Fire detector

In a fire detector that includes a sensor for producing an output voltage corresponding to smoke concentration, temperature, etc., there is provided a charge-and-discharge circuit to be reset by the turning ON of a switching circuit which is made ON at a fixed voltage. The output of the charge-and-discharge circuit and the output of a sensor are applied to the switching circuit to vary the ON period of the switching circuit corresponding to the output voltage of the sensor. Any irregularity in the sensor can be monitored by changes in the ON period.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to an improvement in the fire detector. 
BACKGROUND OF THE INVENTION 
The conventional fire detector simply detects occurrence of a fire by means 
of a sensor which senses smoke concentration and/or temperature change and 
outputs a voltage, the switching circuit of the detector being turned ON 
when the voltage exceeds or falls below a fixed value. An element with 
self-holding function, such as a thyristor, is generally used as the 
switching circuit of a fire detector. 
In testing the conventional fire detector, the operator has to introduce 
smoke into the fire sensor or to raise temperature thereof in order to 
learn whether the sensor is operating normally or whether the set voltage 
for operating the switching circuit is normal. An alternative method for 
testing the operation of the fire sensor is known, in which an equivalent 
voltage is applied to the sensor instead of actual introduction of smoke, 
actual temperature rise, etc. Even in this case, however, the operator 
must go to the place where the fire sensor is installed and the test 
itself is complicated. The need for going to such trouble and for such 
complicated testing is recognized as a defect of the conventional fire 
detector. Another defect is that the conventional fire detector operates 
by a comparison of the output voltage of the sensor with a fixed value 
and, therefore, the operation is merely binary, that is, ON or OFF, and 
degree or variation in smoke concentration and in temperature rise cannot 
be known. A method has also been known in which the output voltage of a 
sensor is sent out after conversion into a digital value, but in such a 
case the sensor unit must be provided independently of the digital 
converting unit so that the overall apparatus is complicated and 
expensive. 
Accordingly, the object of this invention is to provide a fire detector 
which can be checkable for normal operation from a remote location and 
which can ascertain change and variation in parameters, such as smoke 
concentration and temperature, etc., in real time, thus eliminating the 
defects of the prior art fire detectors. 
DISCLOSURE OF THE INVENTION 
This invention provides a fire detector comprising a sensor for producing 
an output voltage corresponding to smoke concentration, temperature, etc., 
and a switching circuit to be turned ON when the output voltage of the 
sensor exceeds or falls below a fixed value, wherein a 
charge-and-discharge circuit is provided to be operated by power supplied 
through a pair of alarm lines and to be reset by the turning ON of the 
switching circuit, and the output of the charge-and-discharge circuit is 
connected with the sensor in series between the alarm lines. 
This invention further provides a fire detector comprising a switching 
circuit which is made ON by a control voltage exceeding a fixed value and 
a charge-and-discharge circuit for starting a charge and discharge 
operation in response to the ON state of the switching circuit wherein the 
switching circuit turns ON in response to a difference between the output 
voltage of the charge-and-discharge circuit and the output voltage of the 
sensor. 
This construction eliminates the defect that it takes a long duration 
before the output of a sensor becomes stable due to the time constant of 
the sensor when the output of the charge and discharge circuit is serially 
connected to the sensor, if the sensor has a high impedance. 
This invention further provides a pulse oscillator to count the number of 
pulses during an ON state of the switching circuit and to send out the 
content of the count to the alarm lines so that the fire parameters can be 
ascertained from moment to moment using a transmission technique of 
conventional time sharing.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
In FIG. 1, a sensor 1, an ionic smoke sensor, for instance a photodiode 
which receives scattered light or a heat sensitive element, generates an 
output voltage corresponding to the smoke concentration, temperature, etc. 
A Zener diode Z conducts when the output voltage of the sensor 1 exceeds a 
fixed value and makes the gate terminal of a thyristor 2 high level to 
turn the thyristor ON. A serially connected circuit consisting of a 
capacitor C.sub.1 and a resistor R.sub.1 is connected between a pair of 
alarm lines L.sub.1 and L.sub.2 and the capacitor C.sub.1 is charged 
gradually by the voltage from the alarm lines. The sensor 1 is connected 
between the alarm line L.sub.1 and one terminal of the capacitor C.sub.1. 
