Automatic Railway Signal System

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Automatic Railway Signal System

1.1 Introduction

The objective of our project is to develop a Railway based system to control railway with signal system. To accomplish the task we have used a 555 timer IC, miniature magnet, Relay SPCO 12V coil with an external circuit. This circuit will used to connect the system with railway line. Our circuit is very simple and easy to understand. We’ve tried to make the device user friendly as much as we can.

1.2 Purpose and scope

Almost everybody today is used to operating a signal system. From the TV, to the cable network, up to the mobile phone, there are just a few appliances left without their own signal system. Oddly, this small superfluous but essential device called signal system is not so common in the railway department. By using this device we can control some of the function of our signal. In our country the railway signal system is very poor. This system will control the signal without any help of human. It will reduce the accident of railway what are happened for the signal system.

Discussion on hardware

Rectifier Diode-Converts AC to DC. An electronic device, such as a rectifier diode or valve, that converts an alternating current to a direct current by suppression or inversion of alternate half cycles .A capacitor (formerly known as condenser) is a passive two-terminal electrical component used to store energy in an field. A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package. Zener Diode- a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage.

2.1 Signal

Fig: signal

In electronics, a signal is an electric current or electromagnetic field used to convey data from one place to another. The simplest form of signal is a direct current (DC) that is switched on and off; this is the principle by which the early telegraph worked. More complex signals consist of an alternating-current (AC) or electromagnetic carrier that contains one or more data streams. a signal is any time-varying or spatial-varying quantity Such information or machine data (for example, the dots on a screen, the ink making up text on a paper page, or the words now flowing into the reader’s mind) must all be part of systems existing in the physical world – either living or non-living Signal modulation can be done in either of two main ways: analog and digital. In recent years, digital modulation has been getting more common, while analog modulation methods have been used less and less. There are still plenty of analog signals around, however, and they will probably never become totally extinct.

2.1.1 Signal of typical operations and applications

2.2 555 timer IC

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation and oscillator applications. The 555 can be used to provide time delays, as an oscillator.

Depending on the manufacturer, the standard 555 package includes 25 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).<href=”#cite_note-1″>[2] Variants available include the 556 (a 14-pin DIP combining two 555s on one chip), and the two 558 & 559s (both a 16-pin DIP combining four slightly modified 555s with DIS & THR connected internally, and TR is falling edge sensitive instead of level sensitive).

2.2.1 The 555 has three operating modes

  • Monostable mode: in this mode, the 555 functions as a “one-shot” pulse generator. Applications include timers, missing pulse detection, bounce free switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) and so on.

Schematic of a 555 in monostable mode

  • Astable: free running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation and so on. Selecting a thermostat as timing resistor allows the use of the 555 in a temperature sensor: the period of the output pulse is determined by the temperature. The use of a microprocessor based circuit can then convert the pulse period to temperature, linearize it and even provide calibration means.

Schematic of a 555 in Astable mode

  • Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bounce-free latched switches.

These specifications apply to the NE555. Other 555 timers can have different specifications depending on the grade (military, medical, etc.).

Supply voltage (VCC) 4.5 to 15 V
Supply current (VCC = +5 V) 3 to 6 mA
Supply current (VCC = +15 V) 10 to 15 mA
Output current (maximum) 200 mA
Maximum Power dissipation 600 mW
Power consumption (minimum operating) 30 mW@5V, 225 mW@15V
Operating temperature 0 to 70 °C

2.2.2 Pin out diagram

Fig: Pinout diagram

The connection of the pins for a DIP package is as follows:

Pin Name Purpose
1 GND Ground, low level (0 V)
2 TRIG OUT rises, and interval starts, when this input falls below 1/3 VCC.
3 OUT This output is driven to +VCC or GND.
4 RESET A timing interval may be interrupted by driving this input to GND.
5 CTRL “Control” access to the internal voltage divider (by default, 2/3 VCC).
6 THR The interval ends when the voltage at THR is greater than at CTRL.
7 DIS Open collector output; may discharge a capacitor between intervals.
8 V+, VCC Positive supply voltage is usually between 3 and 15 V.

2.3 Relay coil

Fig: Relay SPCO 12V coil

Relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contractor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults

2.3.1 Operation of relay

A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.

When an electric current is passed through the coil it generates a magnetic field that activates the armature and the consequent movement of the movable contact either makes or breaks (depending upon construction) a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing.

