15. Signal Circuits

SIGNAL CIRCUITSSIGNAL CIRCUITS

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Starate CameronStarate Cameron

What Is An Electric Bell What Is An Electric Bell ►An electric bell is a mechanical bell that

functions by means of an electromagnet. When an electric current is applied, it produces a repetitive buzzing or clanging sound. Electric bells have been widely used at railroad crossings, in telephones, fire and burglar alarms, as school bells, doorbells, and alarms in industrial plants, but they are now being widely replaced with electronic sounders.

Single stroke BellSingle stroke Bell► The bell is made up of two coils, usually wound The bell is made up of two coils, usually wound

with varnish-insulated or cotton insulated with varnish-insulated or cotton insulated copper wire, wound on an insulated bobbin.copper wire, wound on an insulated bobbin.

► The bobbins are fitted on to a U-shaped soft The bobbins are fitted on to a U-shaped soft iron core and the magnetic circuit is completed iron core and the magnetic circuit is completed by a soft-iron strip riveted to a spring loaded by a soft-iron strip riveted to a spring loaded armature.armature.

► The armature carries a striker which hits the The armature carries a striker which hits the

gong at the end of the movement.gong at the end of the movement.

Single stroke BellSingle stroke Bell

Trembler Bell QuestionsTrembler Bell Questions

http://en.wikipedia.org/wiki/File:Electric_Bell_animation.gif

Chimer Chimer

►The double-note chimer is a variation The double-note chimer is a variation of single-stroke bell. Tubular chimes of single-stroke bell. Tubular chimes replace the gong and the second replace the gong and the second chime is struck by the rebound of the chime is struck by the rebound of the striker. striker.

Trembler BellTrembler Bell

► This type of bell in This type of bell in construction to the single construction to the single stroke bell, with the stroke bell, with the exception of a set of exception of a set of variable contacts at the variable contacts at the armature which are used armature which are used to break the circuit when to break the circuit when the armature is drawn the armature is drawn towards the towards the electromagnetic. The electromagnetic. The contact points are contact points are generally made of German generally made of German silver to minimize the silver to minimize the effect of the continual effect of the continual arcing during operation.arcing during operation.

Principle of OperationPrinciple of Operation► When the push-button is When the push-button is

operated the coils from an operated the coils from an electromagnetic and electromagnetic and attract the soft-iron strip.attract the soft-iron strip.

► The movement of the The movement of the armature to the coils armature to the coils opens the contacts.opens the contacts.

► The coil is then The coil is then demagnetized and the demagnetized and the spring pulls the armature spring pulls the armature back to its original back to its original position.position.

► The cycle is then repeated, The cycle is then repeated, giving a continual make –giving a continual make –and-break action.and-break action.

►The speed of the stroke can be varied The speed of the stroke can be varied by varying the contact screw. The by varying the contact screw. The buzzer is similar in construction to the buzzer is similar in construction to the tremble bell, the only difference being tremble bell, the only difference being the absence of a striker and Gong.the absence of a striker and Gong.

Continuous Ringing BellContinuous Ringing Bell ►Construction (fig 8.3). The continuous Construction (fig 8.3). The continuous

ringing bell differs in construction from ringing bell differs in construction from the trembler bell in that small lever the trembler bell in that small lever place below the contact screw. This place below the contact screw. This lever drops automatically on the first lever drops automatically on the first stroke of the bell, and as it drops it stroke of the bell, and as it drops it shorts out the bell-push, thus causing shorts out the bell-push, thus causing the bell to ring continuously until the the bell to ring continuously until the lever is reset by the cord. Confusion can lever is reset by the cord. Confusion can arise in the drawing of this circuit if the arise in the drawing of this circuit if the operation of the lever is not understood.operation of the lever is not understood.

Principle of OperationPrinciple of Operation

►This is as follows.This is as follows.►When the bell-push is operated the coils When the bell-push is operated the coils

become an electromagnet and the become an electromagnet and the armature is drawn towards them. armature is drawn towards them.

