GB1592475A - Smoke detectors - Google Patents

Description

(54) SMOKE DETECTORS (71) I, CHEUNG KING FUNG a British subject t/a DOREME INDUSTRIAL Co., of 27A Chatham Road, 13/F, Kowloon, Hong Kong, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to smoke detectors.

According to the present invention, a smoke detector comprises a self-luminous light source, and a photosensitive device which is so arranged that the amount of light which it receives from the light source is changed by the presence of smoke in a detecting region. Preferably, the light source is a gaseous tritium light source.

The photosensitive device may conveniently be a phototransistor, and may drive the warning device, for example a hooter, through a directly coupled transistor amplifier, which need only include very few components.

The light source and the photosensitive device are preferably so arranged that light from the light source cannot reach the photosensitive device directly, but can reach the photosensitive device when scattered from smoke or the like in the detecting region.

Such a smoke detector may or may not include lenses to focus the rays from the light source and to focus the scattered light on to the photosensitive device.

The invention may be carried into practice in various ways, but one particular smoke detector and alarm and a modification thereof will now be described by way of example, with reference to the accompanying drawings, of which: Figure 1 is a circuit diagram of the smoke detector and alarm; Figure 2 is a sectional view of the smoke detector and alarm, showing the physical arrangement of the smoke detecting components; Figure 3 is a sectional view, similar to part of Figure 2, but showing a modified arrangement of the smoke detecting components; and Figure 4 is a perspective view showing some of the components of Figure 3.

As can be seen from Figure 2, the smoke detector and alarm includes a light source 10 and a phototransistor 12, which are associated with a smoke chamber 14 mounted inside the outer casing of the alarm. Air from outside the outer casing of the alarm can reach the interior of the smoke chamber 14 through openings (not shown) in outer casing of the alarm, and opening 16 and 18 which lead into the smoke chamber itself. Light from the source 10 is focussed by a lens 20 to form a beam which shines across the smoke chamber. The phototransistor 12 is mounted in a similar manner to the light source 10; that is to say, it 'looks' into the smoke chamber 14 through a lens 22.As can be seen from Figure 2, the optical axes of the light source and lens system 10, 20 and of the phototransistor-lens system 12, 22 lie at about 60C to one another, and each of these systems is fitted with a tubular shield 24; this arrangement means that the beam from the lens 20 cannot reach the lens 22 directly. If the air in the smoke chamber 14 is completely clear, the beam will cross the chamber without being scattered, so that no light reaches the lens 22 either directly or indirectly. The phototransistor 12 remains unilluminated, and passes only a very small current.

If a quantity of smoke should now enter the smoke chamber 14, the beam of light from the lens 20 will be at least partially scattered, and some of this scattered light will fall on the lens 22 and be focussed on to the phototransistor 12. Thus the presence of smoke will result in an increase in the phototransistor currant, and this increase in current is used to start an alarm.

The light source 10 comprises a gaseous tritium light source. This type of light source consists of a bulb of borosilicate glass, having an internal phosphor coating and filled with tritium gas. Since tritium is a radioisotope of hydrogen, it emits radiation in the form of low energy beta particles, which cause the phosphor to glow. Thus, such a light source will continuously emit light, reliably and without an external power source.

The circuit controlling the alarm will now be described, with reference to Figure 1.

The phototransistor 12 is connected in series with a current limiting resistor 26 between the positive power supply line of the circuit and the base of the first transistor 28 of a Darlington arrangement of three transistors 28, 29 and 30. The collector load of this Darlington combination is formed by an alarm hooter 32, which is of a type which will sound whenever a d.c. voltage is applied across it. Thus, the hooter 32 will sound whenever the phototransistor 12 becomes sufficiently illuminated to turn on the Darlington combination 28, 29 and 30. In this way, an audible warning is given when smoke is detected.

A preset adjustable resistor 34 is connected between the base of the transistor 28 and the negative supply line (to which the emitter of the transistor 30 is also connected). This resistor allows the overall sensitivity of the smoke detector to be adjusted, so that the variations in the sensitivity of individual phototransistors can be compensated.

A bypass capacitor 36 and a freewhecl diode 38 are connected across the hooter 32, to protect the transistor 28, 29 and 30 from damage by transients generated by the hooter.

The power for the circuit is supplied from a battery 68, in parallel with a reservoir capacitor 44. Since no electrical power is required to provide the illumination in the smoke chamber, the circuit will draw practically no current from the battery under normal conditions, so that the battery should have a long life. It should be mentioned, however, that it is possible to provide means whereby an alarm is given if the battery voltage should fall below a predetermined value.

The colour of the light emitted by the gaseous tritium light source is determined by the composition of the phosphor coating; for the present purpose, a phosphor giving white or orange light is suitablc. The brightness of the source may typically be up to 3 foot lamberts, and the resulting intensity might be about 0.6 candle power. The half-life of the complete light source would be about 10 years; this results both from the decay of the tritium (half-life 12.3 years) and degradation of the phosphor.

Figures 3 and 4 show a modified arrangement of the smoke detecting components.

