Fire alarm system comprising a plurality of alarms which may be operated by way of an alarm loop

A fire alarm system comprises a plurality of alarms which may be operated via an alarm loop and which, being subject to selective interrogation, each alarm transmits an analog value of a particular characteristic of a fire to a central control, the analog value being tapped from a detector for that characteristic. Each alarm comprises an alarm circuit which has a load resistor which can be connected in parallel to the alarm loop by means of a timing element and which amplifies the current which characterizes the alarm at the instant of interrogation.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to an application of Otto Walter Moser et al, 
Ser. No. 821 839 filed Aug. 4, 1977 and is also related to an application 
of Peer Thilo et al, Ser. No. 821,840 filed Aug. 4, 1977. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a fire alarm system, and more particularly to 
such a system which comprises a plurality of alarms which may be operated 
by an alarm loop, and which are subject to selective interrogation so as 
to feed an analog value of a particular fire characteristic to a central 
control, the analog value being tapped from a measuring transducer for 
that particular characteristic. 
2. Description of the Prior Art 
Fire alarm system are well known in the art and may be supplied by a 
commercial power supply or by batteries. In the event of a breakdown of 
the commercial supply, fire alarm systems are to be supplied for a minimum 
length of time by a second, independent energy source. Batteries generally 
serve this purpose. The requisite capacity of this emergency current 
supply is determined, on the one hand, by the current drain of the alarm 
central control, and, on the other hand, by the number of alarms connected 
to the central control. 
SUMMARY OF THE INVENTION 
The object of the present invention is to considerably reduce the energy 
consumption in the individual alarms, without thereby endangering the 
alarm transmission from the alarms to the central control and to provide 
that the system will operate without disturbances notwithstanding the 
lower energy consumption. 
According to the invention, the above objects are achieved in a fire alarm 
system of the type mentioned above in that each alarm has a load resistor 
which can be connected in parallel to the alarm loop by means of a timing 
element, and which in each case amplifies the current characterized by the 
alarm at the instant of interrogation. Advantageously, the load resistor 
can be the resistor of an RC element which constitutes the timing element. 
It is also advantageous for the load resistor to form part of a monostable 
trigger stage. 
In a further development of the invention, the selective interrogation of 
the individual alarms, that is a request for the alarms to emit their fire 
characteristic analog values which can be tapped from respective measuring 
transducers, can be effected by means of chain synchronization, i.e. by a 
common disconnection of all of the alarms from the alarm loop prior to 
interrogation and a subsequent reconnection of the alarms to the alarm 
loop, the reconnection being made by reconnecting the alarms consecutively 
one after the other.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the fire alarm system illustrated in FIG. 1, a plurality of alarm 
circuits Md1-Md30 and an analysis device Mc have been illustrated in a 
schematic form, detailed illustrations being available in FIGS. 2 and 8. 
The system comprises a central control Ze and an alarm loop Ms formed by 
the alarm circuits. The alarm loop Ms is connected, in the central control 
Ze to a pair of serially connected batteries Ba1 and Ba2 by way of a 
transfer switch Us. A pair of interrogation windings Wi1 and Wi2 are 
symmetrically looped into the supply lines of the battery Ba1, and feed 
pulses occurring in the supply lines, by way of a common core Ke, to an 
output winding Wi3. The windings Wi1-Wi3 on the common core Ke is tuned by 
a capacitor Co to a particular resonant frequency, and it is also strongly 
attenuated by a resistor Re. The interrogation signals emitted from the 
alarm circuits by way of the transformer pass two limiting diodes Di, Di' 
connected in opposite polarity fashion to each other and are received by a 
threshold value switch Sw, the diodes and the switch forming rectangular 
pulses which are then fed to a micro-computer Mc. In the micro-computer 
the rectangular pulses are individually analyzed, as will be described 
below in connection with FIG. 8, to determine if the signals represent a 
fire characteristic which should be considered an alarm condition. 