More particularly, the sensor 1 and the capacitor C.sub.1 are connected in 
series between the two alarm lines and the voltage charged on the 
capacitor C.sub.1 is applied to the negative side of the sensor 1. A 
charge-and-discharge circuit is constructed by the capacitor C.sub.1, the 
resistor R.sub.1 and a normally-open contact S.sub.2 of a relay 3 and the 
voltage on the capacitor C.sub.1 is the output voltage of the 
charge-and-discharge circuit. The cathode electrode of the thyristor 2 is 
connected to the alarm line L.sub.2 and the anode electrode of the 
thyristor 2 is connected to the alarm line L.sub.1 through a 
normally-closed contact S.sub.1 of the relay 3. The normally-open contact 
S.sub.2 is connected across the capacitor C.sub.1. The normally-open 
contact S.sub.2 closes and the normally-closed contact S.sub.1 opens upon 
the operation of the relay 3. A current sensing circuit may be provided in 
place of the relay 3 and the normally-closed contact S.sub.1 and the 
normally-open contact S.sub.2 may be controlled by the current sensing 
circuit. In this case, the contacts S.sub.1 and S.sub.2 may each be 
constituted of a transistor. 
The operation of the circuit shown in FIG. 1 is as follows. The capacitor 
C.sub.1 is charged by the voltage from the alarm lines L.sub.1 and L.sub.2 
through the resistor R.sub.1 and the voltage on the terminal of the 
capacitor C.sub.1 increases gradually. When the sum of the voltage charged 
on the capacitor C.sub.1 and the output voltage of the sensor 1 exceeds a 
fixed value, the Zener diode Z conducts to trigger the thyristor 2. When 
the thyristor 2 turns ON the relay 3 operates to close the normally-open 
contact S.sub.2 and the voltage charged on the capacitor C.sub.1 
discharged. In other words, the charge-and-discharge circuit is reset and 
the Zener diode Z turns OFF. While, when the normally-closed contact 
S.sub.1 is opened, the thyristor 2 turns OFF and the relay 3 is restored 
to the original state. The normally-closed contact S.sub.1 is closed by 
the restoration of the relay 3 and the thyristor 2 remains OFF since the 
Zener diode Z has already turned OFF. While, since the normally-open 
contact S.sub.2 is opened by the restoration of the relay 3, the capacitor 
C.sub.1 is again charged through the resistor R.sub.1 and the operation 
stated above is repeated. In other words, the thyristor 2 is turned ON and 
OFF at a fixed period. The period varies normally according to the output 
voltage of the sensor 1. More particularly, for example, the period 
becomes shorter with a larger output voltage of the sensor 1 and becomes 
longer with a smaller output voltage of the sensor 1. At an extreme 
condition, if the sensor should fault and no output voltage is obtained, 
the Zener diode Z does not contact so that the thyristor 2 does not turn 
ON. Accordingly, it is possible to check the abnormality of the sensor 1 
by monitoring the ON period of the thyristor 2 from the other end of the 
alarm lines L.sub.1 and L.sub.2 . 
When the output voltage of the sensor 1 changes, for example, with 
generation of smoke upon the occurrence of a fire the ON period of the 
thyristor 2 changes according to the degree of change in the output 
voltage. The ON period becomes shorter when the change in the output 
voltage of the sensor 1 adds to the charge voltage of the capacitor 1 and 
becomes longer when the change is subtracted from the charge voltage. 
Therefore, any type of smoke sensor can be used and the sensor 1 may be a 
temperature sensor. 
As will be understood from the above explained operation, the relay 3 may 
desirably have some restore-delay characteristics so that complete 
discharge of the capacitor C.sub.1 and turning OFF of the thyristor 2 are 
assured. One terminal of the sensor 1 may be connected to the alarm line 
L.sub.1 through the normally-closed contact S.sub.1 instead of being 
directly connected to the alarm line L.sub.1. A diode D for prevention of 
reverse current and a capacitor C.sub.2 may desirably be connected to the 
alarm line L.sub.1, as shown by broken lines in FIG. 1, in order to 
prevent the voltage on the sensor 1 from changing precipitantly when the 
thyristor 2 turns ON. These modifications can appropriately be made 
according to the characteristics of the sensor 1 or other conditions. 
