Photoelectric Beam Controlled Security System

Photoelectric Beam Controlled Security System


Nowadays we see a lot of electronic detector project which are used in various ways of applications. Using this low-cost project one can detect the presence of an object when he is in unconscious state. This circuit has two parts one is transmitter and other is receiver. It does not use any wire connection between transmitter and receiver. The connecting link between transmitter and receiver are placed by the beam of light.

In the receiver section, we used an operational amplifier IC LM308. The original used the LM308, which is now obsolete and expensive to buy, though still available from some sources. The datasheet for the LM308 states that the special characteristics of this op-amp are very low input bias current, low supply current, and guaranteed drift characteristics, characteristics not normally important to audio. Further checking of the datasheet shows the slew rate of the LM308 (0.15 V/microsecond) is dismal compared to a typical audio op-amp, like a TL071, which I think is the key to why it sounds great in the circuit.


The resistor is the most common and well-known passive electrical component. A resistor is a device connected into an electrical circuit to introduce a specified resistance. The resistance is measured in Ohms. As stated by Ohms Law (E=IR), the current through the resistor will be directly proportional to the voltage across it and inversely proportional to the resistance.

Resistors have numerous characteristics that determine their accuracy during use. The performance indices affect the accuracy to a greater or lesser extent depending on the application. Some of these indices are: Tolerance at DC, Temperature Coefficient of Resistance (TCR), Voltage Coefficient of Resistance (VCR), Noise, Stability with respect to Time and Load, Power Rating, Physical Size, and Mounting Characteristics. Resistor networks typically require temperature and voltage tracking performance.

Please refer to the application note: Glossary of Resistor Terminology for an expanded explanation of resistor terminology.

Selection Requirements

Determine the resistance in ohms and watts.

Determine the proper physical case size as controlled by voltage, watts, mounting conditions, and circuit design requirements.

Select the resistor that meets your needs for type, termination and mounting.

Resistor Color Code, Tutorial

 Another example for a Carbon 22000 Ohms or 22 Kilo-Ohms also known as 22K at 5% tolerance:

Band 1 = Red, 1st digit

Band 2 = Red, 2nd digit

Band 3 = Orange, 3rd digit, multiply with zeros, in this case 3 zero’s

Band 4 = Gold, Tolerance, 5

Example for a Precision Metal Film 19200 Ohms or 19.2 KiloOhms also known as 19K2 at 1% tolerance:

Band 1 = Brown, 1st digit

Band 2 = White, 2nd digit

Band 3 = Red, 3rd digit

Band 4 = Red, 4th digit, multiply with zeros, in this case 2 zero’s

Band 5 = Brown, Tolerance, 1%

Band 6 = Blue, Temperature Coefficient, 6

If you are a bit serious about the electronics hobby I recommend learning the “Color-Code”. It makes life a lot easier. The same color code is used for everything else, like coils,

 If you are interested in learning the code by memory, try the steps below to help you ‘Learn the Color-code’. Make sure you add the number to the color, like: 0 is black, 1 is brown, 2 is red, etc. etc.

Do not proceed to step 3 until you know the color-code backwards, forwards, and inside-and- out (trust me!)Can you ‘create’your own resistors? Sure thing, and not difficult. Here is how to do it: Draw a line on a piece of paper with a soft pencil, HB or 2HB will do fine. Make the line thick and about 2 inches (5cm) long. With your multimeter, measure the ohm’s value of this line by putting a probe on each side of the line, make sure the probes are touching the carbon from the pencil. The value would probably be around the 800K to 1.5M depending on your thickness of the line and what type of pencil lead is used. If you double the line the resistance will drop considerably, if you erase some of it (length-wise obviously!) the resistance will increase. You can also use carbon with silicon glue and when it dries measure the resistance, or gypsum with carbon mixed, etc. The reason for mentioning these homebrew resistors is that this method was used in World War II to fix equipment when no spare parts were available. My father, who was with the Dutch resistance during WWII, at that time made repairs like this on many occasion.

