bell circuits and indicators

TYPES OF BELLS AND THEIR DESCRIPTION 

To meet the different requirements electric bells of various types are manufactured. Some of those most commonly used are given below: 

• Vibrating bell.
• Single stroke bell. 
• Continuous ringing bell. 
• Combination of vibrating and single stroke bell. 
• Bracket bell. The types of bells which are most commonly used described below: 

1. Vibrating Bell:- It is most commonly used house bell. It consists of two electromagnets, metal projection provided with ad- justing screw, pivot armature, hammer, gong, frame and a contact post etc. When the push button is pressed, the current is passed through the electromagnets to the adjusting screw, down through the armature by means of contact spring and then to the source of supply. When this condition occurs, the current energies the magnets which attract the armature, causing the hammer to strike the bell, but before it reaches the end of the stroke, the contact breaker breaks the circuit. As soon as the circuit is de-energised, the hammer of armature moves back to the original position, thus again closing the circuit. This process is repeated again and again producing vibration of the hammer as long as the push button remains pressed. 

2. Single Stroke Bell:- It is a bell in which the hammer strikes the gong only once, each time the circuit is closed or push button is pressed. It consists of following major parts: 
Two electromagnets (solenoid coils) 

Pivoted armature 

Hammer 

Gong

Iron Frame 

In the single stroke bell, the current flows through the solenoid coils not through the armature. When the push button is pressed for operat- ing the bell, the current is sent through the electromagnets (produced in the solenoid coils) which are connected directly across the terminals 

and energies them. It causes the armature to be attracted towards the iron cores of solenoid coils. The hammer when attracted by electromag. nets, strikes the gong. The armature remains in the attracted position till the push button remains pressed. AS soon as the push button js released, the electromagnets are de-energised causing the hammer to return to its original position with the help of spring power. The action is repeated as and when the push button is pressed. This bell may not be used as a means for providing an acknowledged call or alarm but may be used for the purpose of giving a signal or code of signals such as are required in various types of telegraph and railway signalling apparatus. 

3. Continuous Ringing Bell:- When the push button for operating the bell is pressed, it continues to ring, until its local battery is fully discharged or until it is silenced purposely by pulling down of a cord or chain attached to it. Sometimes, a switch like object is attached near the bell to silence it. This type of bell is used for giving alarms in special circumstances. The above shows detail of parts in the continuous ringing bell alongwith its electrical circuit. The bell consists of parts which are in an ordinary vibrating bell. A few more parts are provided which help to make it continuous ringing. The circuit is completed through solenoid coil, flat spring, armature and adjusting contact screw. The armature is also connected to battery through trigger, contact screw and terminal. 

When push button is pressed, the electromagnet is magnetised which attracts the soft iron armature towards it. During the process, the end of trigger is lifted up by spring action which comes in contact with the contact screw which is further connected to battery. The current which first flows through the solenoid coils as in normal cases is now cut-off owing to the break in the contact. A new path is thus made upon such that the current flows from pivot to the contact breaker via the trigger to the supply mains. Thus the bell continues to ring until the cord operating the trigger is pulled down, so that the later engages with the projection on the contact breaker. 

Bell Transformer 

The bells for office and domestic purposes are designed for 230 volts AC without using any step-down transformers. Some electric bells are designed to operate on 6,9 or 12 volts which can be either obtained from battery or by step down transformer. 

The circuit is easy to understand and operate. If low voltage bell is to be used for domestic purpose supply conductors are extended up to cut-out and from there, the leads are connected to primary winding of the bell transformer. The leads for bell or other equipment are taken from secondary which provides reduced voltage. 

Simple Alarm Circuits Without Relay

The alarm circuits con- nection are used to convey information by means of light or bell signals. The basic principle of wiring connection of bells is same as the lighting circuits. In general practice, the neutral is given direct to bells where as phase is supplied through push button. The alarm of signalling system usually installed in small houses may be divided into 

two general classifications. The first is the audible or a visual type of signalling system which attracts the attention by means of sound, given off by an electric bell or buzzer, or by a visual signal such as the glow of a coloured electric lamp. The second is voice transmission signalling system represented by telephone, by means of which one person may converse with another at any distance. The simple wiring diagram of electric bell, apart from wiring necessary to conduct the electric current on its path consists of three important parts: 

1. Usable load (in this case, a bell). 

2. Means of control (in this case, a push button). 

3. A source of supply (in this case a battery - or 230 V a.c. supply) 

The connections to the bell are made such that the positive path of current of the battery travels through push button to the bell. The negative pole of the battery is connected to other terminal of the bell, completing the path of current through the bell. The bell push, also known as push button is in series with battery and bell. No current will flow through the circuit until the push button is pressed. 