FIG. 2 shows a specific example of the circuit employed when the sensor 1 
is an ionizing smoke sensor. The output of the ionic smoke sensor 1 is of 
high impedance and, therefore, the intermediate electrode M of the ionic 
smoke sensor 1 is connected to the gate terminal of a field effect 
transistor FET so that an impedance conversion is effected. The source 
electrode of the field effect transistor FET is connected to a Zener diode 
Z and the drain electrode of the field effect transistor FET is connected 
to an alarm line L.sub.1 through a normally-closed contact S.sub.1 and a 
relay 3. The voltage supplied from the alarm lines L.sub.1 and L.sub.2 
charges a capacitor C.sub.2 through a diode D, and the charged voltage of 
the capacitor C.sub.2 is applied to the inner electrode I of the smoke 
sensor and the outer electrode O of the sensor 1, the charged voltage 
being applied to the outer electrode through a resistor R.sub.2. The other 
end of the Zener diode Z is connected to the gate electrode of a thyristor 
2, and the anode of the thyristor 2 is connected to the alarm line L.sub.1 
through the normally-closed contact S.sub.1, while the cathode electrode 
of the thyristor 2 is connected to the alarm line L.sub.2. Therefore the 
power for operating the sensor 1 is supplied by the voltage charged on the 
capacitor 2 even when the thyristor is ON. A capacitor C.sub.1 is charged 
through the normally-closed contact S.sub.1 and a resistor R.sub.1 and the 
voltage charged on the capacitor C.sub.1 is applied to the outer electrode 
of the sensor 1. Thus, the voltage on the gate terminal of the field 
effect transistor FET is the sum of the voltage charged on the capacitor 
C.sub.1 and the voltage existing between the intermediate electrode M and 
the outer electrode of the sensor 1. A normally-open contact S.sub.2 is 
connected in parallel to the capacitor C.sub.1, and a serial circuit of a 
resistor R.sub.3 and a capacitor C.sub.3 is connected in parallel with the 
relay 3 which has delayed restoring characteristics. 
The operation of the circuit shown in FIG. 2 is as follows. The capacitor 
C.sub.2 is charged with the voltage supplied from the alarm lines L.sub.1 
and L.sub.2 through the reverse flow prevention diode D, and a voltage (a 
sensed voltage) is present between the intermediate electrode M and the 
outer electrode O of the sensor 1, said voltage corresponding to a voltage 
such as an inter-electrode impedance voltage when no smoke exists. Said 
voltage is applied to the Zener diode Z after undergoing impedance 
conversion by the field effect transistor FET and the Zener diode Z is 
kept OFF by the voltage in the absence of smoke. On the other hand, the 
capacitor C.sub.1 is gradually charged through the resistor R.sub.1 and 
the voltage on the outer electrode O of the sensor 1 increases with the 
increase in the voltage charging the capacitor C.sub.1. The voltage on the 
gate terminal of the field effect transistor FET is the sum of the voltage 
charged on the capacitor C.sub.1 and the voltage sensed by the sensor 1. 
The summed-up voltage is applied to the Zener diode Z after undergoing 
impedance conversion and then the Zener diode Z conducts to turn the 
thyristor 2 ON when the voltage on the capacitor C.sub.1 becomes a fixed 
value. After the thyristor turns ON, the relay 3 operates to close the 
normally-open contact S.sub.2 and the potential on the outer electrode of 
the sensor 1 drops to turn the Zener diode OFF. On the contrary, the 
normally-closed contact S.sub.1 turns OFF and the thyristor 2 turns OFF. 