Step 1: Learn the colors

The color ‘Gold’ is not featured in the above table. If the 3rd band is gold it means multiplying by 0.1. Example, 1.2 ohm @ 5% would be brown-red-gold-gold. 12 multiplied by 0.1 gives 1.2 don’t get confused by gold as a resistance or a tolerance value. Just watch the location/position of the band.

1st band, denominator: Brown

(1)  2nd  band,  denominator: Black (0) 3rd band, how many

zeros (1)

4th  band,  tolerance  in  %:  gold  (5)


A diode is a dispositive made of a semiconductor material, which has two terminals or electrodes (diode), that act like an on-off switch. When the diode is “on”, it acts as a short circuit and passes all current. When it is “off”, it behaves like an open circuit and passes no current. The two terminals are different and are marked as plus and minus in figure 1. If the polarity of the applied voltage matches that of the diode (forward bias), then the diode turns “on”.

When the applied voltage polarity is opposite (reverse bias), it turns “off”. Of course this is the theoretical behaviour of an ideal diode, but it can be seen as a good approximation for areal diode.

A diode is simply a pn junction (see ’Introduction into Semiconductor Physics’) with the following characteristics:

  • Under forward bias, it needs a small voltage to conduct. This voltage drop is maintained during conduction.
  • The maximum forward current is limited by heat-dissipation ability of the diode.

Usually it is around 1000 mA.

  • There is a small reverse current.

Forward current:

It is the current flowing through a forward biased diode. Every diode has a maximum value of forward current which it can safely carry. If this value is exceeded, the diode may be destroyed due to excessive heat.

Peak Inverse Voltage (PIV):

It is the maximum reverse voltage that a diode can withstand without destroying the junction.

Reverse current or Leakage current :

It is the current that flows through a reverse biased diode. The reverse current is usually very small as compared with forward current.

Forward resistance of resistor :

The resistance offered by the diode to the forward bias is known as forward resistance.

Reverse resistance of resistor :

The resistance offered by the diode to the reverse bias is known as reverse resistance.

Ripple factor :

The ratio of r.m.s. value of a.c. component to the d.c. component in the rectifier output is known as ripple factor.

diode. (c) IV curve for a real diode.

Breakdown voltage :  The breakdown voltage of a diode is the minimum reverse

voltage to make the diode conduct in reverse. The breakdown voltage depends upon the amount of doping. If the diode is heavily doped, depletion layer will be thin and consequently the breakdown of the junction will occur at a lower reverse voltage. On the other hand, a lightly doped diode has a higher breakdown voltage.

Knee voltage : The point in the forward operating region of the characteristic

curve where conduction starts to increase rapidly is called Knee voltage of a PN

Junction Diode.          

Rectifier diodes

A rectifier is a dispositive that ideally transforms the AC input voltage into a DC voltage (voltage is always positive or zero). These diodes have the largest ratings and sometime can be quite big in volume. As a rule of thumb, the bigger the diode (more pn surface junction available for heat dissipation), the higher the ratings.

Half-wave rectifier

A half-wave rectifier is composed of a single diode that connects an AC source to a load. In figure 3 the load is represented by a resistor. The diode conducts on AC voltage only when its anode is positive with respect to the cathode (i.e. greater than 0.7 V for a silicon diode). The output has therefore only a positive component with an average value.The output peak voltage is the AC source minus the voltage drop of the diode, that in most cases can be neglected.

Full-wave rectifier

In half-wave rectifiers, half of the power provided by the source is not used. To solve this problem, we have to use full-wave rectifiers. The minimum full-wave rectifier is composed of two diodes, but it requires a center tapped transformer. Figure 4 shows a bridge rectifier, composed of four diodes, that can use a “normal” transformer.

The AC current, according to its direction, flows either in the top or in the bottom part of the bridge in each half-cycle. In the output voltage we will have a component for both negative and positive parts of the input voltage. In both cases the current passes through two forward-biased diodes in series, what produces a voltage drop of 1.4 V.



The gain won’t be identical even in transistors with the same part number. The gain also varies with the collector current and temperature. Because of this we will add a safety margin to all our base current calculations (i.e. if we think we need 2mA to turn on the switch we’ll use 4mA just to make sure).