►The lever drops on the first stroke and The lever drops on the first stroke and shorts push.shorts push.

►The bell now operates as a trembler bell.The bell now operates as a trembler bell.►To stop the bell pull the cord, thus To stop the bell pull the cord, thus

resetting the lever.resetting the lever.

What is a RelayWhat is a Relay►The term Relay generally refers to a device that provides The term Relay generally refers to a device that provides

an electrical connection between two or more points in an electrical connection between two or more points in response to the application of a control signal.response to the application of a control signal. It is an It is an electrical switch that opens and closes under the control of electrical switch that opens and closes under the control of another electrical circuit.another electrical circuit.

► The most common and widely used type of electrical relay The most common and widely used type of electrical relay is the electromechanical relay or EMR is the electromechanical relay or EMR that is operated by that is operated by an electromagnet to open or close one or many sets of an electromagnet to open or close one or many sets of contacts.contacts.

Relay ConstructionRelay Construction

Relay ConstructionRelay Construction

Operation of the RelayOperation of the Relay►A relay has at least two circuits, one to be A relay has at least two circuits, one to be

used to control the other. The 1st circuit used to control the other. The 1st circuit (input circuit) supplies current to the (input circuit) supplies current to the electromagnet that magnetized and electromagnet that magnetized and attracts one end of the iron armature. attracts one end of the iron armature.

►The armature then closes the contacts The armature then closes the contacts (2nd switch) and allows current flows in the (2nd switch) and allows current flows in the second circuit. When the 1st switch is open second circuit. When the 1st switch is open again, the current to the electromagnet is again, the current to the electromagnet is cut, the electromagnet loses its magnetism cut, the electromagnet loses its magnetism and the 2nd switch is opened. Thus current and the 2nd switch is opened. Thus current stop to flow in the 2nd circuit.stop to flow in the 2nd circuit.

Reed SwitchReed Switch

Reed SwitchReed Switch

Alarm Circuit Alarm Circuit

►There are two types of alarm circuit:There are two types of alarm circuit:► (a) the closed circuit; and(a) the closed circuit; and► (b) the open circuit.(b) the open circuit.► Both types are made up of two Both types are made up of two

distinct sections:distinct sections:►The battery-operated coil circuit. The battery-operated coil circuit. ►The mains-operated bell circuit which The mains-operated bell circuit which

is operated by the relay contacts.is operated by the relay contacts.

Closed Burglar Alarm Closed Burglar Alarm SystemSystem

Closed Burglar Alarm Closed Burglar Alarm SystemSystem

► This Closed burglar alarm system shows that Circuit (1) is a series circuit which is operated when one of the special switches fitted on doors and windows is opened. All switches are connected in series.

Note. The relay coil in this circuit is Note. The relay coil in this circuit is normally normally energized: any break in energized: any break in this circuit (for this circuit (for example, the cutting example, the cutting of a wire) will:of a wire) will:

1.) Open the circuit. 1.) Open the circuit. 2.) Demagnetize the coil. 2.) Demagnetize the coil. 3.) Cause the contacts to come 3.) Cause the contacts to come together, together, thus operating the alarm thus operating the alarm bell.bell.

Closed Burglar Alarm System

Open Alarm Circuit.Open Alarm Circuit.

►Fig. 8.11 shows an open alarm circuit. The relay coil is only energized when one of the switches is closed. All the switches are connected in parallel. The drawback with this circuit is that it becomes ineffective if any wire is cut (the opposite from a closed circuit system), or if a bad connection occurs in the closed circuit.

FIRE ALARM BELLSFIRE ALARM BELLS

FIRE ALARM BELLSFIRE ALARM BELLS

►The above circuit is generally used for Fire The above circuit is generally used for Fire alarm bells and they are divided into two alarm bells and they are divided into two categories: vibrating, and single-stroke. categories: vibrating, and single-stroke. The only difference is the use of special The only difference is the use of special heat-operated switches such as: heat-operated switches such as:

1)1) Bending of bimetallic strip cause by heat Bending of bimetallic strip cause by heat and close two contacts.and close two contacts.