Primed reference numerals will be used to denote components corresponding to components shown in Figure 2. The smoke chamber 14' is a simple rectangular shape, with a self-luminous light source 10' mounted outside an opening 16' in one of its walls; this opening, together with an opening 18' in the opposite wall, allows smoke to penetrate into the chamber 14'. The light source 10' is not provided with either a shield or a lens. A tubular shield 24' for a phototransistor 12' is mounted in one of the other walls of the chamber 14'; again, no lens is used.As can be seen from Figure 3, light from the source 10' cannot reach the phototransistor 12' directly, but the 'fields of view' of the light source and the phototransistor overlap in the area which is shaded in Figure 3, and if any appreciable amount of smoke is present in this area, light from the light source 10' will be scattered and picked up by the phototransistor 12', in much the same way as with the arrangement of Figure 2.

WHAT I CLAIM IS: 1. A smoke detector comprising a selfluminous light source, and a photosensitive device which is so arranged that the amount of light which it receives from the light source is changed by the presence of smoke in a detecting region.

2. A smoke detector as claimed in Claim 1, which also includes a warning device which is connected to be controlled by the photosensitive device through a direct-coupled transistor amplifier.

3. A smoke detector as claimed in Claim 1 or Claim 2, in which the photosensitive device is a phototransistor.

4. A smoke detector as claimed in Claim 1 or Claim 2 or Claim 3, in which the light source, and the photosensitive device are so arranged that light from the light source cannot reach the pElotosensitive device directly, but can reach the photosensitive device when scattered from smoke or the like in the detecting region.

5. A smoke detector as claimed in Claim 4 which is so arranged that, apart from the said scattering, light from the light source reaches the photosensitive device without refraction or reflection.

6. A smoke detector as claimed in Claim 4 which includes at least one lens in the path of light from the light source to the photosensitive device.

7. A smoke detector as claimed in any of the preceding claims, in which the light source is a gaseous tritium light source.

8. A smoke detector substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. The circuit controlling the alarm will now be described, with reference to Figure 1. The phototransistor 12 is connected in series with a current limiting resistor 26 between the positive power supply line of the circuit and the base of the first transistor 28 of a Darlington arrangement of three transistors 28, 29 and 30. The collector load of this Darlington combination is formed by an alarm hooter 32, which is of a type which will sound whenever a d.c. voltage is applied across it. Thus, the hooter 32 will sound whenever the phototransistor 12 becomes sufficiently illuminated to turn on the Darlington combination 28, 29 and 30. In this way, an audible warning is given when smoke is detected. A preset adjustable resistor 34 is connected between the base of the transistor 28 and the negative supply line (to which the emitter of the transistor 30 is also connected). This resistor allows the overall sensitivity of the smoke detector to be adjusted, so that the variations in the sensitivity of individual phototransistors can be compensated. A bypass capacitor 36 and a freewhecl diode 38 are connected across the hooter 32, to protect the transistor 28, 29 and 30 from damage by transients generated by the hooter. The power for the circuit is supplied from a battery 68, in parallel with a reservoir capacitor 44. Since no electrical power is required to provide the illumination in the smoke chamber, the circuit will draw practically no current from the battery under normal conditions, so that the battery should have a long life. It should be mentioned, however, that it is possible to provide means whereby an alarm is given if the battery voltage should fall below a predetermined value. The colour of the light emitted by the gaseous tritium light source is determined by the composition of the phosphor coating; for the present purpose, a phosphor giving white or orange light is suitablc. The brightness of the source may typically be up to 3 foot lamberts, and the resulting intensity might be about 0.6 candle power. The half-life of the complete light source would be about 10 years; this results both from the decay of the tritium (half-life 12.3 years) and degradation of the phosphor. Figures 3 and 4 show a modified arrangement of the smoke detecting components. Primed reference numerals will be used to denote components corresponding to components shown in Figure 2. The smoke chamber 14' is a simple rectangular shape, with a self-luminous light source 10' mounted outside an opening 16' in one of its walls; this opening, together with an opening 18' in the opposite wall, allows smoke to penetrate into the chamber 14'. The light source 10' is not provided with either a shield or a lens. A tubular shield 24' for a phototransistor 12' is mounted in one of the other walls of the chamber 14'; again, no lens is used.As can be seen from Figure 3, light from the source 10' cannot reach the phototransistor 12' directly, but the 'fields of view' of the light source and the phototransistor overlap in the area which is shaded in Figure 3, and if any appreciable amount of smoke is present in this area, light from the light source 10' will be scattered and picked up by the phototransistor 12', in much the same way as with the arrangement of Figure 2. WHAT I CLAIM IS:

1. A smoke detector comprising a selfluminous light source, and a photosensitive device which is so arranged that the amount of light which it receives from the light source is changed by the presence of smoke in a detecting region.

2. A smoke detector as claimed in Claim 1, which also includes a warning device which is connected to be controlled by the photosensitive device through a direct-coupled transistor amplifier.

3. A smoke detector as claimed in Claim 1 or Claim 2, in which the photosensitive device is a phototransistor.

4. A smoke detector as claimed in Claim 1 or Claim 2 or Claim 3, in which the light source, and the photosensitive device are so arranged that light from the light source cannot reach the pElotosensitive device directly, but can reach the photosensitive device when scattered from smoke or the like in the detecting region.

5. A smoke detector as claimed in Claim 4 which is so arranged that, apart from the said scattering, light from the light source reaches the photosensitive device without refraction or reflection.

6. A smoke detector as claimed in Claim 4 which includes at least one lens in the path of light from the light source to the photosensitive device.

7. A smoke detector as claimed in any of the preceding claims, in which the light source is a gaseous tritium light source.

8. A smoke detector substantially as herein described with reference to the accompanying drawings.