The transistor has three regions, namely; Emitter, base and Collector. The base is much thinner than the emitter while Collector is wider than both.

The emitter is heavily doped so that it can inject a large number of charge carriers (electron or holes) into the base. The base is lightly doped and very thin, it passes most of the emitter injected charge carriers to the collector. The collector is moderately doped.

The transistor has two p-n junctions. It is like two diodes. The junction between emitter and base may be called emitter-base diode. The junction between the base and collector may be called collector-base diode.


Figure 5 shows a simplified structure for the BJT. A practical transistor structure will be shown later (see also Appendix A, which deals with fabrication technology).As shown in Fig. 5, the BJT consists of three semiconductor regions: the emitter region (n type), the base region (p type), and the collector region (n type).

Such a transistor is called an npn transistor. Another transistor, a dual of the npn as shown in Fig. 6, has a p-type emitter, an n-type base, and a p-type collector and is appropriately called a pnp   transistor.

The transistor consists of two pn junctions, the emitter–base junction (EBJ) and the collector–base junction (CBJ). Depending on the bias condition (forward or reverse) of each of these junctions, different modes of operation of the BJT are obtained. The active mode, which is also called forward active mode, is the one used if the transistor is to operate as an amplifier. Switching applications (e.g., logic circuits) utilize both the cutoff and the saturation modes. The reverse active (or inverse active) mode has very limited application but is conceptually important. As we will see shortly, charge carriers of both polarities—that is, electrons and holes— participate in the current conduction process in a bipolar transistor, which is the reason for the name bipolar .

 Operation of the npn Transistor in the Active Mode

(Reverse current components due to drift of thermally generated minority carriers are not shown.)

Let us start by considering the physical operation of the transistor in the active mode.1 This situ- ation is illustrated in Fig. 7 for the npn transistor. Two external voltage sources (shown as batteries) are used to establish the required bias conditions for active-mode operation. The voltage VBE causes the p-type base to be higher in potential than the n-type emitter, thus forward-biasing the emitter–base junction. The collector–base voltage VCB causes the n-type collector to be higher in potential than the p-type base, thus reverse-biasing the collector–base junction.

Current Flow In the following description of current flow only diffusion-current compo- nents are considered. Drift currents due to thermally generated minority carriers are usually very small and can be neglected. Nevertheless, we will have more

to say about these reverse- current components at a later stage.

The pnp  Transistor

The pnp transistor operates in a manner similar to that of the npn device described above. Figure 8 shows a pnp transistor biased to operate in the active mode. Here the voltage VEB causes the p-type emitter to be higher in potential than the n-type base, thus forward-biasing the base–emitter junction. The collector–base junction is reverse-biased by the voltage VBC, which keeps the p-type collector lower in potential than the n-type base.

Unlike the   npn transistor, current in the  pnp device is mainly conducted by holes injected from the emitter into the base as a result of the forward-bias voltage.

EB. Since the component of emitter current contributed by electrons injected from base to emitter is kept small by using a lightly doped base, most of the emitter current will be due to holes. The electrons injected from base to emitter give rise to the first component of base current,  


FIGURE 8 :    Current flow in a    pnp transistor biased to operate in the active mode.

The holes that succeed in reaching the boundary of the depletion region of the collector– base junction will be attracted by the negative voltage on the collector. Thus these holes will be swept across the depletion region into the collector and appear as collector current.

Common Base Amplifier

The common-base amplifier can provide a reasonable level of voltage gain but suffers from low input impedance and a current gain of less than one. However, this circuit is used extensively for high-frequency applications because its terminal characteristics at high frequencies are better than those of a common-emitter configuration using the same transistor.

The standard common-base configuration also requires two DC supplies rather than the one needed for the common-emitter and common-collector configurations. There is, however, a method to be introduced in this experiment that permits the proper biasing of a common-base amplifier with only a single supply.

The ac small signal, hybrid (h-parameter) equivalent circuit for a common base transistor is shown in Figure 9a. The simplified equivalent circuit is shown in Figure 1b.

