[LIGHT ACTIVATED ALARM]By: OGUNNUSI Olaoluwa Olalekan.

Electrical and Electronics Engineering .

120403076.

12th June, 2014.

EEG 305

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 speaker is 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 loudspeaker SP1. Switch.

2 (L.E.D’s) Light emitting diode

Soldering gun.

CIRCUIT DIAGRAM

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 essential elements 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 material is arranged in the form of a long strip, zip-zagged across a disc shaped base with protective sides. For additional protection, a glass or plastic cover may be included. The two ends of the strips are bought out to connecting pins below the base as shown below.

The commercial photoconductive materials include cadmium sulphide (Cds), cadmium selenide (CdSe), Lead sulfide (Pbs) and Indium antimonide (InSb) e.t.c, there is a large change in the resistance of a cadmium sulphide remains relatively stable. Moreover, the spectral response of a cadmium sulphide remains

relatively stable. Moreover, the spectral response of a cadmium sulphide cell closely matches to that of a human eye. Hence, LDR is used in applications where human vision is a factor such 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 differential amplifier 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.

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 between the 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 gain of 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 symbol just like an operational amplifier. 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 factor of 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 approach to 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 gain were 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. 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 loudspeaker load. 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 between 500 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-cycles of the signal when it is necessary to source current to the load. The bottom output 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-emitter voltage 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 crossover region 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 spreader is set to provide a very large output stage idle bias current, both the top and 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 dissipation as 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-down version of the output voltage and acts to make them essentially the same. This action applies to both stabilization of 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.

PRINCIPLE OF OPERATIONThe 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 capacitor drop 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 loudspeaker would 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 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 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.