GB2030288A - Photo-electric fire detectors - Google Patents

Abstract

A fire-radiation detector (Figure 3) comprises sensitive elements (R, B) having different spectral response ranges. The a.c. output of each element is amplified by a respective multistage amplifier (17, 20; 57, 60). The outputs of each pair of corresponding stages are applied to respective comparators (34, 44) each yielding an alarm output when the output of the element (R) sensitive to longer wavelengths exceeds that of the other element. To avoid malfunctions due to the circuitry saturating in the presence of strong radiation, a further comparator 23 produces an alarm signal if the d.c. level of such radiation received by element R exceeds a pre-set level. The detector can thus signal fires at a large distance without the danger of failure to signal a nearby fire due to saturation. <IMAGE>

Description

SPECIFICATION Optical radiation detector This invention relates to optical radiation detectors of the type which are responsive to radiation in two different spectral ranges and are arranged to provide an alarm signal denotive of the presence of a fire when the ratio of the magnitudes of the responses within these two spectral ranges exceeds a predetermined value.

Such radiation detectors are conveniently referred to as "two-wavelength" detectors.

Differences between the present invention and the prior art will now be described with reference to the accompanying drawings, in which: Figure lisa block diagram of one known type of radiation detector; Figure 2 is a block diagram of another known type of radiation detector, and Figure 3 is a circuit diagram of a radiation detector embodying the present invention.

In the known two-wavelength type of radiation detector shown in Figure 1, which may be used for detecting fires in road tunnels, there are provided a pair of radiation sensitive elements B and R having different spectral response ranges such that they respond to radiation from a fire. Element B is responsive to radiation in a shorterwavelength range and element R is responsive to radiation in a longer-wavelength range. Lowfrequency output signals provided by element B and R are amplified by respective amplifiers Ab and Arand when the ratio of the amplitudes of the output signals of the two amplifiers exceeds a predetermined value, a comparator K responds and actuates a switching circuit S to give rise to an alarm signal.

This simple form of two-wavelength detector has, however, the disadvantage that if its sensitivity is increased so that the detector will respond appropriately to weak radiation emanating from a fire located at a great distance from the detector, or from a small fire located near to the detector, the amplifiers are immediately saturated by intense radiation from a fire, or from a fire very close to the detector, so that the detector becomes inoperative and does not yield an alarm despite the presence of the fire.

In order to eliminate this drawback, an improved two-wavelength radiation detector has been proposed in Japanese Utility Model Application Sho 51-85853 and is illustrated in Figure 2 of the accompanying drawings.

Radiation-sensitive elements B and R, again respectively responsive in a shorter-wavelength range and in a longer-wavelength range, are coupled to respective similar amplifier chains Abi, Ab2, Ab3 and Ar1, Ar3, Ar3. The two outputs of each pair of amplifiers are compared in respective comparators K1, K2 and K3. If a predetermined amount of imbalance exists between the outputs of any pair of amplifiers that is not saturated, this condition is detected by the respective comparator and the resultant signal is fed through an OR gate G to actuate a switching circuit S yielding an alarm signal.

If, however, the number of amplifier stages is made large, so that the detector can respond to radiation over a very wide range of intensities, then the apparatus becomes very costly. It is found in practice that it is difficult to avoid an inoperative range for a large-scale fire or a fire which is very close to the detector.

It is the object of the present invention to eliminate the disadvantage discussed above by the addition to the known detector of a simple circuit.

According to the present invention there is provided a radiation detector responsive to radiation generated by fire and comprising a pair of radiation-sensitive elements having mutually different spectral response ranges, a respective signal or multi-stage low-frequency alternatingcurrent amplifier arranged to amplify the output of each of said radiation-sensitive elements, a respective comparator arranged to compare the amplitudes of the output signals from the or each stage of said amplifiers and to yield an alarminitiating signal when the amplitude of the signal representing the longer-wavelength component of radiation exceeds that of the signal representing the shorter-wavelength component, and a further comparator arranged to respond to the presence of a d.-c. output in excess of a predetermined magnitude from the element sensitive to longerwavelength radiation by developing an alarminitiating signal.

A two-wavelength radiation detector embodying the invention will now be described with reference to Figure 3 of the accompanying drawings. Power is supplied to the illustrated apparatus by a d.-c. source (not shown) of which the positive pole is connected to terminal Vc and the negative pole to terminal Vo. A resistor 11 and a Zener diode 1 2 are connected in series between terminals Vc and Vo. The junction point between the resistor 11 and the Zener diode 12 thus provides an intermediate potential V'.An element B sensitive to radiation within a shorter wavelength range is provided by a solar cell 13, arranged for example to respond to blue light, by being provided with an optical filter or otherwise being suitably arranged for receiving shorterwavelength light, for example, within the wavelength range of 0.7 to 0.75 ssm. An element R sensitive to radiation within a longer wavelength range is provided by a solar cell 53 arranged for example to respond to red light, by being provided with an optical filter or otherwise being suitably arranged for receiving longer-wavelength light, for example within the wavelength range of 0.8 to 1.1 Um.

Each of the solar cells 1 3, 53 is shunted by a respective resistor 14, 54 and provides a signal, by way of a respective series combination of a resistor 15, 55 and a capacitor 16, 56 to the inverting input of a respective low-frequency amplifier 17, 57. These amplifiers, and other devices similarly represented in the drawing, receive power supplies from the Vc and Vo lines, the connections being omitted, as is conventional.

Each of amplifiers 17, 57 may be an operational amplifier having a frequency response range of 3 to 20 Hz. The non-inverting inputs of these ampifiers are connected to the V' line, as are the cathodes of the solar cells 13, 53.