2.3.2 Relays are used to and for

  • Operating environment – minimum and maximum operating temperatures and other environmental considerations such as effects of humidity and salt
  • Assembly – Some relays feature a sticker that keeps the enclosure sealed to allow PCB post soldering cleaning, which is removed once assembly is complete.
  • Mounting – sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
  • Switching time – where high speed is required
  • “Dry” contacts – when switching very low level signals, special contact materials may be needed such as gold-plated contacts
  • Contact protection – suppress arcing in very inductive circuits
  • Coil protection – suppress the surge voltage produced when switching the coil current
  • Isolation between coil circuit and contacts
  • Aerospace or radiation-resistant testing, special quality assurance
  • Expected mechanical loads due to acceleration – some relays used in aerospace applications are designed to function in shock loads of 50 g or more
  • Accessories such as timers, auxiliary contacts, pilot lamps, test buttons
  • Regulatory approvals
  • Stray magnetic linkage between coils of adjacent relays on a printed circuit board.

2.4 Magnetic field

Fig: Magnetic field

A magnetic field is generated when electric charge carriers such as electrons move through space or within an electrical conductor. In other word magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic field tensor; the aspect of the electromagnetic field that is seen as a magnetic field is dependent on the reference frame of the observer. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons

2.4.1 Magnetic field due to moving charges and electric currents

All moving charged particles produce magnetic fields. Moving point charges, such as electrons, produce complicated but well known magnetic fields that depend on the charge, velocity, and acceleration of the particles. Magnetic field lines form in concentric circles around a cylindrical current-carrying conductor, such as a length of wire. The direction of such a magnetic field can be determined. The strength of the magnetic field decreases with distance from the wire. A current-carrying wire into a loop concentrates the magnetic field inside the loop while weakening it outside. Bending a wire into multiple closely spaced loops to form a coil. A device so formed around an iron core may act as an electromagnet, generating a strong, well-controlled magnetic field. An infinitely long cylindrical electromagnet has a uniform magnetic field inside, and no magnetic field outside. A finite length electromagnet produces a magnetic field that looks similar to that produced by a uniform permanent magnet, with its strength and polarity determined by the current flowing through the coil. The magnetic field generated by a steady current I (a constant flow of electric charges in which charge is neither accumulating nor depleting at any point)<href=”#cite_note-ex12-25″>[nb 12] is described by the Biot–Savart law:

where the integral sums over the wire length where vector d? is the direction of the current, ?0 is the magnetic constant, r is the distance between the location of d? and the location at which the magnetic field is being calculated, and r? is a unit vector in the direction of r. relating the current I to the B-field is through Ampere’s law:

2.4.2 Force on moving charges and current

A charged particle moving in a B-field experiences a sideways force that is proportional to the strength of the magnetic field, the component of the velocity that is perpendicular to the magnetic field and the charge of the particle. This force is known as the Lorentz force, and is given by

Where F is the force, q is the electric charge of the particle, v is the instantaneous velocity of the particle, and B is the magnetic field (in teslas).

The Lorentz force is always perpendicular to both the velocity of the particle and the magnetic field that created it. When a charged particle moves in a static magnetic field it will trace out a helical path in which the helix axis is parallel to the magnetic field and in which the speed of the particle will remain constant. Because the magnetic force is always perpendicular to the motion, the magnetic field can do no work on an isolated charge. It can only do work indirectly, via the electric field generated by a changing magnetic field. It is often claimed that the magnetic force can do work to a non-elementary magnetic dipole, or to charged particles whose motion is constrained by other forces, but this is incorrect<href=”#cite_note-Deissler-28″>[16] because the work in those cases is performed by the electric forces of the charges deflected by the magnetic field.

2.5 Resistor

Fig: Resistor

A resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element. The current through a resistor is in direct proportion to the voltage across the resistor’s terminals. Thus, the ratio of the voltage applied across a resistor’s terminals to the intensity of current through the circuit is called resistance. This relation is represented by Ohm’s law:

Resistors are common elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel-chrome). Resistors are also implemented within integrated circuits, particularly analog devices, and can also be integrated into hybrid and printed circuits.

2.5.1 Power dissipation

The power P dissipated by a resistor (or the equivalent resistance of a resistor network) is calculated as:

The first form is a restatement of Joule’s first law. Using Ohm’s law, the two other forms can be derived.

