This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A very serviceable fire and burglar alarm may be installed by anyone who can work with carpenters' tools and who has an elementary knowledge of electricity. Fire and burglar alarms are divided into two general types, called "open circuit" and "closed circuit," respectively.
In the open-circuit type of alarm all the windows, doors, and places to be protected are equipped with electrical alarm springs which are in circuit with an ordinary vibrating bell and battery, and these alarm springs are all normally open. When a window or door is disturbed or moved more than a predetermined amount, the bell circuit is closed and the alarm sounded. The arrangement of such an alarm is shown in Fig. 1. A switch, A, is placed in circuit so that the alarm may be disconnected during the day and the opening and closing of doors and windows will not operate the bell. It is best not to place a switch in the fire-alarm circuit as this circuit should be in an operating condition at all times.
The alarm switch controlled by the window consists of a narrow metal plate, B, and a spring, C, mounted in a recess cut in the side of the window frame. The spring C is bent into such a form that its upper end is forced into contact with the plate B, when the window is raised past the outwardly projecting part of the spring C, and the bell circuit is thus closed. The position of the alarm switch can be adjusted so that the window may be opened a sufficient distance to permit the necessary ventilation but not allow a burglar to enter.
The alarm switch controlled by the door is arranged in a different manner. In this case the free end of the spring D is held away from contact with the spring E by the edge of the door, which forces the spring D back into the recess cut in the door jamb. When the door is opened the spring E is permitted to move out and come into contact with the spring or plate E, and the alarm circuit is thus closed. The form of the spring D can be so adjusted that the door may be opened some distance, but not enough to allow a person to enter, before the alarm is sounded.
Ill: Fig.1 Connections and Wiring Diagram Showing an Open-Circuit Fire and Burglar Alarm
An alarm switch, identical with that just described for the door, should be mounted in the upper part of the window frame to take care of the upper sash. This alarm switch may be located low enough to permit the window to be lowered for the purpose of ventilation without sounding the alarm.
The wires for these various alarm switches should be run as near completely concealed as possible to prevent them being tampered with by curious parties, who may unintentionally break one of the conductors and thus make some part of the system inoperative. It might be best to test the system occasionally, to make sure all switches are in operating condition.
The fire-alarm switch consists of two springs that are held from contact with each other by means of a thin cord. This switch is placed in the location to be protected, or wherever a fire is most likely to break out, such as over the furnace, in the coal bin, etc. When the cord is destroyed the springs make contact and the alarm is sounded. A metal having a very low melting temperature may be used instead of the cord, and the alarm will be sounded when the temperature exceeds a certain amount and the actual occurrence of a fire thus prevented. In some cases, the fire-alarm switch may be completely destroyed and the alarm circuit will then be opened and the bell will cease ringing. To prevent this trouble a small electric drop may be placed in the circuit, the arrangement being similar to that shown in Fig. 2. When the shutter of the drop falls, due to the closing of the alarm circuit, there is a second circuit closed, and this second circuit remains closed until the shutter is restored to its vertical or normal position, or the switch, A, is thrown to the open point. The addition of the drop in the burglar-alarm circuit may prove to be an advantage, as a burglar cannot stop the alarm, after he has once closed any of the alarm switches and operated the drop, by simply restoring the window or door to its original position.
Ill: Circuit Equipped with Drop to Ring the Bell in Case the Switch is Destroyed
In the closed-circuit type, the alarm switches are all normally closed and the alarm is sounded by opening the circuit at some point. The arrangement of such an alarm is shown in Fig. .1. The alarm switches are all connected in series in this case and in circuit with a closed-circuit battery And relay or drop. The drop or relay controls a local circuit composed of an open-circuit battery and an ordinary vibrating bell.
Ill: Alarm Switches Gravity Battery
The operation of a drop on a closed circuit is a little different from its operation on a normally open circuit. The drop for the closed circuit must be so constructed that its latch holds the shutter in a vertical position when there is a current in the drop winding, but allows it to fall as soon as the drop circuit is opened.
An ordinary telegraph relay may be used in connection with the closed-circuit alarm. The connections to the relay are such that the bell circuit is normally open and remains so until the armature of the relay is released, which does not occur until the circuit of which its winding is a part is opened at one of the alarm springs. A special switch, A, and resistance, B, are shown connected in circuit in Fig. 3, the object of which is as follows : When it is desired to disconnect the alarm springs or make them inoperative they must be replaced by another circuit which will permit a sufficient current to pass through the relay winding at all times, to prevent its armature from being released and sounding the alarm. The switch A is so constructed that either the alarm switches or the resistance B is in series with the battery and relay winding at all times, there being no open-circuit position for the switch.
The fire-alarm switch for this type of signal may be made from a narrow piece of tin foil, or some metal having a low melting temperature, mounted between two insulated clips that are connected in the alarm circuit.
Strips of gold or silver foil may be placed on windows and connected in the alarm circuit, which will give A protection from theft by breaking the glass.
Two or three gravity cells will serve very nicely for the closed-circuit battery, while several dry cells will do for the open-circuit or bell battery.
All types of alarm switches can be purchased at any up-to-date electrical supply house, but their construction and operation is so simple that they may be easily made by almost anyone. A detailed description of the construction of the various parts of the above circuits will not be given here, but such details can be safely left to the ingenuity of the person installing the system.
It is easily seen from the above description that a burglar who might discover that a house was wired for alarm would be greatly perplexed to know what to do, for the very thing that would prevent one kind of alarm from ringing would cause the other to ring.