How to Install Electric Bells, Annunciators, and Alarms. / Including Batteries, Wires and Wiring, Circuits, Pushes, Bells, Burglar Alarms, High and Low Water Alarms, Fire Alarms, Thermostats, Annunciators, and the Location and Remedying of Troubles

by Norman H. Schneider

Burglar Alarms.

A burglar alarm is a device for indicating the opening of a door or window, by the ringing of a bell or operation of an annunciator. The contact apparatus at the points to be protected may either open an electrical circuit or close one, in the latter case being mere modifications of push buttons. The simplest form is the latter or open-circuit method.

The spring contact to be inserted in the door jamb or window frame is so constructed that while under pressure the contacts are kept apart and the circuit is open. But when the door or window is opened, the pressure is released and a spring forces the contacts together.

[Fig. 44] is an open-circuit window spring fitted in the window frame so that when the window is closed the spring lug S is pressed inwards, breaking contact with the base B.

If the window is raised, the lug flies to the position shown by the dotted lines, and making contact with B, completes the circuit through bell and battery. These springs are fitted in the side of the window frame in a vertical position and are entirely concealed when the window is shut.

Fig. 44

In the closed-circuit system the reverse happens. The pressure of the closed door or window keeps the contacts together and its opening enables them to spring apart.

Fig. 45

In [Fig. 45] is a diagram of a closed-circuit burglar alarm, C a cell of gravity battery, R a relay, F the fixed contact and M the movable contact of the spring, S a stud projecting through the base of the spring and pushed in by the closed door.

When the door is closed, S being pushed in, the circuit of C, R, F and M is closed. The magnets of the relay hold the armature arm A forward against a hard rubber contact. But when S is released, the relay circuit is opened, R loses its power and A flies back, making contact, and throwing in circuit bell B and battery L.

Fig. 46

A form of bell and relay combined is shown in [Fig. 46]. Here the armature A is held against the magnets while the circuit through the spring F and battery G is closed. But on opening this circuit the armature flies back and makes contact with an adjustable contact screw S putting in circuit a local battery C. The bell is now practically a vibrating bell; on a closed circuit it rings until the circuit is again closed or the battery runs down.

Fig. 47

A different connection of the same scheme is [Fig. 47], where only one battery is used. This must be a gravity battery or some other closed-circuit battery. The circuit can be easily traced in the figure and needs no special description.

Both of the latter schemes are inferior to one using a separate relay. If the circuit at the spring were quickly closed again the bell would either stop ringing, or be so hampered as to ring very weakly.

Fig. 48

A relay made as in [Fig. 48] has no spring support to the armature A, which falls down by gravity. The adjustable contact C is screwed far back, so that the armature must fall a considerable distance away from the electromagnets before it makes contact. This ensures that the armature will not be attracted and the bell stopped from ringing by a re-closing of the circuit at the door or window spring.

A shade spring ([Fig. 49]), is made for either open or closed circuits. In operation, the shade is pulled down and its string or ring hooked on to H. This draws H up a trifle against a spiral spring and its lower end makes contact with an insulated spring S closing the circuit. If the shade is disturbed, the spiral spring on the lower part of H is released and it causes a break of contact with S in the direction of the arrow.

Fig. 49

When made for open circuit, S is bent so that while under tension no contact is made, but release of tension causes the contact.

[Fig. 50] gives the wiring of two windows and a door on the closed-circuit system. It will be seen that the contact springs are all in series, opening a window or the door will thus break the circuit.

When setting the alarm at night by connecting up the batteries, relay and bell, should any one of these springs be open the relay armature will not hold, and the bell rings.

Fig. 50

In this figure the relay is replaced by an electromagnet holding up a drop shutter by magnetic attraction. Upon the circuit opening, this shutter falls, exposing a number painted on it. At the same time it hits a spring contact placed below it and closes the bell and local battery circuit.