When the relay 3 restores with some delay, the normally-closed contact 
S.sub.1 closes and the normally-open contact S.sub.2 opens, whereafter the 
above operation is repeated. In other words, the thyristor 2 turns ON at a 
fixed period. The period is constant if the output voltage of the sensor 1 
is constant under the no smoke condition. But the period varies with 
change in the output voltage of the sensor 1 due to insufficient 
insulation even under the no smoke condition. Accordingly, normal 
operation and abnormal operation of the sensor 1 can be distinguished by 
monitoring the ON period of the thyristor 2 in the ordinary state. 
The output voltage of the sensor 1 increases according to the smoke 
concentration when a fire occurs, and thus the thyristor 2 turns ON at a 
lower voltage charged on the capacitor C.sub.1. In other words, the ON 
period of the thyristor 2 becomes shorter. The reduction in the period 
corresponds to the smoke concentration, so that the smoke concentration 
can be learned by measuring the period. If the connection of the inner 
electrode and the outer electrode of the sensor 1 is reversed, the period 
becomes longer with greater smoke concentration and the smoke 
concentration can be known in this case as well. 
In FIG. 3, a serial circuit consisting of a sensor 1 and a capacitor 
C.sub.1 is connected between the alarm lines L.sub.1 and L.sub.2 and is 
charged through a resistor R.sub.1 by the alarm line L.sub.1. More 
particularly, a charge-and-discharge circuit is formed by the resistor 
R.sub.1 and the capacitor C.sub.1, and the capacitor C.sub.1 for producing 
an output voltage of the charge and discharge circuit is connected 
serially to the sensor 1. The output voltage of the sensor 1 is applied to 
the gate electrode of a thyristor 2 through a Zener diode Z. A switching 
circuit which turns ON at a fixed voltage is formed by the Zener diode Z 
and the thyristor 2. The anode electrode of the thyristor 2 is connected 
to the junction of the capacitor C.sub.1 and the resistor R.sub.1 and the 
cathode electrode of the thyristor 2 is connected to the alarm line 
L.sub.2. Accordingly, when the thyristor 2 turns ON, the 
charge-and-discharge circuit is reset by the discharge of the capacitor 
C.sub.1. After the capacitor C.sub.1 discharges completely, the thyristor 
2 turns OFF because of the lack of holding current and the capacitor 
C.sub.1 starts again to be charged. In this embodiment, the relay 3 and 
the normally-closed contact S.sub.1 as shown in FIG. 1 are omitted, and 
the thyristor 2 which acts as a switching circuit serves as the 
normally-open contact S.sub.2. The circuit of the embodiment shown in FIG. 
3 is extremely simple and can achieve an effect similar to the effect of 
the embodiment shown in FIG. 1 by an operation similar to the operation 
stated above. 
In FIG. 4, is shown the circuit diagram of an embodiment of the fire 
detector characterized in that a switching circuit is turned ON by the 
difference in the output voltage of a charge-and-discharge circuit and the 
output voltage of a sensor. In the embodiment, an ionic smoke sensor is 
employed but other types of sensors, such as a photoelectric smoke sensor 
or a temperature sensor, can also be applied. In FIG. 4, positive and 
negative voltage is supplied to an alarm line L.sub.1 and a common line 
L.sub.2c from a receiving unit, not shown in the figure. A capacitor 
C.sub.2 is charged from the alarm line L.sub.1 through a reverse flow 
prevention diode D and the voltage charged on the capacitor C.sub.2 is 
applied between the inner electrode and the outer electrode of an ionic 
smoke sensor 1. An intermediate electrode is located between the inner 
electrode and the outer electrode and the voltage of the intermediate 
electrode varies according to smoke concentration. The output of the 
sensor appearing on the intermediate electrode is applied to an impedance 
transducer 12 to be converted to a low impedance as the inner impedance is 
very high. An impedance transducer 12 is, in this embodiment, exemplified 