For the single supply common-base configuration of Figure 9, the small signal equivalent circuit is as shown in Figure10. Note that the small signal equivalent circuit drawn assumes the emitter resistor Re is much bigger than the transistors’ internal emitter resistance, re (usually of the order of 20 Ω), and is therefore not needed in this circuit.

The equations describing the dc biasing and ac operation of the common base amplifier are therefore derived below.

DC Biasing

Since the base capacitor Cb is an open circuit at dc, this circuit is like a commonemitter

circuit at dc and the same biasing equations therefore hold. That is, Voltage

at the emitter, Ve = Ie Re

Voltage at the base, Vb = Ve + 0.7

Assuming the base current to be small, Vb ≈ Vcc R2/(R1+R2)

The collector voltage is Vc = Vcc – Rc Ic

The collector and emitter currents are almost equal, Ic ≈ Ie.

AC Operation

The open circuit voltage gain is and since the typical range of the current gain α is, 0.98<α<0.998.

where re is approximately given by 26mV / Ie(mA).

The input impedance is ZI = Re || re

≈ re if Re is reasonably big (>100Ω say).

Common Collector (or Emitter Follower) Amplifier

The emitter-follower or common collector configuration is used primarily for impedance matching purposes. It has high input impedance to draw as much of the available signal to the base of the transistor as possible. The output impedance however is almost equal to the emitter resistance and can therefore be easily matched to a transmission line or load of known impedance. Its voltage gain is less than one, but the current gain can approach the AC beta ( β) of the transistor. The output voltage is also in phase with the applied signal rather than 180 ° out of phase as encountered for the common-emitter configurations.

For the emitter-follower configuration of Figure 12, the applicable equations are given below.



This introduction includes a brief review of the design of a single stage common emitter transistor amplifier. In order to complete this practical the student should have a basic understanding of the operation of an npn silicon transistor. Readers who are unfamiliar with this device are referred to the texts listed under the reference section below.

There are two parts to transistor amplifier design.

  1.  dc biasing
  2.  ac amplifier design.

To ensure linear amplification by a transistor amplifier, the amplifier is normally designed so that under quiescent (no input or dc) conditions it will be operating at the centre of a linear region, as normally determined from the transistor output characteristics (Ic vs Vce).

The dc bias design part of the amplifier design will ensure that the amplifier operates about an appropriate quiescent point. Subsequent to this the ac amplifier design ensures that the amplifier provides the correct ac signal gain.


The biasing technique consists of supplying the base of the transistor with a fraction of the supply voltage Vcc via the resistive divider network R1, R2. In addition RE is used to place the emitter at its correct voltage (determined by the value chosen for Ie) and Rc is chosen to place the collector at it’s optimum operating point.



Figure 15 shows the complete circuit for a classical single-stage transistor amplifier employing the bias arrangement just described. A signal source Vs with output resistance Rs is coupled via Cin to the base of the transistor. Cin should be chosen large enough so that it appears as an ac short circuit over the frequency band of interest. The output from the collector is coupled to the load RL via the coupling capacitor Cout. Cout should also appear as a short circuit over the frequency band of interest. The detrimental effect of RE on the ac performance of the amplifier is eliminated by Ce. Ce acts as a short circuit to the frequencies of interest, effectively shorting RE as far as ac signals are concerned. Thus while the dc emitter current will continue to flow through RE, the ac signal current ie will flow through Ce, bypassing RE. For this reason Ce is called an “Emitter bypass capacitor” and the circuit is called a “Grounded emitter” or “Common-emitter amplifier”.

We shall analyse the circuit of Figure 5 to determine the amplifier gain and input resistance for ac signals in the frequency range of interest. Note in this circuit the emitter resistance has been split in two. One part, R E2, is shunted by an ac bypass capacitor Ce and will not play a role in the ac circuit analysis. The total resistance in the emitter RE1 + RE2, will need to be considered in the dc biasing design though.


Phototransistors are photodiode-amplifier combinations integrated within a single silicon chip. These are combined to overcome the major fault of photodiodes: unity gain. Many applications demand a greater output signal from the photodetector than can be generated by a photodiode alone. While the signal from a photodiode can always be amplified through use of an external op-amp or other circuitry, this approach is often not as practical or as cost-effective as the use of phototransistors.

