected to each other. If you have a component that is
plugged into one row it is electrically isolated from the component that is connected on
the next row.
7. On each side of the breadboard are two columns of sockets. These are called bus strips.
The sockets in one column are all electrically connected. The columns are not
electrically connected to each other. In the above Figure, the blocks of sockets are
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separated from each other by a blank space. The gap in the marking indicates that the
top set and bottom set are electrically isolated from each other.
Can you Light up
an LED??
Equipment/Materials:





5 Volt DC Power Supply
Digital multimeter
Breadboard
470 Ohm resistor
Red LED
Procedure:
1. We are going to build the following circuit.
2. You know how to measure resistance!
3. Remember the +/- leads of the Red LEDs.
4. Your kit should contain two 470 Ohm resistors (the band colors are YELLOW-PURPLEBROWN). For this circuit you will need to use only one.
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5. Just to confirm that you are in possession of 470 Ohm resistor, measure the resistance
using Digital Mulimeter with knob set to Ω (Ohms). Document the measured resistance
on your work sheet.
6. To construct the circuit shown in Step 1 we will need 5 Volts.
7. For 5 Volts we will use a DC power supply – you can think of this as a variable battery.
We are going to use the fixed 5V output on the right side of the power supply.
8. Turn the power supply on by pressing the Orange Power Button.
9. Measure the output of the power supply using a Digital Multimeter.
a. For voltage measurement we need to adjust the knob to Volts.
Volts
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b. Take the multimeter leads and place the red lead on the red side of the power
supply and the black lead on the black side of the power supply. The voltage
should read 5 Volts.
Side Note:
Your cell phone requires
a battery. (how many v
is this?) Which means
that all the components
inside the cell phone are
powered by that XX volts
battery. Not every
10. Connect the circuit as shown in the photos below.
component takes away
a. Step 1 – place a resistor and red LED (pay attention to +/-). The longer lead
the same fraction of the
should connect to a resistor as shown in this figure.
XX voltage. Some will
require more and some
will require less ,
however it turns out
that those components
that complete a loop
(similar to our loop with
the LED and resistor –
where there is no break
in the circuit – we go
from a terminal and end
b. Step 2 – Connect +5 (the red terminal of the power supply) to the resistor.
up in a terminal) will add
Nothing should happen – the circuit is not complete.
up to XX voltage.
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c. Step 3 – Finish the loop – Connect the black terminal of the 5V power supply to
the shorter side of the LED. You should see the LED light.
Where did the 5 Volts go? Who used the 5 Volts? Measure the voltage
across your resistor and LED and see if they add up to 5 Volts!!
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Lyles College of Engineering, California State University, Fresno
Steps to Make the Night Light –
The night light circuit diagram from Figure 1 is repeated below in Figure 2. The night light circuit
performs three distinct operations. The first block (SENSOR CIRCUIT) senses the light intensity in the
room. The left branch of the SENSOR CIRCUIT serves as a reference branch and it never changes, while
the right branch of the circuit contains the light dependant resistor (LDR) and therefore it will change
depending on the light levels in the room. The second block (COMPARATOR) compares the right branch
of the sensor circuit to the left branch and makes a decision to turn the RED LED ON if the current flow
through the LDR is less than the current flow through the reference branch. On the other hand if the
current flow through LDR is greater than the current flow through the reference branch, the YELLOW
LED will stay LIT. You are already familiar with the third block (LIGHT). Because the orientation of the
LEDs is not the same (and depending on the output of the comparator) the light block will have only one
LED lit at the time.
Figure 2. Night light circuit diagram.
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Shown is the night light circuit constructed on the breadboard. This is what we are going for…
Light
Comparator
Sensor Circuit
Let’s build the SENSOR CIRCUIT –
1. You kit already contains an LDR. What is missing in your kit are the three resistors that complete the
sensor circuit. The two resistors labeled RX will have the same value. The equation for finding the
value of resistor RX is
2.
3.
4.
5.
6.
7.
𝑅𝑥 = √(𝑅𝑠𝑒𝑛𝑠𝑜𝑟(𝑙𝑖𝑔ℎ𝑡) ∗ 𝑅𝑠𝑒𝑛𝑠𝑜𝑟(𝑑𝑎𝑟𝑘)
By this point on your worksheet you should have resistor values for Rsensor(light) and
Rsensor(dark). Using the calculator and the above equation find the value of RX.
