Sunday, August 13, 2006









1. I.C. (Integrated Circuit) – Two
IC1 555
IC2 555

2. Resistors – Six
R1 10 KW
R2 100 KW
R3 2.7 KW
R4 4.7 KW
R5 4.7 KW
R6 4.7 KW

3. Diode – One
D1 IN400Z

4. PNP-Transistor – One
T1 BC167

5. Capacitances – Three
C1 10 mF
C2 0.1 mF
C3 47 mF

6. Speaker – One



Resistor is a component used to control the size of the current that flows in an electronic or electric circuit. The size of the current that flows through a conductor depends on its resistance. The greater the resistance, the smaller is the current.

It is the property of a conductor due to which it poses an obstruction to the flow of current through it.

Resistance of a conductor is defined as the ratio of potential difference (v) across the ends of the conductor to the current flowing through it. i.e. R = V/I

Where, 1ohm = 1 volt = 1V
1 ampere 1A

Thus, one ohm is the resistance of the conductor through which a current of 1 ampere flows when a potential difference of 1 volt is applied across the ends of the conductor.

SI UNIT : Ohm (W).

DIMSENSIONS: [M1 L T-3 A –2 ].

Cause of resistance of a conductor:
Resistance of a given conducting wire is due to the collisions of free electrons with the ions or atoms of the conductor while drifting towards the positive end of the conductor

R = r L ___(1)
Thus from equation (1) we get that resistance of a material is
1. Directly proportional to the length of the conductor.
2. Inversely proportional to the area of cross section
3. It also depends on the nature of material and temperature of conductor.

Resistances in series:
Resistors are said to be in series if the same current is flowing through each resistor when some potential difference is applied.
If three resistances R1 ,R2 ,R3 are connected in series then the net resistance of the combination is given by Rs :
Rs = R1 + R2 + R3

Therefore , the resistors are to be connected in series if the effective resistance in the circuit is to be increased.
Resistances in parallel:
A number of resistances are said to be in parallel if potential difference across each of them is same and is equal to the applied potential difference.
If three resistances R1 ,R2 ,R3 are connected in parallel then the net resistance of the combination is given by Rp :

Therefore , the resistors are to be connected in parallel if the effective resistance in the circuit is to be decreased.

Integrated Circuit (I.C.)

Integrated Circuit is a tiny electronic circuit used to perform a specific electronic function, such as amplification; it is usually combined with other components to form a more complex system.
It is formed as a single unit by diffusing impurities into single-crystal silicon, which then serves as a semiconductor material, or by etching the silicon by means of electron beams. Several hundred identical integrated circuits (ICs) are made at a time on a thin wafer several centimetres in diameter, and the wafer is subsequently sliced into individual ICs called chips. In large-scale integration (LSI), as many as 5,000 circuit elements, such as resistors and transistors, are combined in a square of silicon measuring about 1.3 cm (0.5 in) on a side. Chips are assembled into packages containing external electrical leads to facilitate insertion into printed circuit boards for interconnection with other circuits or components.

Testing an Integrated Circuit Wafer :
In its final stage of design, an integrated circuit wafer is tested by probes. Each gold square in the wafer is an individual integrated circuit. At one time, circuits consisted of separate electronic devices (such as inductors and capacitors) mounted on a chassis and strung together with wire.


If we connect a variable resistance in series to an inductor and an E.M.F. is applied, the resistance is varied , the current flowing through the inductor will also vary.the varying current flowing through an inductor will produce a varying magnetic field around it. If another coil is kept near its varying magnetic field, a varying E.M.F. will be induced in the second coil.

The E.M.F. induced in the second coil is directly dependent upon the variations in the resistance. Using the above principle, we can convert an audio frequency(sound) into electrical impulses of the same frequency.

If the variable resistance is made out of an aluminium cup filled with carbon granules which is inturn connected to a piston and a diaphragm. By compressing the carbon granules, the resistance decreases and by decompressing the carbon granules the resistance increases. Now if sounds waves are produced in front of the diaphragm, it will push the piston in and out hence varying the resistance accordingly. This variation of resistance will induce an E.M.F. in the second inductor or coil. The frequency of the E.M.F. induced is directly proportional to the variation of resistance (i.e. the sound causing the variation). This is called carbon microphone which converts audio energy into electrical impulses.

Now, if a coil is wound loosely on a permanent magnet and a varying E.M.F. is applied to the coil, the coil will start moving in and out of the magnet depending upon the E.M.F. applied. This is due to attraction and repulsion of the magnetic field. If the E.M.F. applied to the coil is of a particular audio frequency the movement of the coil will also be of the same frequency. If a diaphragm is attached to the coil, it will also move along with the coil as per the frequency. This movement will produce the same audio frequency. Hence, we have converted the electrical impulse back to audio energy. This component is called the speaker

A junction transistor is obtained by growing a thin layer of one type semiconductor in between two thick layers of other similar type semiconductor. Thus a junction transistor is a semiconductor device having two junctions and three terminals.

The two types of junction transistors are p-n-p transistor and n-p-n junction transistor.

A p-n-p junction transistor is obtained by growing a thin layer of n-type semiconductor in between two relatively thick layers of p-type semiconductor.

A p-n-p junction transistor is obtained by growing a thin layer of p-type semiconductor in between two relatively thick layers of n-type semiconductor.

Two PN junctions can be assembled as shown in figure where there are two regions of N-type materials, one on each side of a very thin wafer of P type material. Here, the wafer of the P type material is known as the base and the N type materials are known as Emitter and collector. The whole device is called as transistor.

Diode, electronic device that allows the passage of current in only one direction.

Junction-type diodes consist of a junction of two different kinds of semiconductor material..