The total amount of heat energy released over a period of time can be determined from the integral of the power over that period of time:

Practical resistors are rated according to their maximum power dissipation. The vast majority of resistors used in electronic circuits absorbs much less than a watt of electrical power and require no attention to their power rating. If the average power dissipated by a resistor is more than its power rating, damage to the resistor may occur, permanently altering its resistance; this is distinct from the reversible change in resistance due to its temperature coefficient when it warms. Excessive power dissipation may raise the temperature of the resistor to a point where it can burn the circuit board or adjacent components, or even cause a fire. There are flameproof resistors that fail (open circuit) before they overheat dangerously.

2.6 Capacitor

Fig: Capacitor

A capacitor (formerly known as condenser) is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric (insulator); for example, one common construction consists of metal foils separated by a thin layer of insulating film. Capacitors are widely used as parts of electrical circuits in many common electrical devices. When there is a potential difference (voltage) across the conductors, a static electric field develops across the dielectric, causing positive charge to collect on one plate and negative charge on the other plate. Energy is stored in the electrostatic field. An ideal capacitor is characterized by a single constant value, capacitance, measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them.

2.6.1 Theory of Operation

A capacitor consists of two conductors separated by a non-conductive region.<href=”#cite_note-Ulaby_p168-7″>[8] The non-conductive region is called the dielectric. In simpler terms, the dielectric is just an electrical insulator. Examples of dielectric mediums are glass, air, paper, vacuum, and even a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from any external electric field. The conductors thus hold equal and opposite charges on their facing surfaces,<href=”#cite_note-Ulaby_p157-8″>[9] and the dielectric develops an electric field. In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage of one volt across the device.<href=”#cite_note-Ulaby_p169-9″>[10]

The capacitor is a reasonably general model for electric fields within electric circuits. An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of charge ±Q on each conductor to the voltage V between them:<href=”#cite_note-Ulaby_p168-7″>[8]

Sometimes charge build-up affects the capacitor mechanically, causing its capacitance to vary. In this case, capacitance is defined in terms of incremental changes:

2.7 Zener Diode

Fig: Zener Diode

Zener Diode- a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown volt . A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by circuitry, the diode will be permanently damaged due to overheating. In the case of a large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations. A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage

2.8 Reed switch

Fig: Reed Switch

The reed switch is an electrical switch operated by an applied magnetic field. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied. The switch may be actuated by a coil, making a reed relay or by bringing a magnet near to the switch. Once the magnet is pulled away from the switch, the reed switch will go back to its original position. A magnetic field (from an electromagnet or a permanent magnet) will cause the reeds to come together, thus completing an electrical circuit. The stiffness of the reeds causes them to separate, and open the circuit, when the magnetic field ceases. Another configuration contains a non-ferrous normally-closed contact that opens when the ferrous normally-open contact closes. Good electrical contact is assured by plating a thin layer of non-ferrous precious metal over the flat contact portions of the reeds; low-resistivity silver is more suitable than corrosion-resistant gold in the sealed envelope. There are also versions of reed switches with mercury “wetted” contacts. Such switches must be mounted in a particular orientation otherwise drops of mercury may bridge the contacts even when not activated.

2.9 Push-switch

Fig: Push-switch

A push switch is a momentary or non-latching switch which causes a temporary change in the state of a electrical circuit only while the switch is physically actuated. An automatic mechanism (i.e. a spring) returns the switch to its default position immediately afterwards, restoring the initial circuit condition. There are two types:

  • A push to make switch allows electricity to flow between its two contacts when held in. When the button is released, the circuit is broken.
  • A push to break switch does the opposite, i.e. when the button is not pressed, electricity can flow, but when it is pressed the circuit is broken.


Background Study

3.1 Block Diagram

Fig: Block Diagram of Railway Signal System

3.2 Circuit Diagram

Fig: Circuit Diagram

3.2.1 How does it actually Work

  • Connect the reed switches to push-switches A and B (see the strip board layout).
  • The switches can be held in place between the rails with a small piece of blu tac.
  • Connect the track wires to the COM and NC contacts of the relay.
  • When soldering to the track makes sure you solder to the outside of the rail.
  • Each locomotive will need a miniature magnet glued to its underside – test first with blu tac, but superglue is probably best once you are sure it is in the correct position.
  • Note that railway signals have red at the bottom, unlike road traffic lights where red is at the top.