The phototransistor can be viewed as a photodiode whose output photocurrent is fed into the base of a conventional small-signal transistor. While not required for operation of the device as a photodetector, a base connection is often provided, allowing the designer the option of using base current to bias the transistor. The typical gain of a phototransistor can range from 100 to over 1500.

Phototransistors can be used as ambient-light detectors. When used with a controllable light source, typically an IRED, they are often employed as the detector element for optoisolators and transmissive or reflective optical switches.

Light Emitting Diodes (LEDs)


To explain the theory and the underlying principle behind the functioning of an LEDBriefHistory: The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round. In the mid 1920s, Russian Oleg Vladimirovich Losev independently created the first LED, although his research was ignored at that time.

In 1955, Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys. Experimenters at Texas Instruments, Bob Biard and Gary Pittman, found in 1961 that gallium arsenide gave off infrared radiation when electric current was applied. Biard & Pittman received the patent for the infrared light-emitting diode. In 1962, Nick Holonyak Jr., of the General Electric Company and later with the University of Illinois at Urbana-Champaign, developed the first practical visible-spectrum LED. He is seen as the “father of the light-emitting diode”. In 1972, M. George Craford, Holonyak’s former graduate student, invented the first yellow LED and 10x brighter red and red-orange LEDs.



A Light emitting diode (LED) is essentially a pn junction diode. When carriers are injected across a forward-biased junction, it emits incoherent light. Most of the commercial LEDs are realized using a highly doped n and a p Junction.

To understand the principle, let’s consider an unbiased pn+ junction (Figure16 shows the pn+ energy band diagram) . The depletion region extends mainly into the p-side. There is a potential barrier from Ec on the n -side to the Ec on the p-side, called the built-in voltage, V0. This potential barrier prevents the excess free electrons on the n+ side from diffusing into the p side.When a Voltage V is applied across the junction, the built-in potential is reduced from V0 to V0 –V. This allows the electrons from the n+ side to get injected into the p-side. Since electrons are the minority carriers in the p-side, this process is called minority carrier injection. But the hole injection from the p side to n+ side is very less and so the current is primarily due to the flow of electrons into the p-side.

 LED Structure:

The LED structure plays a crucial role in emitting light from the LED surface. The LEDs are structured to ensure most of the recombinations takes place on the surface by the following two ways.By increasing the doping concentration of the substrate, so that additional free minority charge carriers electrons move to the top, recombine and emit light at the surface. By increasing the diffusion length L = √ Dτ, where D is the diffusion coefficient and τ is the carrier life time. But when increased beyond a critical length there is a chance of re-absorption of the photons into the device.

The LED has to be structured so that the photons generated from the device are emitted without being reabsorbed. One solution is to make the p layer on the top thin, enough tocreate a depletion layer. Following picture shows the layered structure. There are different ways to structure the dome for efficient emitting(See Appendix 6).

Operational Amplifier

Operational amplifiers are high-gain amplifiers The Op-amp is used for many amplifier varieties such as Inverting, Non-inverting, differential, voltage follower and summing amplifier. In addition to amplifiers, op amps are used as switches and even in some digital applications as comparators or A/D converters. Op amps make use of what is called open loop gain. This open loop gain is used to for the purposes of negative feedback. Negative feedback is when the output signal is feed back to the input terminals and the gain of the op amp can be controlled. This is done because the properties of the op amp become more predicable. Negative feedback also creates a more customizable frequency response for the desired amplifier. In turn there is also and increase in the input impedance of the amplifier is negative feedback is used. There is also what is called positive feedback, and the main use for this is to create an oscillator. The way this idea works is that instead of canceling the input to reduce gain, the output is combined in phase with the input to create oscillations. There are many different types of oscillators that can be created with op amps, one ofwhich is the Colpitts Oscillator. In many cases, the op amp is thought of as an Ideal Op Amp. The Ideal Op Amp has a few basic rules that apply. These rules are as follows:

  1.  Infinite voltage gain
  2.  Infinite input impedance
  3.  Zero output impedance
  4.  Infinite bandwidth

Unfortunately there is no such device, and there are limits to the parameters of a real op amp. There are two rules of which an op amp will follow, too. These are that the output of the op amp will do whatever is necessary to make the input differential between the two input terminals exactly zero, and that the input terminals draw no current. Again, since there is no such device, the real op amp does not fit these rules. There is a limit to the gain on a real op amp (~106 ) and the input terminals do draw current (~.08 µA). The input current is so small, that it is thought to be zero.

