1. [LIGHT ACTIVATED ALARM]
By: OGUNNUSI Olaoluwa Olalekan.
Electrical and Electronics Engineering .
120403076.
12th
June, 2014.
EEG 305

2. AIM OF THE EXPERIMENT
When light rays hit the surface of the L.D.R(Light Dependent Resistor) while
the switch is closed, the 8 Ω speakeris triggered to give a sequential sound.
APPARATUS USED
 An electronic breadboard / Ferro board.
 Connecting wires / lead wires.
 9V battery and a battery clip.
 Three 10k resistors (brown - black - orange) R3, R4, R7.
 One 4k7 resistor (yellow - violet - red) R6.
 One 2k2 resistor (red - red - red) R8.
 One 1k5 resistor (brown - green - red) R1.
 One 1k resistor ( brown - black - red) R2.
 One 330Ωresistor (orange - orange - brown) R5.
 Two 100nF C1, C2 Capacitor.
 Three PN2222 or KSP2222 Q2, Q3, Q4 NPN transistors.
 One PN2907 Q1 PNP transistor.
 One (L.D.R) Light dependent resistor.
 One 8Ω loudspeakerSP1.
 Switch.
 2 (L.E.D’s) Light emitting diode
 Soldering gun.

3. CIRCUIT DIAGRAM

4. THEORY BEHIND THE EXPERIMENT.
We are conversant with most of the Electrical components,but for
better understanding i would place emphasis on certain components
referencing the circuit.
THE LIGHT DEPENDENT RESISTOR:
A light dependent resistor is a resistor whose resistor changes with the
intensity of incident light. The working principle of light dependent resistor is
photoelectric effect.A light dependent resistor is made of a high resistance
semiconductor.If the energy of the incident light is greater than the band
gap of the semiconductor,electron-hole pairs are generated. The
photogenerated electron-hole pair transits the device giving rise to
photoconductivity.
The essentialelements of a photoconductive cell are the ceramic
substrate, a layer of a photoconductive material, metallic electrodes to
connect the device into a circuit and a moisture resistant enclosure. Light
sensitive materialis arranged in the form of a long strip, zip-zaggedacross a
disc shaped base with protective sides.For additionalprotection, a glass or
plastic covermay be included. The two ends of the strips are bought out to
connecting pins below the base as shown below.
The commercialphotoconductive materials include cadmium sulphide
(Cds),cadmium selenide (CdSe),Lead sulfide (Pbs) and Indium antimonide

5. (InSb) e.t.c, there is a large change in the resistance of a cadmium sulphide
remains relatively stable. Moreover, the spectralresponse of a cadmium
sulphide remains relatively stable.Moreover, the spectralresponse of a
cadmium sulphide cell closely matches to that of a human eye. Hence, LDR is
used in applications where human vision is a factorsuch as street light control
or automatic iris control for cameras.The above mentioned features drive us
to opt for Cds based LDR in our electronic circuit for automatic street light
controller.
Basic Amplifier Topology:
In figure 1.5 diagram below shows a simplified three-stage audio power
amplifier design. This is a direct descendant of the Lin topology introduced in
the 1950s. Although other arrangements have appeared through the years,
this one and its many derivatives account for the vast majority of power
amplifier designs, and it will be the focus of most of this book. Transistors Q1
and Q2 form the input differential pair. This arrangement is often called a
long-tailed pair (LTP) because it is supplied with a so-called tail current from a
very high-impedance circuit like the current source shown. We will often take
the liberty of referring to the amplifier’s input stage as the IPS. The input
differentialamplifier usually has a fairly low voltage gain, typically ranging
from 1+ to 15.The IPS compares the applied input signal to a fraction of the
output of the amplifier and provides the amount of signal necessary for the
remainder of the amplifier to create the required output.