Reverse Biasing:
If we connect a battery to a PN junction, Positive to N type and negative to P a continuous flow of electrons or current will not be there. Connecting the battery in this way a PN junction where a continuous flow of electrons is not there is called reverse biasing.

Forward Biasing:
If we connect the battery to a PN junction- negative to N type and positive to P type current flows through the circuit continuously. Biasing this way is called forward Biasing.

The PN junction is called a diode because it allows current to flow in one direction only. So the current in the diode will flow from P type to the N type.

Capacitor is a device for storing an electrical charge, sometimes called a condenser. A capacitor consists of two metal plates separated by a non-conducting layer called the dielectric. When one plate of a capacitor is charged using a battery or other source of direct current, the other plate becomes charged with the opposite sign.

Electrical capacitance:
Electrical capacitance of a conductor is the measure of the ability of the conductor to store charge on it.
If Q is charge and V is potential of a conductor, then

Q = CV

Where C is the constant of proportionality and is called capacity or capacitance of the conductor.
The value of C depends on the shape and size(dimensions) of the conductor and also on the nature of the medium in which the conductor is located

SI UNIT : Farad(F)

Capacitance of an isolated spherical conductor:
C = 4pÎor

Where, r is the radius of the conductor.

Capacity of a capacitor:

Capacitance of a parallel plate capacitor:

C = ÎoA

Where, A is the area of the plate.
and d is the distance between the two plates.

Grouping of capacitors:

1. Capacitors in series
If three capacitors C1 ,C2 ,C3 are connected in series then the net capacitance of the combination is given by Cs :

2. Capacitors in parallel
If three capacitors C1 ,C2 ,C3 are connected in parallel then the net capacitance of the combination is given by Cp :
Cp = C1 + C2 + C3


1. An appropriate circuit diagram was made which showed the appropriate placement of components.

2. Now a rectangular copper clad sheet was taken.

3. A piece of paper with the size of the copper clad sheet was taken.

4. Now the etch diagram was designed on that paper.


1. On another paper of the same size, the drill guide was mapped.

2. This drill guide was stuck on the copper clad sheet.

3. Using the drill machine the holes were drilled on the copper clad sheet by punching the sheet on the marked points on drill guide.

4. After drilling, the drill guide sheet was removed from the copper clad sheet.


1. Now to etch the copper clad sheet the etch diagram made earlier was referred.

2. Using a permanent marker the paths as drawn in the etch diagram were made neatly on the copper clad sheet so that all the punched holes were connected properly.

3. After this the copper clad sheet was dipped in an acidic medium so that the copper where the pen was not marked is removed and we get the circuit tracks that we marked.

4. To remove the marks of the pen the copper clad sheet is rubbed with iron filings so that the black marks of the tracks were removed and the shining copper outside was obtained.

5. Now we apply any lubricant on the copper clad sheet so that the copper does not react with the oxygen in the air to form copper oxide and thus prevent it from degrading.

Component fixation:

1. Now we solder the components on the copper clad sheet using the solder wire according to the circuit diagram that we made.

2. Now we connect the battery and the switch to complete the circuit.

3. Thus we get the required police siren circuit with all its components.


The police siren circuit is made in accordance with the circuit diagram which uses two 555 timer ICs and thus a high pitch frequently varying sound is produced . It can be used in different ways depending upon the requirement.


It can be used in many ways:

1. It can be used as a siren in the police vehicles, fire brigade vehicles and ambulances as it attracts the attention of the people on the road immediately.

2. It can also be used as a security system in banks and other such places to alert the police.

3. It can also be used as a part of a burglar alarm system as its frequently varying high pitch sound grabs the attention of the nearby people as well as alerts the burglar.

4. It can be used as a call bell.

5. It can be fitted in the vehicles of the VIPs.


1. The current flows through the diode D1 connected near the positive terminal of the circuit. The P region of the diode is connected to the positive terminal and the N region of the diode is connected to the negative terminal of the battery. Thus, the circuit is in forward Bias mode.

2. It makes use of two 555 timer ICs used as astable multivibrators. The frequency is controlled by the pin 5 of the IC2.

3. The first IC2 (right) is wired so that it works around the frequency of 1Hz.

4. The capacitor C3 which is having the capacitance of 47uF is charged and discharged periodically.

5. Thus the voltage across this capacitor C3 gradually increases and decreases periodically.

6. This varying voltage modulates the frequency of the First IC i.e. IC1(Left).

7. The IC1 feeds the speaker coils and thus a sound is produced whose frequency varies alternatively with time i.e. it produces high pitch sound and then low pitch sound alternatively.

8. This process repeats and what we hear is the sound remarkably similar to the police siren.

9. Two resistors R2 and R5 are provided to vary the siren period of repetition and the tone of the siren.

10. By changing the resistance of the resistor R5 we can set how
fast the siren changes from high frequency to low frequency.

11. The resistor R2 sets the siren frequency.


1. All the plugs and connections should be neat, clean, and tight.

2. While drilling in the copper clad sheet the drill holes should be made carefully keeping in mind the sizes of the components to be fitted in.

3. While Etching the copper clad sheet the circuit track linings should be made carefully so that they don’t meet each other.

4. The circuit track linings using the pen should be made such that all the punched holes for the components are connected properly.

5. The copper clad sheet should be dipped in the acidic medium carefully so that the track linings are not removed due to reaction with the acid.

6. Soldering should be done carefully so that the components are in appropriate contact with the circuit board.


1. Basic Electronics - Compiled By
Education Wing VIT Museum

2. Fundamental Physics - By
Resnick, Halliday & Walker

3. Physics Lab Manual - By
J.N. Jaiswal

Sources of error

1. While etching the copper clad sheet there may be a gap or disconnection in the tracks.

2. Soldering may not be proper and there may be a contact due to intermixing.

3. A component may not be in proper contact with the circuit board.