as a field effect transistor FET, etc. and the output voltage of the 
sensor 1 applied to the gate electrode of the field effect transistor FET 
is taken out as a voltage which is nearly equal to the applied voltage 
from a source electrode of the field effect transistor FET. The source 
electrode is connected to the common line L.sub.2c through a resistor 
R.sub.3 and a capacitor C.sub.1. The output voltage of the source 
electrode is applied to one end of a Zener diode Z and the other end of 
the Zener diode Z is connected to the base electrode of a transistor 
T.sub.1. The collector electrode of the transistor T.sub.1 is connected to 
the alarm line L.sub.1 through a resistor R.sub.2 and the emitter 
electrode of the transistor T.sub.1 is connected to the junction of the 
resistor R.sub.3 and the capacitor C.sub.1. Thus, when the voltage across 
the resistor R.sub.3 exceeds a fixed voltage, the Zener diode Z conducts 
and the transistor T.sub.1 turns ON. In other words, in this embodiment, a 
switching circuit that turns ON at a fixed voltage is formed by the Zener 
diode Z and the transistor T.sub.1. The emitter electrode and the 
collector electrode of a transistor T.sub.2 are connected across the 
capacitor C.sub.1 and the base electrode of the transistor T.sub.2 is 
connected to the collector electrode of a transistor T.sub.3. A parallel 
circuit consisting of a resistor R.sub.1 and a capacitor C.sub.12 is 
connected between the base electrode of the transistor T.sub.2 and the 
common line L.sub.2c. The emitter electrode of the transistor T.sub.3 is 
connected to the alarm line L.sub.1 through a Zener diode Z.sub.2 and the 
base electrode of the transistor T.sub.3 is connected to the collector 
electrode of the transistor T.sub.1 through a capacitor C.sub.3 and an 
inductance L. The base electrode of the transistor T.sub.3 is also 
connected to the alarm line L.sub.1 through a resistor R.sub.4. Thus, the 
charge-and-discharge circuit formed by the transistors T.sub.2 and 
T.sub.3, the capacitors C.sub.1 and C.sub.12 and the resistors R.sub.1 and 
R.sub.4, etc. starts the charge-and-discharge operation when the 
transistor T.sub.1 turns ON. 
The operation of the circuit shown in FIG. 4 is explained in reference to 
FIG. 5. The Zener voltage of the Zener diode Z is so selected that the 
transistor T.sub.1 is ON when the voltage on the capacitor C.sub.1 is 0 
under the condition where no smoke is present. Accordingly, the transistor 
T.sub.1 turns ON first and the transistor T.sub.3 turns ON next. By the 
turning ON of the transistor T.sub.3, the capacitor C.sub.12 is charged to 
a fixed voltage through the Zener diode Z.sub.2, the voltage on the 
emitter electrode of the transistor T.sub.2 increases, the transistor 
T.sub.1 turns OFF and the transistor T.sub.3 turns OFF successively. That 
is to say, the transistor T.sub.1 is ON for a short time and waveforms of 
pulses as shown in FIG. 5(a) appear across the resistor R.sub.2. As an 
output voltage V.sub.s of the sensor in the absence of smoke as shown in 
FIG. 5(b) is applied to the gate electrode of the field effect transistor 
FET, the potential of the source electrode of the field effect transistor 
FET is nearly equal to the output voltage V.sub.s. Accordingly, the 
voltage V.sub.R across the resistor R.sub.3 is expressed by the following 
equation: 
EQU V.sub.R .apprxeq.V.sub.s -V.sub.c 
wherein V.sub.c is the voltage of the capacitor C.sub.1. In other words, 
the voltage V.sub.R takes a fixed minimum value when the capacitor C.sub.1 
is charged to a fixed value. The transistor T.sub.1 is then OFF. 
Next, when the transistor T.sub.3 turns OFF the charge on the capacitor 
C.sub.12 discharges through the resistor R.sub.1 and the voltage on the 
base electrode of the transistor T.sub.2 decreases gradually due to the 
time constant determined by the capacitor C.sub.12 and the resistor 
R.sub.1, and accordingly the voltage V.sub.c, that is the voltage on the 
emitter electrode of the transistor T.sub.2, decreases as shown in FIG. 
