Electric Bells and Alarms

A simple bell circuit consists of a push-button and dry batteries. When the circuit is ‘made’ by pressing the pushbutton P the current flows from the batteries through the coils and the contact breaker B. This sets up a magnetic field about the magnet poles and attracts the armature A which breaks the circuit at B and at the same time causes the hammer to strike the gong. The break in the circuit cuts off the magnetic field and allows the armature to spring back and resume the position of contact at B. This cycle is repeated rapidly and continuously until pressure on the push-button ceases.

Adjustment

The only adjustment to be made is by turning a screw at contact breaker B. If this screw is unscrewed far enough to leave a gap between the contact points, no current can flow and the bell will not ring. If, on the other hand, it is screwed in too fai the contacts will not part when the armature is drawn to the magnet poles and will remain jammed. The screw should be adjusted, while the push-button is being pressed, to a position between these two extremes to give the most satisfactory ringing. There is usually a locking screw to keep the contact screw in adjustment and this should be tightened when a satisfactory adjustment has been made.

Power Supply

The power for ringing bells may be derived from wet or dry batteries or A.C. House mains. For general convenience, bell circuits are designed to operate at low voltage, and one or two dry batteries are sufficient for most house circuits. In cases where the wiring run is very long, more batteries may be needed to cope with the voltage drop in the line.

The batteries are wired in series, I.e. the tail of the first is connected to the centre terminal of the second, tail of the second to centre terminal of the third, and so on. The odd terminals at the end of the series, one tail and one centre terminal, connect up to the two wires from the bell. One of these two wires is cut at some convenient point, the two severed ends being attached to the terminals in the bell-push.

The batteries may be positioned at any convenient point in the circuit, generally on a shelf near the bell. Modern batteries have a long life but must necessarily be replaced from time to time, and it is cheaper in the long run to use the A.C. Mains supply if available.

For this purpose a small transformer is utilized to step down the mains voltage of 200/240 A.C. To the low voltage required for the bell circuit. The transformer has a primary winding which is permanently connected to the mains through fuses, and a secondary winding with tappings to give, at option, 4, 6, or S volts, in some cases, 12 volts, to feed the bell circuit. No current is consumed except when the bell is ringing and even then the consumption is negligible.

Wiring

The transformer should be fixed at the nearest convenient point to the fuse box carrying the house lighting circuits, and wired into the selected pair of fuses with vulcanized India rubber.

Twin bell wire is used for the low tension circuit to the pushbutton and it is usual to carry this on the top of skirting boards or picture rails, fixing it at suitable points with insulated staples.

The voltage required to operate the ringing circuit will depend partly on the bell windings and partly on the length and resistance of the circuit. The correct transformer tapping can be found by trial. The array of terminals on the transformer may be somewhat confusing at first sight. Figs. 4 and 5 show two typical arrangements of terminals and the voltage to be obtained between various pairs. The primary winding will be marked either ‘Mains’, ‘Primary’ or ‘200/240 volts A.C,’, and the secondary winding will have three or more terminals giving various voltages from 2 to 12.

Multi-Point Circuits

If it is desired to install a bell that can be rung from any number of pushbuttons in different parts of the house, an indicator panel should be provided to show the point of origin of any particular call. A diagrammatic circuit of such an arrangement is, but the layout of the wiring must be governed by the disposition of the points.

In the diagram************ the push-buttons, marked thus (P), are numbered to correspond with their respective indicator posif’ins. Tracing the provided with switches of the normally closed type, in contrast to the normally open type shown in the previous system. These switches are all connected in series with a magnetic relay. In principle, the relay is a switch actuated by an electromagnet. The current flowing in the electromagnet maintains the switch contacts in the open position, and.as soon as the current is cut off the contacts close under spring tension. These contacts are wired in the secondary circtiit and operate the alarm bell.

The current consumption of the relay is very low, but since it must be continuously energized for long periods while protection is required, it is advisable to supply it from the mains through a transformer. The bell should be supplied from a battery and not the mains, otherwise the whole system could be defeated by deliberate interference with the mains on the part of an inside confederate. If such a contingency is possible, the bell, battery, and relay must be housed in a cupboard that is itself protected by the system. As it stands, the system is still not perfect, since a failure of the power supply would release the relay and give rise to a false alarm and cause unnecessary consternation. This defect can be overcome if desired by feeding the relay circuit from an accumulator instead of the mains transformer. The same accumulator would serve to supply the bell. It would be necessary to install a trickle charger to keep the accumulator charged from the mains. A trickle charger can be made with a suitable transformer and rectifier but it is beyond the scope of this article to give details of its design. Circuit from any selected pushbutton, it will be seen that current flows to the bell via its associated coil on the indicator panel. By this means, the coil is energized simultaneously with the bell and causes a white disk to flutter at the numbered indicator window below it. When the call is answered, the disk is restored to normal by hand, or drops to normal position automatically.

Burglar Alarms

A simple bell circuit may be installed as a burglar alarm, and I special concealed switches are obtainable for this purpose for fitting to door jambs, window sashes, etc. In situations where the wiring can be laid out of reach of an intending thief’s wire cutters.

The master switch shown adjacent to the battery is a single pole switch such as used for ordinary electric lighting. Its purpose is to render the system inoperative during the daytime. It is, however, subject to human fallibility, forgetfulness or even collusion. This objection can be overcome by using a time switch which can be set to cut in and out at predetermined times. In the case of shops or offices a time switch with a seven-day dial may be used and can be set to make due allowance for different closing times at week-ends and early closing days.

In locations where any part of the wiring is liable to be cut, the system should be adopted. In this case, the main circuit is normally energized and the alarm bell is on a secondary circuit which operates only when a break occurs in the main circuit.

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