In a state of readiness for operation, the alarm loop Ms is connnected to 
the higher voltage of the batteries Ba1 and Ba2, as illustrated in FIG. 1 
and as shown on the voltage curve of FIG. 3 in the range 00. For an 
interrogation, the transfer switch Us must first be opened so that a 
voltage gap is formed, as indicated by the range 01 in FIG. 3. Then the 
transfer switch must be closed to the operating position, that is to the 
lower contact illustrated in FIG. 1 so as to connect the lower voltage of 
the battery Ba1 to the loop and initiate the interrogation range 02 in 
FIG. 3. As a result, voltage again is applied to a pair of attenuating 
resistors Re1 and Re2 of the alarm loop Ms. Finally, the transfer switch 
must be returned to its rest position, and thus to the higher voltage of 
the two batteries Ba1 and Ba2 in series to again reach a rest state 00. 
As a result of the disconnection of the voltage from the measuring loop Ms, 
the timing elements Zg1-Zg30 in the respective alarm circuits open the 
respective interrogation switches, schematically illustrated as switches 
Sc1-Sc30 in the individual alarm circuits so that all of the alarm 
circuits are disconnected from the central control Ze in the range 01. If 
voltage is again applied to the alarm circuit Md1, the detector, in the 
form of a measuring transducer Wd1 is powered to control the timing 
element Zg1 in accordance with the fire characteristic value, which timing 
element closes the interrogation switch Sc1 after a predetermined length 
of time, and thus connects the alarm circuit Md2 to the central control 
Ze. In this manner, all of the alarm circuits Md1-Md30 are sequentially 
connected to the central control Ze in the form of a chain, for different 
lengths of time. Here, the individual alarm circuits Md1-Md30 are 
characterized by the sequence of their reconnection to the central control 
Ze and the fire characteristic values are characterized by the time 
differences t.sub.1 -t.sub.30 between the activation of the individual 
alarm circuits. The function of the series connection of a diode Di1-Di30 
and the associated capacitors Co1-Co30 in the individual alarm circuits is 
simply to supply the transducers and possibly also the timing elements 
with voltage for the time at which the voltage is disconnected from the 
central control Ze. 
FIG. 2 illustrates in detail an alarm circuit Md. A Zener diode D1 serves 
only as a protection against excess voltages, and when the alarm circuit 
Md is connected to an incorrect polarity the Zener diode protects the 
individual components, in particular the transistors T1, etc. A diode D2 
allows a capacitor C1 to charge for such time as the high voltage of the 
two batteries Ba1 and Ba2 is connected to the alarm loop Ms in the range 
00. On the other hand, it prevents the capacitor C1 from discharging when 
the alarm loop Ms is disconnected from the central control Ze in the range 
01, or is supplied by the battery Ba1 in the range 02. However, the 
capacitor C1 itself supplies the requisite operating voltage for the alarm 
Md, and thus bridges the voltage gaps, that is the range 01. A transistor 
T1, in association with a resistor R1 and a Zener diode D3, serves to 
stabilize the voltage for an ionization chamber J. A field effect 
transistor F, in combination with a load resistor R2, amplifies the output 
voltage of the ionization chamber J. Thus, the voltage across a measuring 
point M changes in dependence upon the particular characteristic of a 
fire, here the smoke concentration in the ionization chamber J. 
In FIG. 2, the timing element Zg which has been illustrated in FIG. 1 
comprises a plurality of resistors R3-R6, a capacitor C2 and a pair of 
transistors T2 and T3. The transistors T2 and T3 are conductive for such 
time as the capacitor C2 is charged. Following the disconnection of the 
voltage from the central control Ze, the capacitor C2 had been discharged, 
and a diode D4 blocked the voltage at the measuring point M. After 
reconnection of the alarm circuit to the voltage of the battery Ba1, the 
capacitor C2 is recharged to the voltage prevailing at the measuring point 
M. During this period of time, a pair of interrogation transistors T4 and 
T5 are in a blocking condition. When the voltage across the capacitor C2 
has reached the value predetermined by the measuring point M, the 
transistors T2 and T3 block and render the transistors T4 and T5 
conductive, by which action these transistors connect the next alarm 
circuit, in this example the alarm circuit Md2, to the alarm loop Ms. A 
resistor R7 determines the base current for the transistor T5. A capacitor 
C3 prevents the transistor T4 from being temporarily switched through, as 
a result of transients, when the voltage is connected across the terminals 
1 and 2. Finally, a diode D5 serves only to assist the drive of the 
transistor T4, but does not form a part of the present invention and is a 
primary feature of the aforementioned Moser et al application. When the 
next alarm circuit is connected to the alarm loop Ms, the series 
arrangement of a resistor R8 and a capacitor C4 is also connected to the 
alarm loop Ms, so that the latter is recharged; on the occasion of the 
last voltage disconnection it had discharged by way of the alarm loop Ms. 