Record your result on the worksheet.
From the components cabinet select two resistors which have resistor values that are closest to
your calculated resistance RX.
Use Digital Multimeter (with the knob set to Ohms) to measure the actual resistance of the select
RX.
Record measured resistances on your worksheet.
For resistor R1 use the following equation
𝑅1 = 𝑅𝑠𝑒𝑛𝑠𝑜𝑟(𝑙𝑖𝑔ℎ𝑡) + 500 Ohms
8. Record your result for R1 on the worksheet.
9. From the components cabinet select a single resistor that will serve as R1 and has the value that is
closest to your calculated resistance.
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10. Use Digital Multimeter (with the knob set to Ohms) to measure the actual resistance of the selected
R1.
11. Record measured resistance on your worksheet.
12. Now you have all the part to construct the SENSOR CIRCUIT.
13. Construct the following circuit using the LDR provided in the kit and resistors obtained in Step 4 and
Step 9. (IMPORTANT! The color band for your RX and R1 resistors may be different than what is
shown in the figure below.)
Rx
R1
Rx
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Let’s build the COMPARATOR circuit –
1. The LM741 operational amplifier will serve as a comparator. You should already
have one in your kit.
2. Construct the circuit shown below. Make sure to place your LM741 opamp correctly
(pay attention where Pin 1 is located).
3. The opamp Pins 2 and 3 will serve as the comparator input.
4. The opamp Pin 6 will serve as the comparator output.
5. The opamp Pins 4 and 7 will be supplied -10 V and +10 V, respectively.
2
4
3
Circle designating Pin 1.
6
7
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6. In the circuit below, the connection between SENSOR CIRCUIT and COMPARATOR is made
by red and orange wires. Once you complete this step, have lab instructor double check
your wiring.
Let’s build the LIGHT circuit –
1. You have already built the circuit in the LIGHT section of the night light in the ‘Can
you light up an LED’ section of these instructions.
2. This time is slightly different. Pay close attention to the + and – sides of the LEDs as
you construct the below circuit. The resistors are both 470 Ohm in value.
3. One side of the LEDs is connected to ground and the other side is connected to the
470 Ohm resistors. Use the terminal buses at the side of the breadboard to
connect to a common ground.
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Let’s complete this circuit!
Complete circuit – before we proceed with powering up our night light, have lab instructor
check your complete circuit.
GROUND
- 10V
VVVV
+ 10V
V
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1. Adjust the Voltage knob on the right side (MASTER) of the power supply to 10 Volts.
Repeat and adjust the voltage knob on the left side (SLAVE) of the power supply to 10
Volts. The display on both sides of the power supply should be 10.
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2. The right side of the power supply is the positive voltage. The left side of the power
supply is the negative voltage.
8. Measure the right side of the power supply.
a. Multimeter lead Red connect to the Red of the power supply.
b. Multimeter lead black connect to the green of the power supply.
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9. Measure the left side of the power supply.
a. Connect the red lead to the yellow of the power supply.
b. Connect the black lead to the green of the power supply.
10. In both cases, document the voltage readings.
11. Connect the yellow alligator clip to -10 V; connect the green alligator clip to ground;
connect the red alligator clip to +10 V.
- 10V
VVVV
GROUND
+ 10V
V
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12. If everything is connected correctly a yellow LED will light up. (If the yellow LED does not
light up have lab instructor look at your circuit).
13. If you cover the LDR (or if we turn the lights off in the room) the red LED will light up.
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Girls SEE 2010
Turn on a Fan!
Equipment List:
Temperature Sensor – LM35
NPN222A - Transistor
Op-Amp – LM741 – Operational Amplifier
LEDs
DC Fan
Multimeter
Dual DC Power Supply
Resistors
Jumpers
Wire Cutters
Wire
Breadboard
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Girls SEE 2010
Convert Celsius to Farenheit
Equipment List:
Temperature Sensor – LM35
NPN222A - Transistor
Op-Amp – LM741 – Operational Amplifier
LEDs
Multimeter
Dual DC Power Supply
Resistors
Jumpers
Wire Cutters
Wire
Breadboard
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