5(c). The voltage V.sub.c on the capacitor C.sub.1 is the output voltage 
of the charge-and-discharge circuit. With the decrease of the voltage 
V.sub.c of the capacitor C.sub.1, the voltage across the resistor R.sub.3 
increases as shown in FIG. 5(d) and the transistor T.sub.1 turns ON when 
the voltage V.sub.R reaches a fixed Zener voltage. In this embodiment, as 
the voltage V.sub.c does not affect the output voltage V.sub.s of the 
sensor 1, the operation of the sensor is stable (While, when the voltage 
V.sub.c is applied between the sensor 1 and the common line L.sub.2c, the 
change in the voltage V.sub.c does affect the output voltage of the sensor 
1 and it takes a long time before the sensor 1 reaches a stable state). 
Accordingly, the sensor 1 operates stably even with a shorter charge and 
discharge period of the capacitor. As the charge and discharge circuit 
repeats the charge and discharge at a period similar to the period stated 
above with the turning ON of the transistor T.sub.1, the transistor 
T.sub.1 turns periodically ON. Accordingly, a pulse train with a fixed 
period appears across the resistor R.sub.2 as shown in FIG. 5(a). The 
period of the pulse train becomes shorter with a larger output voltage 
V.sub.s of the sensor 1 and becomes longer with a smaller output. Thus, 
any degradation of the sensor 1 due to such a factor as deteriorating 
insulation can be sensed by monitoring of the pulse period. 
When the output voltage of the sensor 1 rises, for example, due to the 
occurrence of a fire, the period of the pulse train becomes shorter and as 
the smoke concentration increases, the output voltage of the sensor 1 
rises and the period of the pulse train accordingly becomes shorter. This 
means that the smoke concentration can be distinguished by measuring the 
period of the pulse train. If a sensor such that the output voltage drops 
as the smoke concentration increases, is used, the period of the pulse 
train, on the contrary, becomes longer with increasing concentration. Thus 
the smoke concentration can be known in this case as well. Accordingly, 
the fire detector of this invention can be applied to any kind of sensor. 
As changes in the output voltage of a sensor due to defective insulation 
or the influence of ambient temperature occur very slowly, the changes can 
be easily distinguished from a change due to the occurrence of a fire by 
monitoring of the changes per unit time. Not only the intensity of a fire, 
but also how it spreads and how it is being extinguished can be determined 
by monitoring of the period of the pulse train in time series before and 
after the fire occurrence. Therefore, proper measures for extinguishing 
the fire can be taken in accordance with the situation. 
In FIG. 6, a resistor R.sub.3 is connected between the source electrode of 
a field effect transistor FET and a common line L.sub.2c and the emitter 
electrode of a transistor T.sub.1 is connected to the common line L.sub.2c 
through a parallel circit consisting of a resistor R.sub.1 and a capacitor 
C.sub.1. The collector electrode of the transistor T.sub.1 is connected to 
an alarm line L.sub.1 through a resistor R.sub.2. A sensor 1, a reverse 
flow prevention diode D, a capacitor C.sub.1, etc. are connected similarly 
to the cases of the preceding figures. A Zener diode Z is connected 
between the source electrode of the field effect transistor FET and the 
base electrode of the transistor T.sub.1. A capacitor C.sub.4 is connected 
between the base electrode and the emitter electrode of the transistor 
T.sub.1. 