 Non-Inverting Amplifiers

The first op amp circuit that will be analyzed is that non-inverting amplifier. The non-inverting amplifier is called this because the input signal is connected to the non-inverting terminal. Also the output is in phase with the input. A special case of the non-inverting amplifier is that of the Voltage Follower. The voltage follower has the output signal connected to the inverting input terminal of the op-amp as shown in Figure 18. The analysis of this device shows that Vout= Vin. The common use for a voltage follower is to create a buffer in a digital circuit. The follower isolates the output signal from the signal source with the very large input impedance. This is where the term ‘buffer’ came from. Notice that in the picture of the Voltage Follower the pin numbers of the device are listed. This is important for when the device is connected on a breadboard that the device pins

are connected to the correct locations. The pin assignments for any device can be found on the data sheets that are available online or in paper form. This information will be provided one way or another.

The voltage follower does not hold much interest right now, so the next amplifier that will be looked at is a non-inverting amplifier with a gain. This amplifier is shown in Figure 19. By doing the analysis of this device using KCL and KVL, the transfer function, or gain, can be found.

Therefore the gain of an inverting amplifier does not have an automatic gain of 1 in the system. The resistor values have to be chosen such that Rf= R1to get the inverting gain of 1. There are many uses for the inverting amplifier configuration some ofwhich will be discussed further. Two other uses of the inverting configuration are the integrator and the differentiator. The integrator (as the name suggests) integrates the input signal over time. The integral of the input is the output waveform. And the counterpart of the integrator is the differentiator which as the name implies again, differentiates the input as the output. These two configurations as well as the input and output waveforms are shown below in Figure 21.

 The Golden Rules

The golden rules are idealizations of op-amp behavior, but are nevertheless very useful for describing overall performance. They are applicable whenever op-amps are configured with negative feedback, as in the two amplifier circuits discussed below. These rules consist of the following two statements:

  1.  The voltage difference between the inputs +V, -V is zero. (Negative feedback will ensure that this is the case.)
  2.  The inputs draw no current. ( This is true in the approximation that the Zin of the op-amp is much larger than any other current path available to the inputs.)

When we assume ideal op-amp behavior, it means that we consider the golden rules to be exact. We now use these rules to analyze the two most common op-amp configurations.

LM308 Operational Amplifiers

General Description

The LM108 series are precision operational amplifiers having specifications a factor of ten better than FET amplifiers over a 55C to 125C temperature range. The devices operate with supply voltages from 2V to 20V and have sufficient supply rejection to use unregulated supplies. Although the circuit is interchangeable with and uses the same compensation as the LM101A, an alternate compensation scheme can be used to make it particularly insensitive to power supply noise and to make supply by- pass capacitors unnecessary. The low current error of the LM108 series makes possible many designs that are not practical with conventional amplifiers. In fact, it operates from 10 MX source resistances, introducing less error than devices like the 709 with 10 kX sources. Integrators with drifts less than 500 mV/sec and analog time delays in one excess of one hour can be made using capacitors no larger than 1 mF.

The LM108 is guaranteed from 55C to 125C, the LM208 from 25C to 85C, and theLM308 from 0C to 70C.Features Maximum input bias current of 3.0 nA over temperature Offset current less than 400 pA over temperature Supply current of only 300 mA, even in saturation Guaranteed drift characteristics.

DC Motors

Almost every mechanical movement that we see around us is accomplished by an electric motor. Electric machines are a means of converting energy. Motors take electrical energy and produce mechanical energy. Electric motors are used to power hundreds of devices we use in everyday life. Motors come in various sizes. Huge motors that can take loads of 1000’s of Horsepower are typically used in the industry. Some examples of large motor applications include elevators, electric trains, hoists, and heavy metal rolling mills. Examples of small motor applications include motors used in automobiles, robots, hand power tools and food blenders. Micro-machines are electric machines with parts the size of red blood cells, and find many applications in medicine.