6. This operation forms the essence of the negative feedback loop. The
fraction of the output to which the input is compared is determined by the
voltage divider consisting of R3 and R2. If the fraction is 1/20 and the forward
gain of the amplifier is large, then very little difference need exist betweenthe
input and the fed back signal applied to the IPS in order to produce the
required output voltage.The gain of the amplifier will then be very nearly 20.
This is referred to as the closed-loop gainof the amplifier(CLGor Acl).
This simplified explanation of how negative feedback works is illustrated
in Figure 1.6. The core of the amplifier that provides all of the open-loop gain
(OLGor Aol) is shown as again block symboljust like an operationalamplifier.
For purposes of illustration, it is shown with a gain of 1000. The feedback
network is shown as a block that attenuates the signal being fed back by a
factorof 20. Suppose the output of the amplifier is 20 V, the amount fed back
will then be 1 V. The input across the differential inputs of the gain block will
be 20 mV if the forward gain is 1000. The required input from the input
terminal will then be 1.02 V. This simplified approachto looking at a feedback
circuit is sometimes referred to as input-referred feedback analysis because
we start at the output and work our way back to the input to see what input
would have been required to produce the assumed output. The closed-loop
gain is thus 20/1.02 =19.6. This is just 2% shy of what we would get if we
assumed that the closed-loop gainwere just the inverse of the attenuation in
the feedback path.
Transistor Q3 in the first diagram below forms what is called the voltage
amplifier stage (VAS).It is a high-gain common-emitter(CE) stage that
provides most of the voltage gain of the amplifier. Notice that it is loaded
with a current source rather than a resistor so as to provide the highest
possible gain. It is not unusual for the VAS to provide a voltage gain of 100 to
10,000. This means that the difference signal needed to drive the input stage
does not need to be very large to drive the output to its required level. If the
difference signal is close to zero, and 1/20 of the output is compared to the
input, it follows that the output would be almost exactly 20 times the input.

7. The output stage (OPS) is composed of transistors Q4 through Q7. Its main
job is to provide buffering in the form of current gain between the output of
the VAS and the loudspeakerload. Most output stages have a voltage gain of
approximately unity.
The output stage here consists essentially of two pairs of emitter
followers (EF), one for each polarity of the output swing. This is called a
complementary push-pull output stage. Transistors Q4 and Q5 are referred to
as the drivers, while Q6 and Q7 are the output devices. The two-stage OPS,
like this one, will typically provide a current gain between500 and 10,000. This
means that an 8-Ωload resistance will look like a load resistance between
4000 and 80,000 Ω to the output of the VAS. Other output stages,like so-
called triples, can provide current gain of 100,000 to 1 million, greatly
reducing the load on the VAS.
This OPS is the classic class B output stage used in most audio power
amplifiers. The upper output transistor conducts on positive half-cyclesof the
signal when it is necessary to source current to the load. The bottom output

8. transistor conducts on the negative half-cycle when it is necessary to sink
current from the load. The signal thus follows a different path through the
amplifier on different halves of the signal. This of course can lead to
distortion.
The box labeled bias provides a DC bias voltage that overcomes
the turn-on base-emittervoltage drops (Vbe) of the driver and output
transistors. It also keeps them active with a small quiescent bias current even
when no current is being delivered to the load. This bias circuit is usually
referred to as the bias spreader.The output stage bias current creates a small
region of overlapping conduction between the positive and negative output
transistors. This smoothens the transition from the upper transistors to the
lower transistors (and vice versa) when the output signal goes from positive
to negative and the output stage goes from sourcing current to the load to
sinking current from the load.
We’ll have much more to say about this crossoverregion and the
distortion that it can create in Chapters 5 and 10. Because there is a small
region of overlap where both transistors are conducting, this type of output
stage is often referred to as a class AB output stage. If the bias spreaderis set
to provide a very large output stage idle bias current, both the top and