The operation of the circuit shown in FIG. 6 is as follows: The voltage 
V.sub.R across the resistor R.sub.3 is nearly equal to the output voltage 
V.sub.s of the sensor 1 in the absence of smoke. So the Zener diode Z is 
made to conduct by the output voltage V.sub.s to turn ON the transistor 
T.sub.1 and the capacitor C.sub.1 is charged to the fixed voltage which 
has been divided by the resistors R.sub.2 and R.sub.1. When the voltage on 
the emitter electrode of the transistor T.sub.1 rises due to the charge on 
the capacitor C.sub.1, the Zener diode Z turns OFF while the transistor 
T.sub.1 maintains the ON state for a short duration due to the discharge 
of the capacitor C.sub.4 to complete the charging of the capacitor 
C.sub.1. The pulse waveforms appearing across the resistor R.sub.2 are 
shown in FIG. 7(a). The discharge of the capacitor C.sub.4 stops and the 
transistor T.sub.1 turns OFF. The charge on the capacitor C.sub.1 is then 
discharged through the resistor R.sub.1 and the voltage V.sub.c on the 
capacitor C.sub.1 drops according to the time constant C.sub.1 R.sub.1 as 
shown in FIG. 7(c). The voltage V.sub.R appearing across the resistor 
R.sub.3 is maintained at a fixed value which is nearly equal to the 
voltage V.sub.s, as shown in FIG. 7(c). Accordingly, when the voltage 
V.sub.c of the capacitor C.sub.1 drops and the voltage V.sub.R -V.sub.c 
reaches the Zener voltage, the Zener diode Z conducts and the transistor 
T.sub.1 turns ON to charge the capacitor C.sub.1 to a fixed voltage. A 
pulse train as shown in FIG. 7(a) appears across the resistor R.sub.2 when 
the operation stated above is repeated. The period of the pulse train 
becomes shorter as the time required for the voltage V.sub.R -V.sub.c to 
exceed the Zener voltage is shorter when the output voltage V.sub.s of the 
sensor 1 is high, and therefore the voltage V.sub.R across the resistor 
R.sub.3 is high. And the period becomes longer when the output voltage 
V.sub.s of the sensor 1 is lower. Accordingly, any disorder in the output 
voltage due to an insulation defect in the sensor 1 can be checked by 
monitoring the period of the pulse train. Any change in the smoke 
concentration when a fire occurs can be learned from changes in the period 
of the pulse train as well. Thus, the circuit shown in FIG. 6 has an 
effect similar to that obtained in the preceding embodiment. 
In FIG. 8, a Zener diode Z.sub.3 is used in place of the resistor R.sub.2 
shown in FIG. 6 and a thyristor SCR in place of the transistor T.sub.1. 
The output of the Zener diode Z is connected to the gate electrode of the 
thyristor SCR. When a voltage V.sub.R which is nearly equal to the output 
voltage V.sub.s of the sensor is applied to the Zener diode Z and the 
thyristor SCR turns ON, the capacitor C.sub.1 is charged up to a fixed 
voltage through the Zener diode Z.sub.3. The thyristor SCR is kept in the 
ON state by the charging current even if the potential on the cathode 
electrode of the thyristor SCR rises during the charging cycle. The 
thyristor SCR turns OFF when the capacitor C.sub.1 is charged to the fixed 
voltage and no charging current flows. As the charge on the capacitor 
C.sub.1, then discharges through the resistor R.sub.1 and the output 
voltage V.sub.c of the capacitor C.sub.1 drops gradually to a point where 
the voltage difference V.sub.s -V.sub. c reaches the Zener voltage, the 
thyristor SCR turns ON again and the operation stated above is repeated. 
In other words, the thyristor SCR turns ON at a constant period in the 
same way as the operation of the circuit shown in FIG. 6 to attain an 
affect similar to that in the preceding case. 
As stated above, the switching circuit is constructed to be turned ON or 
OFF in accordance with the difference between the output voltage of the 
fire sensor and the output voltage of the charge-and-discharge circuit 
which is charged and discharged at a fixed time constant and, therefore, 
the ON period of the switching circuit varies corresponding to the output 
voltage of the sensor. Accordingly, any disorder of the sensor and the 
smoke concentration can be learned by monitoring or by measuring the 
period. The output voltage of the charge-and-discharge circuit does not 
exert any influence on the operation voltage of the sensor. Therefore, 
differently from the prior art, a long time is not required before the 
operation of the sensor becomes stable after a test voltage is applied. In 
other words, quick checking and measurement is possible. 
In FIG. 9, is shown the circuit of a further improved fire detector in 
accordance with this invention. The preceding embodiment is characterized 
in that the ON period of the switching circuit is varied in correspondence 
to the output voltage of the fire sensor. In the embodiment, shown in FIG. 
9, there are provided one circuit for counting the number of oscillation 
pulses from a pulse generator 5 during the OFF period of the switching 
circuit, thus the output voltage of the sensor is digitized, counts of the 
counting circuit being applied to the alarm line, and another circuit for 
sending the output information of the sensor out to a receiver, not shown 
in the figure. The output information of a plurality of fire sensors can 
be simultaneously monitored. The operation of the embodiment shown in FIG. 