The charging current of the capacitor C4 produces switch-on current peaks 
as illustrated in the current curve I.sub.M of FIG. 3 at the end of the 
times t.sub.1, t.sub.2, etc. respectively for each alarm circuit, and thus 
clearly characterizes the switching on of the particular next alarm 
circuit. 
In FIG. 4, a transistor T6 is connected by way of a resistor R9 to a 
connection point N of the capacitor C4 and the resistor R8 discussed above 
with respect to FIG. 2. Here, a collector resistor R10 produces a current 
amplification in the measuring loom Ms. 
FIG. 5 illustrates the current curve produced on the alarm loop Ms by the 
monostable trigger circuit of FIG. 4, for the respective intervals 
t.sub.11, t.sub.12, etc and out to the end of a time interval t.sub.E at 
which, in comparing back to FIG. 3, the system is placed back in the range 
OO. 
Referring now to FIGS. 6-9, apparatus for establishing the ranges 00, 01 
and 02, and the apparatus for reading and analyzing the resulting signals 
will be discussed in greater detail. In FIG. 6, the transfer switch Us is 
illustrated as being mechanically linked by a push rod St to a cam N which 
is driven by a synchronous motor Sy. The cam N is illustrated as having 
three portions N00, N01 and N02, the portion N01 being a lobe slightly 
raised from the portion N00, and the portion N02 being raised from the 
portion N00 a greater amount. As the cam is rotated by the synchronous 
motor, in the direction illustrated by the arrow and from the position 
illustrated in FIG. 6, the rod St rides along the periphery of the portion 
N00 and permits the movable contact of the transfer switch Us to remain 
closed to its upper stationary contact. As the rod St engages the lobe 
N01, it is depressed to open that circuit for an interval of time, for 
example 100 msec, until the rod is engaged by the lobe N02, whereupon the 
movable contact is pushed into engagement with the lower stationary 
contact of the switch Us. The lobe N02 is dimensioned to provide an 
interrogation interval of, for example, 300 msec, as indicated on the 
camming diagram portion of FIG. 6. The action of one revolution of the cam 
N therefore provides a voltage curve illustrated in FIG. 3 for the ranges 
00, 01 and 02. 
Referring to FIG. 7, the interrogation and signaling of FIG. 3 is 
illustrated in greater detail wherein in the curve a, the voltage U.sub.Ms 
over the ranges 00, 01 and 02 is illustrated once more as is the resulting 
signaling current I.sub.M in a curve b. In the curve b, the height of the 
individual current steps i.sub.1, i.sub.2 etc is constant as the current 
rise per signal from an alarm station is almost independent from its 
measuring value. The length of the individual steps t.sub.11 ', t.sub.12 ' 
etc is respectively a measure for the measuring value of the appertaining 
current signal from the alarm circuits. The index line was selected in 
order to indicate that the values t.sub.11 ' etc are not directly related 
to the preceding intervals. As the signals from the alarm circuits are 
connected in the sequence of their arrangement along the loop Ms, each 
signal can be identified by including the current steps, as will be 
readily apparent to those skilled in the art from FIGS. 7 and 8. 