Types of Diagrams 

There are two ways of drawing alarm circuits, (a) The schematic diagram or circuit diagram which is an explanatory diagram for easy understanding of details of operation of an electric circuit. (b) The wiring diagram showing connections actually applied and the layout of the wiring. 

1. Draw a wiring diagram, single line representation and schematic diagram of one bell controlled by two push buttons located at two different places. The bell should ring by pressing any of the two buttons. Looping system of wiring should be adopted. This circuit is useful when there are two officers to be attended by one peon. The bell should ring by pressing any of the two push buttons located in two different offices. However, the peon cannot know, which of the two officers has called him. In these circumstances, the peon can be made to understand only if different modes of sound are adopted by the officers. 

The phase wire first enters push buttons No.1 and from there it is looped-in to push button No. 2.The terminals from a push button No. 1 and push button No. 2 is connected to the bell. The bell should ring when either of the two push buttons is pressed.

2. Draw a wiring diagram, single line diagram and schematic diagram of two electric bells controlled by their respective push buttons located at two different places, such that the bell should ring when its respective push button is pressed. Looping system of wiring should be adopted. This is a very common circuit used in offices. The circuit is useful when there are two officers having their individual bell pushes and bells to call the peon or to pass on some information through bell signal system. The basic idea of this circuit is to create two signal systems from one supply. 

The phase wire is taken to one push button P1, from Pi it is looped-in to P2. The second terminal of Pi is connected to its own bell and similarly second terminal of P2 is connected to its own bell-2. The bell-1 should ring when P1 is pressed and Bell-2 should when Pa is pressed. The two push buttons and bells may be located at different places. Each bell thus has its individual control. 

3. Draw a wiring diagram, single line representation and a schematic diagram to control three bells connected in series with three push buttons located at different places. All three bells should ring simultaneously when any of the three push buttons is pressed. Adopt joint box system of wiring. The circuit may be very useful when some information is to be conveyed through bell signals simultaneously at three different places from either of the three different locations. For instance, one push button is provided in officer's office, the other in assistant's office and the third one in the time keeper's office, 

The bells are similarly located one each in three different sections of the office or industry. When ever some information is to be conveyed through bell sigmals i.e. observance of recess timings, interval etc., it will be conveyed from time keeper office. Some special information for instance for calling three section incharges etc. may be conveyed from officer's office. Some other similar information may have to be conveyed by assistant by pressing a push button from his office. The circuit can also be used for similar other purposes. 

4. Draw a bell circuit in three different ways to control two bells (located at different places) operated by their respective push buttons. There should be a provision to temporarily put out of circuit one of the bells by adding a switch. Adopt looping system of wiring. Two push buttons are located in the officer's office to call e assistant and peon by call signals. 

When push button Pi is pressed, the Bell Bi in the assistant's office rings to provide a call to him by the Three bells in series to ring together when any of the three push buttons is pressed (loint box system of wiring). Two bells each controlled independently by two push buttons with provision to temporanily dislocate one bell when required. officer. 

The push button P2 is pressed for calling the peon by ringing of the bell B2. When peon is to attend some other side, he will dislocate the bell by a switch before going, so that the bell does not ring uneces- sarily in his absence. When he comes back, he again puts the switch to ON position to attend to that office again.

5. Draw a bell indicating circuit in which an officer can call any one of his five subordinates sitting at different places by pressing a common push button after adjusting a selector switch installed by the side of the push button. 

The bell of the subordinate room should ring only for the period for which the push button remains pressed. A four terminal selector switch is used, one each for an individual bell. A push button is provided before the selector switch i.e. the phase will first enter into push button and then connected to incoming terminal of the selector switch. The bells are installed in different rooms of the building to provide signals. For example, the person wants to ring bell B2 he will first set the moving contact of the selector switch to No. 2 terminal and then press the push button, the bell B2 will ring. Similarly any particular bell can be operated by setting the moving contact on its respective contact and pushing the push buttons. Several bells say 10 or even more can be controlled from one push button by using selector switch having out going terminal equal to the number of bells installed. A plan of an office comprising four rooms and a verandah The installation of wiring of bells and selector switch is also shown in single line representation. 

Any of the various bells located at different places can be operated by setting the selector switch to that particular number and pressing the bell push installed near to the selector Single line and wiring diagram of seloctor switch circuit. The selector switch is installed in the manager's office from where he can call his attendant, typist, cashier or head clerk to his office by operating selector switch and pressing a single push button.