1 in which the above circuits are added is explained below. But it will be 
understood that these circuits can naturally be applied to the other 
embodiments, too. In FIG. 9, a serial circuit consisting of a sensor 1 and 
a capacitor C.sub.1 is connected between alarm lines L.sub.1 and L.sub.2, 
a resistor R.sub.1 for charging the capacitor C.sub.1 is connected between 
one end of the capacitor C.sub.1 and the alarm line L.sub.1 and a 
normally-open contact S.sub.2 is connected across the capacitor C.sub.1. 
The output voltage of the sensor 1 is applied to the gate electrode of a 
thyristor 2 through a Zener diode Z, the anode electrode of the thyristor 
2 is connected to the alarm line L.sub.1 through a normally-closed contact 
S.sub.1 and a relay 3, and the cathode electrode of the thyristor 2 is 
connected directly to the alarm line L.sub.2 (common line). The anode 
electrode of the thyristor 2 is connected to one terminal of an AND gate 
4, and a pulse train of a fixed period is applied to the other terminal of 
the AND gate 4 from a pulse oscillator 5. With this arrangement, the AND 
gate 4 is opened only during the OFF periods of the thyristor 2 and output 
pulses of the oscillator 5 are applied to a counter 6. The counter 6 
counts input pulses. The anode electrode of the thyristor 2 is further 
connected to the strobe terminal of a latch circuit 8 and the reset 
terminal of the counter 6 through an inverter 7. The counter 6 is reset 
when the thyristor 2 turns ON and, thereafter, counts the output pulses of 
the oscillator 5. The count of the counter 6 is latched in the latch 
circuit 8 when the thyristor 2 turns ON next and, therefore, the count 
corresponds to the ON period of the thyristor 2. In other words, the count 
corresponds to the output of the sensor 1. When the counter 6 is reset, it 
begins to count again the pulses in the next period. If the reset pulse of 
the counter 6 is set to be delayed relative to the reset pulse of the 
latch circuit 8, the contents in the latch circuit 8 will, without fail, 
reflect the contents in the counter 6. 
If the latch circuit 8 simultaneously constitutes a shift register, the 
shift register can send out the contents stored in the latch circuit 8 to 
the alarm lines L.sub.1 and L.sub.2 in succession and thus the output of 
the sensor can be learned at the other end (the receiving unit side) of 
the alarm lines by receiving the count. In this situation, the information 
can be transferred in a short period of time and, therefore, signals can 
be sent out in the time sharing fashion from a plurality of sensors so 
that the sensors can be connected to one pair of alarm lines. 
It is a known method, although not shown in FIG. 9, that each sensor's own 
address information is sent out in addition to the count so that there is 
no interference among the sent-out data when a plurality of sensors are 
connected. In addition, the sensors are controlled at the receiving unit 
side or the sensor side so that the sensors do not send out data 
simultaneously. 
MERITS OF THE INVENTION 
As has been described above, in this invention, a charge-and-discharge 
circuit is provided which is reset by the turning ON of a switching 
circuit that turns ON at a fixed voltage, and the output of the 
charge-and-discharge circuit and the output of the sensor are input to the 
switching circuit, and the switching circuit is arranged to have its ON 
period vary in correspondence to the output voltage of a sensor. Any 
irregularity in the sensor can accordingly be monitored by changes in the 
ON period of the switching circuit. In other words, remote monitoring can 
easily be conducted. Further, the period varies with change in smoke 
concentration, etc. and, therefore, smoke concentration, etc. can be 
learned by measurement of the period or by count of the number of pulses 
during the period, etc. More particularly, it is possible to learn the 
extent and progress of a fire so that the operator has a broad 
understanding of the situation on which he can base his judgment regarding 
appropriate countermeasures. 
In this invention, further, as the output voltage of the 
charge-and-discharge circuit does not exert any influence on the operation 
voltage of the sensor, it does not take a long time before the sensor 
operation becomes stable after a test voltage is applied to the sensor, as 
often encountered in the prior art. This means that quick check and 
measurement are possible.