2)2) Expansion or air in a cylinder, due to Expansion or air in a cylinder, due to temperature and close two contacts. temperature and close two contacts.

The TelephoneThe Telephone

►A telephone is a device that conducts a A telephone is a device that conducts a conversation from one person to other conversation from one person to other persons to be heard directly.persons to be heard directly.

►A telephone converts sound into A telephone converts sound into electronic signals suitable for electronic signals suitable for transmissions via cables or other transmissions via cables or other transmission media over long distances, transmission media over long distances, and replays such signals simultaneously and replays such signals simultaneously in audible form to its user. in audible form to its user.

The TelephoneThe Telephone

Telephone Principle of Telephone Principle of OperationOperation

►If the resistance of the variable resistor If the resistance of the variable resistor is varied, the current flowing in the is varied, the current flowing in the circuit will vary (ohms law).circuit will vary (ohms law).

►These variations in current will cause These variations in current will cause variation in the strength of the variation in the strength of the electromagnet.electromagnet.

►Variation in the strength of the magnetic Variation in the strength of the magnetic field will vary the ‘pull’ on the soft-iron field will vary the ‘pull’ on the soft-iron diaphragm (or disc). The disc can be diaphragm (or disc). The disc can be made to vibrate by varying the made to vibrate by varying the resistance in the circuit.resistance in the circuit.

Simple telephone circuitSimple telephone circuit

►In the simplest telephone circuit a In the simplest telephone circuit a transmitter (or microphone) takes the transmitter (or microphone) takes the place of the variable resistor. place of the variable resistor. Variations in resistance are caused by Variations in resistance are caused by the pressure of sound waves from the the pressure of sound waves from the voice acting on layer of carbon voice acting on layer of carbon granules, thus varying the current in granules, thus varying the current in the circuit. The electromagnet of the the circuit. The electromagnet of the receiver changes these variation in receiver changes these variation in current back into sound waves by current back into sound waves by attracting and repelling the soft-iron attracting and repelling the soft-iron diaphragm. Soft iron is used because diaphragm. Soft iron is used because it does nit retain magnetism.it does nit retain magnetism.

Construction of a Construction of a transmitter transmitter

Construction of a receiverConstruction of a receiver

ConstructionConstruction►The transmitter consist of a sealed The transmitter consist of a sealed

chamber containing carbon granules chamber containing carbon granules (powered Carbon). The cone-shaped (powered Carbon). The cone-shaped diaphragm presses on the granules diaphragm presses on the granules which are placed between two which are placed between two electrodes. electrodes.

►The receiver contains the following:The receiver contains the following:

Soft-iron diaphragm and two coils Soft-iron diaphragm and two coils (connected in series) which are wound (connected in series) which are wound round a U-shaped permanent magnet.round a U-shaped permanent magnet.

Two-positioned Telephone Two-positioned Telephone Circuit. Circuit.

The circuit of a simple Telephone used for communicating over short distances.

Two-positioned Telephone Two-positioned Telephone Circuit. Circuit.

Induction CoilInduction Coil

Induction CoilInduction Coil► An induction coil is used to ‘boost’ the An induction coil is used to ‘boost’ the

speech waves in long distance circuits speech waves in long distance circuits where the line resistance is high. The where the line resistance is high. The induction coil is a double-wound step-up induction coil is a double-wound step-up transformer with a primary winding of a transformer with a primary winding of a few turns and a multi-wound secondary. few turns and a multi-wound secondary. The fluctuating current in the primary, The fluctuating current in the primary, due to the speech waves, induces a due to the speech waves, induces a current into the secondary at a higher current into the secondary at a higher voltage, thus minimizing the effect of voltage, thus minimizing the effect of high resistance in the circuit. high resistance in the circuit.