Electric motors are broadly classified into two different categories: DC (Direct Current) and AC (Alternating Current). Within these categories are numerous types, each offering unique abilities that suit them well for specific applications. In most cases, regardless of type, electric motors consist of a stator (stationary field) and a rotor (the rotating field or armature) and operate through the interaction of magnetic flux and electric current to produce rotational speed and torque. DC motors are distinguished by their ability to operate from direct current.

There are different kinds of D.C. motors, but they all work on the same principles. In this chapter, we will study their basic principle of operation and their characteristics. It’s important to understand motor characteristics so we can choose the right one for our application requirement. The learning objectives for this chapter are listed below.

 DC Motor Basic Principles

Energy Conversion

If electrical energy is supplied to a conductor lying perpendicular to a magnetic field, the interaction of current flowing in the conductor and the magnetic field will produce mechanical force (and therefore, mechanical energy).

Value of Mechanical Force

There are two conditions which are necessary to produce a force on the conductor. The conductor must be carrying current, and must be within a magnetic field. When these two conditions exist, a force will be applied to the conductor, which will attempt to move the conductor in a direction perpendicular to the magnetic field. This is the basic theory by which all DC motors operate. The force exerted upon the conductor can be expressed as follows.

F = B i l Newton                                                                 (1)

Where B is the density of the magnetic field, l is the length of conductor, and i the value of current flowing in the conductor. The direction of motion can be found using Fleming’s Left Hand Rule.

The first finger points in the direction of the magnetic field (first -field), which goes from the North pole to the South pole. The second finger points in the direction of the current in the wire (second – current) . The thumb then points in the direction the wire is thrust or pushed while in the magnetic field (thumb – torque or thrust).


  1.   Resistor
  2.   Ceramic capacitor
  3.   Diode
  4.   Transistor
  5.   Phototransistor
  6.   IC LM308
  7.   LED
  8.   DC Motor
  9.   Battery
  10. .SPST Switch
  11. PCB


At first we consider a common emitter phototransistor. The common emitter phototransistor circuit configuration is possibly the most widely used, like its more conventional straight transistor circuit. The collector is taken to the supply voltage via a collector load resistor, and the output is taken from the collector connection on the phototransistor. The circuit generates an output that moves from a high voltage state to a low voltage state when light is detected.

The phototransistor circuits can be used on one of two basic modes of operation. They are called active or linear mode and a switch mode.

We use the switch mode. The operation of the phototransistor circuit in the switch mode is more widely used in view of the non-linear response of the phototransistor to light. Eventually a point is reached where the phototransistor becomes saturated and the level of current cannot increase. In this situation the phototransistor is said to be saturated. The switch mode, therefore has two levels: -“on” and “off” as in a digital or logic system. This type of phototransistor mode is useful for detecting objects, sending data or reading encoders, etc.

At the primary state of operation, when transmitter remains off or the sensor element that means phototransistor does not get light, the output of IC LM308 becomes high that output power feed to the driver transistor 2N2222 via 100k resistor. The transistor has a high current rating about 750mA that causes the relay circuit to turn on and the output of the relay circuit causes switch on the separated circuit. That circuit contains the load which has to operate. Here we use a DC Motor as a load.

But when photocurrent is fed into the base of phototransistor it generates an output that moves from a high voltage state to a low voltage state and the output of IC LM308 becomes very low and that’s why relay switch cannot conduct therefore the output is zero. We can increase range by using a high output LED for LED1.


This is a very effective electronic circuit which we can use as a security project. We can also use this circuit for detecting object, Burglar alarm and sending data using encoder and decoder etc.


The receiver section can be processed by biasing any lighting source to the base of phototransistor thus the transmitter circuit can be made unnecessary and burglar can do their work easily.

Future Aspects

Using the knowledge gathered for the assigned work now we can make some better electronic security projects. By further modification of this project it can be used in high technology sectors. An example of that LASER technology.