Signals appearing at the outputs of amplifiers 17, 57 are applied by way of respective series combinations of resistors 1 8, 58 and capacitors 19, 59 to the inverting inputs of further amplifiers 20, 60, of which the non-inverting inputs are connected to the V' line.

Between the Vc and V' lines there is also connected a voltage divider formed by series connected resistors 21,22, the voltage appearing at the junction of these two resistors being applied to one input of a comparator 23. The other input of comparator 23 receives the d.c. signal developed by the red-sensitive element R. The comparator 23 is so arranged that when the output of red-sensitive solar cell 53 exceeds the potential set by resistors 21, 22 an output signal is fed through a diode 24 toan output terminal 25, which is returned to the Vo line by way of a resistor 26.

The output signals of amplifiers 17, 57 are applied by way of respective capacitors 27, 67 to the tappings of voltage dividers, formed by resistors 28, 29 and 68, 69 respectively, each of which dividers is connected between the Vo and V' lines. Each of these tappings is connected to the Vo line by way of a respective series combination of a diode 30, 70,e resistor 31, 71 and the parallel combination of a resistor 32, 72 and a capacitor 33, 73. Signals appearing across these parallel combinations are applied to respective inputs of a comparator 34, the output of which is applied to the output terminal 25 by way of a diode 35.

The output signals of amplifiers 20 and 60 also are applied by way of respective capacitors 37, 77 to the tappings of voltage dividers, formed by resistors 38, 39 and 78, 79 respectively, each of which dividers is connected between the Vc and Vo lines. Each of these tappings is connected to the Vo line by way of a respective series combination of a diode 40, 80, a resistor 41, 81 and the parallel combination of a resistor 42, 82 with a capacitor 43, 83. Signals appearing across these parallel combinations are applied to respective inputs of a comparator 44, the output of which is applied to the output terminal 25 by way of a diode 45.

The mode of operation of the circuit .arrangement described above is as follows: In normal environmental light the intensity of the blue component exceeds that of the red component. Even if these components flicker at a frequency within the range of 3-20 Hz, for some reason, so that the outputs of the light-sensitive elements B and R are amplified by amplifiers 1 7.

20 and 57, 60 and are applied to the inputs of comparators 34, 44, after being rectified by diodes 30, 40 and 70, 80 and smoothed by the respective parallel combinations, the relation "blue light > red light" remains unchanged and consequently neither of the comparators 34 and 44 yields an output signal and an alarm is not given. Comparator 23 will also be inoperative, because the normai intensity of the red light component does not exceed the predetermined value set by voltage divider 21 22. There is, therefore, no signal appearing at the output terminal 25 and an alarm switching circuit (not shown) responsive to the signal appearing at output terminal 25, will not be operated.

On the other hand, when a fire breaks out and radiation having a flicker frequency within the range of 3-20 Hz and a spectral composition such that the red light component is greater than the blue light component reaches the solar cells B and R, the resultant signals, if these are of small amplitude resulting from a fire at a distance from the detector, are amplified by amplifiers 1 7, 20 and 57,60 and the rectified outputs of the latter amplifiers are applied to comparator 44, which yields an output signal to output terminal 25 by way of diode 45, thus initiating an alarm.

In the case of a fire located fairly close to the detector, the intensity of radiation reaching the detector is comparatively large. This intense radiation yields signals of such amplitudes that the amplifiers 20, 60 become saturated and therefore inoperative. Amplifiers 1 7 and 57 in the first stage, on the other hand, operate normally without being saturated and their output signals are rectified and smoothed and applied to comparator 34, which yields an output signal via diode 35 to output terminal 25, thus causing an alarm to be given.

If a fire breaks out close to the detector, or a very intense fire occurs elsewhere, even the first stage amplifiers 17, 57 become saturated and therefore inoperative, owing to the large amplitude input signals which they receive from solar cells 1 3, 53, so that neither of comparators 34 or 44 yields an alarm output. Comparator 23, however, is arranged to compare only d.c.

signals, and since under these conditions the output of red-sensitive solar cell 53 exceeds the predetermined voltage level set by voltage divider 21, 22, the comparator yields an output signal to terminal 25 by way of diode 24.

By the provision of comparator 23 it thus becomes possible to detect without fail a fire of great intensity or a fire located close to the detector. Moreover, the signal input to comparator 23 is obtained from the solar cell 53 which responds to red light, i.e. light of long wavelength, which is rarely included in environmental and interfering light For this reason the detector embodying the invention has a low susceptibility to false operation and is reliable in operation.

Although the embodiment of the invention which has been described above uses two amplifier stages in the channel connected to each of the differently radiation-sensitive elements, it will be understood that either one amplifier only, or a number of amplifiers greater than two, may be used in each channel, if appropriate to the particular application.

Claims (4)

1. A radiation detector responsive to radiation generated by fire and comprising a pair of radiation-sensitive elements having mutually different spectral response ranges, a respective signal or multi-stage low-frequency alternating current amplifier arranged to amplify the output of each of said radiation-sensitive elements, a respective comparator arranged to compare the amplitudes of the output signals from the or each stage of said amplifiers and to yield and alarminitiating signal when the amplitude of the signal representing the longer-wavelength component of radiation exceeds that of the signal representing the shorter-wavelength component, and a further comparator arranged to respond to the presence of a d.-c. output in excess of a predetermined magnitude from the element sensitive to longerwavelength radiation by developing an alarminitiating signal.

2. A radiation detector in accordance with claim 1 wherein said radiation-sensitive elements respond respectively to light in the wavelength ranges of 0.6 to 0.75 ,um and 0.8 to 1.1 film.

3. A radiation detector in accordance with claim 1 or claim 2 wherein said amplifiers are arranged to respond to signals within the frequency range of 3 to 20 Hz.

4. A radiation detector in accordance with claim 1 and substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.