9. bottom output transistors will conduct on both half-cycles of the signal. One
will be increasing its current as the other decreases its current, with the
difference flowing into the load. In this case we have a so-called class A
output stage.The fact that the signal is then always taking the same path to
the output (consisting of two parallel paths) tends to result in less distortion
because there is no cross-overfrom one half of the output stage to the other
as the signal swings from positive to negative.The price paid is very high
power dissipationas a result of the high output stage bias current. Actual
operation of the amplifier of Figure 1.5 is quite simple. The input differential
amplifier compares the input voltage to a scaled-downversion of the output
voltage and acts to make them essentially the same. This action applies to
both stabilizationof the DC operating points and the processing of AC
signals. In the quiescent state transistors Q1 and Q2 are conducting the same
amount of current, in this case 1 mA each.
The resulting voltage drop across R1 is just enough to turn on Q3 to
conduct 10 mA, balancing the current supplied to its collector by the 10-mA
current source. Now suppose the output is more positive than it should be,
the voltage at the base of PNP transistor Q1will then be negative with respect
to the scaled version of the output voltage at the base of Q2. A more
negative voltage at the base of a PNP transistor causes it to conduct more
current. Transistor Q1will thus conduct more current and increase the voltage
drop across R1. This will increase the voltage at the base of NPN transistor Q3.
A more positive voltage at the base of an NPN transistor causes it to conduct
more current. Transistor Q3will thus turn on harder. This will cause an
imbalance between Q3’s collector current and the 10-mA current
source. Q3’s increased collector current will thus pull the voltage at its
collector node more negative.This will drive the bases of the driver and
output transistors more negative.
Their emitters will follow this negative voltage change, causing the
output of the amplifier to go more negative. The result will be that the
initially assumed positive error in the output voltage will be corrected.

10. PRINCIPLE OF OPERATION
The light dependent resistor has a range of 0.00cm to ∞cm range of
activation,it is triggered when rays of lights hits the surface of the ‘’resistor’’
housing the light sensitive material these rays can be solar or florescent e.t.c.
The light ray makes the L.D.R act as short circuit closing the circuit. This allows
current flow through the components hence a potential difference would
exist between the components.
The resistors and capacitordrop the p.d between components and
stores charges respectively; the transistors are arranged in such a way to
amplify the output of the loudspeaker. So it’s basically from the source being
the 9v battery when the switch is closed with the presence of light rays
through the resistors and capacitors to the transistors and finally to the
speaker.
NOTE: The siren operates like a push button bell, in the sense that the
loudspeakerwould only give out sound when the button is pushed and
would stay on as long as the switch is pressed in the presence of light, once
the switch is left the siren trips off.
CHALLENGES
 Some of the values of the resistors were not standard values

11.  During the soldering part of the transparent liquid part got to another
path of the circuit causing a bridge on the network
 On transferring the circuit from the bread board to the Ferro board
some unknown problems came up in which the cause was unknown
and the circuit was not working as it was working on the bread board.
 It was not also easy learning the soldering and making it straight and
shiny at the end
PROCEDURE
 Initially I sourced a circuit diagram from the internet.
 The circuit diagram was drawn using “multisim” and it was
simulated to ensure it was working perfectly and that none of
the components would blow up.
 I then coupled each component on a bread-board as shown in
the sourced circuit diagram to test the authenticity in a real life
situation.
 I checked the continuity of each component with my
multimeter.
 After I ensured the circuit was working perfectly on the bread-
board I started mounting each component on a Ferro-board
 I used the soldering iron and lead to solder the components
on the Ferro-board.
 I then used my cutting knife or plier to smoothen the back of
the Ferro –board by clipping the sharp end to avoid injuries.
PRECAUTIONS TAKEN

12.  I ensured each component was correctly placed on the
breadboard.
 I used exactly the same materials as the sourced circuit
diagram.
 I ensured all connections made were firm.
 I avoided parallax error when reading the analog meter.
 During soldering I ensured that the soldering iron was placed
in the soldering rack in order to harm me or my surrounding.
 I ensured that during soldering the molten lead did not smear
in order not to make a wrong connection.
REFRENCES
 www.youtube.com.
 www.google.com.
 Department of Electrical and Computer Engineering, Federal
University Technology, P.M.B. 65, Minna, Nigeria.
 Audio power amplifiers by Bob Cardell.
 Circuits and system by K.M Soni.
 Electricity and magnetism with electronics by K.K Tewari.
 The Electronics laboratory of the University of Lagos.