As the primary windings Wi.sub.1 and Wi.sub.2 of the transformer Ue are 
symmetrically arranged in the loop, each current alteration effects a 
voltage pulse in the primary windings which is induced in the secondary 
winding Wi3. At the secondary winding, the transformer is tuned to a 
particular resonant frequency by a capacitor Co; and it is, moreover, 
strongly damped by the resistor Re. The output signal illustrated in curve 
c of FIG. 7 is fed to a threshold value switch Sw by way of the two 
limiter diodes Di and Di'. A representative circuit for the threshold 
value switch is the Schmitt trigger circuit illustrated in FIG. 9, which 
can be constructed in accordance with the teachings of Phil Schrrod in his 
article "Comparator Circuit Makes Versatile Schmitt Trigger", published in 
the Feb. 19, 1976 issue of the periodical "Electronics" at Page 128 et seq 
and the National Semiconductor Data Sheet on Operational Amplifiers, 
identified as LM741/LM741C. The threshold value switch and the limiting 
diodes convert the damped signals into voltage pulses as illustrated in 
the curve d in FIG. 7 and feeds the pulses to the micro-computer Mc which 
evaluates the pulse spacings. 
A functional schematic circuit diagram of a microcomputer Mc is illustrated 
in FIG. 8 in a simplified form. The rectangular pulses are fed to an 
excitation winding Dr of a rotary switch having a movable contact arm dr. 
Pulsing of the winding Dr steps the selector contact arm dr from its home 
or zero position sequentially through its stationary contacts and back to 
the zero position. A pulse generator Tg provides pulses of, for example, 
50 .mu.us, to a plurality of counters Z1-Zx to measure the time spacing 
between two rectangular pulses of the curve d in FIG. 7. Each of the 
counters has an associated comparator which may be set to a predetermined 
pulse count by a dial Ek. When this pulse count is reached, an associated 
relay is operated, as will be apparent from the description below. 
Assuming that the counter Z1 has its associated comparator set to a value 
of 40 pulses, and that a pulse from the first alarm circuit Md1 is 
converted to the first pulse of the train in the curve d of FIG. 7 and 
causes the excitation winding Dr to move the selector contact arm dr to 
the contact 1, the pulse generator Tg feeds pulses to the counter Z1 until 
such time as the excitation winding Dr receives the next succeeding pulse 
from the curve d of FIG. 7. If and when 40 pulses are counted by the 
counter Z1, the comparator causes associated relay U to operate and close 
its contact u which prepares an alarm generator Ag1 for operation, an 
intervening contact v1 being opened. The second pulse received from the 
threshold switch Sw causes the pulses of the pulse generator Tg to be fed 
to the counter Z2 which, as illustrated in FIG. 8, is set to 70 pulses. 
Upon receipt of 70 pulses, the associated comparator causes a relay V to 
operate and close its contacts v1 and v2. Closure of the contact v1 causes 
the alarm generator Ag1 to operate, and closure of the contacts v2 
prepares an alarm generator Ag2 for operation. The additional alarm 
generators Ag2-Ag3 can be connected with one another by way of 
corresponding contacts u-x of the relays U-X, and thus alarm signaling is 
safeguarded. The selector contact arm dr provides pulses to the reset 
input of the counters Z1-Zx when it has again reached its zero position. 
If the orderly functioning of the apparatus is to be examined, since no 
alarm was given for some time, a key Ta can be pushed and the resetting of 
the counters Zl-Zx can thus be delayed. An observer then recognizes 
whether the individual counters Zl-Zx reacted, or whether they remained in 
their zero position, and thus a defect of the apparatus can be determined. 
The circuit illustrated in FIG. 8 serves only as a functional model. In 
order to provide the prescribed switching times, the electromechanical 
switching elements illustrated would be replaced by suitable electronic 
components. 
Although we have described our invention by reference to particular 
illustrative embodiments thereof, many changes and modifications of the 
invention may become apparent to those skilled in the art without 
departing from the spirit and scope of the invention. We therefore intend 
to include within the patent warranted hereon all such changes and 
modifications as may reasonably and properly be included within the scope 
of our contribution to the art.