Project Overview:

Fig: Circuit diagram Fig: Strip board layout

Fig: Track connections

Fig: Railway Line

Fig: Strip Board Layout


Railway signal

A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal’s indication and acts accordingly. Typically, a signal might inform the driver of the speed at which the train may safely proceed or it may instruct the driver to stop.

Application and positioning of signals

Originally, signals displayed simple stop/proceed indications. As traffic density increased, this proved to be too limiting and refinements were added. One such refinement was the addition of distant signals on the approach to stop signals. The distant signal gave the driver warning that he was approaching a signal which might require a stop. This allowed for an increase in speed, since trains no longer needed to be able to stop within sighting distance of the stop signal.

Under timetable and train order operation, the signals did not directly convey orders to the train crew. Instead, they directed the crew to pick up orders, possibly stopping to do so if the order warranted it.

5.1 Signals are used to indicate one or more of the following

  • That the line ahead is clear (free of any obstruction) or blocked.
  • That the driver has permission to proceed.
  • That points (also called switch or turnout in the US) are set correctly.
  • Which way points are set.
  • The speed the train may travel.
  • The state of the next signal.
  • That the train orders are to be picked up by the crew.

5.2 Signals can be placed

  • At the start of a section of track.
  • On the approach to a movable item of infrastructure, such as points/switches or a swing bridge.
  • In advance of other signals.
  • On the approach to a level crossing.
  • At a switch or turnout.
  • Ahead of platforms or other places that trains are likely to be stopped.
  • At train order stations.

5.3 Aspects and indications

Signals have aspects and indications. The aspect is the visual appearance of the signal; the indication is the meaning. In American practice the indications have conventional names, so that for instance “Medium Approach” means “Proceed at not exceeding medium speed prepared to stop at next signal”. Different railroads historically assigned different meanings to the same aspect, so it is common as a result of mergers to find that different divisions of a modern railroad may have different rules governing the interpretation of signal aspects

It is important to understand that for signals that use colored aspects, the color of each individual light is subsumed in the overall pattern. In the United States, for example, it is common to see a “Clear” aspect consisting of a green light above a red light. The red light in this instance does not indicate “Stop”; it is simply a component of a larger aspect. Operating rules normally specify that when there is some imperfection in the display of an aspect (e.g., an extinguished lamp), the indication should be read as the most restrictive indication consistent with what is displayed.

5.4 Position light signals

A position light signal is one where the position of the lights, rather than their color, determines the meaning. The aspect consists solely of a pattern of illuminated lights, which are all of the same color (typically amber or white). In many countries, small position light signals are used as shunting signals, while the main signals are of color light form. Also, many tramway systems (such as the Metro of Wolverhampton) use position light signals.

On the Pennsylvania Railroad (PRR) as on other railroads, initial efforts were made to replace the semaphore with illumination of the position of the blade rather than by color lamps alone. This had the advantage of eliminating any and all effects of even slight color blindness by the train crew. Lamps with inverted half toxic optic lenses, covered with a light yellow tinted conical cover glass with a frosted tip to avoid phantom indications were displayed in rows of three, corresponding to the positions of a semaphore blade. Multiple signal heads were used at interlocking where four aspects did not suffice. The PRR chose to use their Superintendent of Signaling, A.H.Rudd’s, in-house developed position light signals to both replace the semaphores and their moving parts, also because the intense lemon yellow light provided superior visibility in adverse weather conditions such as rain or fog.



6.1 Components used & cost

Name of the component Number used Price (Tk)
1. Resistors 4 4tk
2. Capacitors 1 10tk
3. 1N4001 diode 1 5tk
4. 1N4148 diode 1 5tk
5. Red LED (3mm best) 1 1tk
6. Green LED (3mm best) 1 1tk
7. 555 timer IC 1 6tk
8. 8-pin DIL socket for IC 1 10tk
9. Push-switch ×2 2 20tk
10. Reed switch ×2 2 80tk
11. Relay SPCO 12V coil 1 35tk
12. Stripboard 11 rows × 24 holes 1 50tk
TOTAL 15 217 tk

6.2 Advantage

# Low cost

# Better security

# Automatic & manual both controlled

6.3 Future Improvement

• We can improve it to the whole railway system like level crossing, line crossing.

• We can improve the security section of the system.

• We can improve it as a wireless project.

6.4 References