IE46081B1 - Smoke detector - Google Patents

Abstract

An alarm device (K) is actuated by an integrator (T) only if a photocell (C) receives a plurality of successive light pulses, reflected by smoke. In the disconnected mode, a light source (LED) is fed by a pulse generator (P) with low frequency. A first light pulse reflected by smoke occurring triggers via the photocell (C) a flip-flop (F), the output signal of which closes a switch (S1) in order to reduce the distance from the next pulse emitted by the pulse generator (P). Thus, if smoke persists, the number of pulses which the integrator (T) requires to set off the alarm is received in a shorter time. In another embodiment, the pulse frequency is increased by the first light pulse reflected by the smoke for the duration of a predetermined number of pulses or for the duration of a predetermined period of time. In both embodiments, the time required for the alarm to be set off is thus reduced, without increasing the power consumption of the detector device in the disconnected mode and without reducing the service life of the pulsating light source.

Description

This invention relates to smoke detectors, and one object is to provide a design of smoke detector which operates by use of reflected light, and which can give an alarm promptly on detection of the appearance of smoke while yet having reasonable immunity from false alarms.

According to the present invention a smoke detector comprises a light-producing device arranged normally to pulse at a first predetermined interval, a light detector arranged to produce a signal pulse in response to a light pulse when smoke is present, a circuit responsive to a predetermined number greater than one of signal pulses to provide an output signal, and means responsive to a first signal pulse to decrease the interval to at least the next light pulse to less than that of the first predeterm15 ined interval.

One of the problems with a smoke detector using reflected light, is to provide a source of light which is capable of operating for a long time without failure, and the light-produeing device is conveniently a light20 emitting diode because a light-emitting diode can operate for a long time without failure, and can provide sufficient light for detecting smoke if it is operated in high energy pulses with a reasonable interval between pulses. For example pulses of 20 microseconds duration with a repetit25 ion of 1 pulse for every few seconds can give adequate light and still allow the light-emitting diode to have a long life without failure. 4608 1 In accordance with the invention as soon as a signal pulse is generated, the interval between light pulses is reduced, and an output signal or alarm signal is only produced in response to two or more signal pulses to avoid the danger of a false alarm as much as possible. If for example, four or more pulses are required to produce an alarm, an alarm can still be given within one second of the conditions being detected if the pulse repetition rate is increased to five pulses per second when the first signal pulse is produced.

If any succeeding pulse does not produce a response indicating that smoke is present, it may be arranged that the pulse repetition rate returns to the slower or standby rate, or alternatively it may be arranged that the faster pulse rate is maintained for a certain number of pulses or for a certain interval of time. In either of the latter cases even if smoke is being detected continually, the alarm can be sounded intermittently because after each set period of faster rate pulses, there will be one interval at the slower rate during which the alarm is not sounded. That tends to make the alarm more effective.

An alarm may be of the kind which when switched on is automatically locked into an alarm condition regardless of its input signals until a manual switch is operated. Alternatively an alarm may be of a kind which is switched off as soon as the input to it falls, and the input may be arranged to decay naturally once smoke stops being detected.

The invention may be carried into practice in various ways and two embodiments will be described by way of example with reference to the accompanying drawings, in which:46031 - 4 Figure 1 is a schematic diagram of an electrical circuit for use in a smoke detector; Figure 2 is a diagram illustrating the time-spacing of the pulses applied to the various components of Figure 1; Figure 3 is a diagram illustrating the time-spacing of the pulses occurring in a modification of the circuit of Figure 1 when only a single pulse has detected smoke; Figure 4 is a diagram similar to that of Figure 3 10 illustrating the response when smoke is continuously present; and Figure 5 is a diagram illustrating the time-spacing of the pulses occurring in a further modification of the circuit of Figure 1 and the response when smoke is continuously present.

Figure 1 shows an electronic circuit for use in a smoke detector operating on the reflected light principle. Certain portions of the illustrated circuit are disclosed and claimed in U.S. Patent Specification No. 3,964,241 issued on March 23, 1976.

The circuit includes a light-emitting diode LED and a photo-voltaic cell C positioned out of the direct line of the beam of light from the LED. In a preferred embodi ment of the invention the cell C is positioned to view a portion of the beam in front of the LED at an angle of about 135° from the axis of the beam, to take advantage of the well-known forward scatter effect.

The output of cell C is utilised as the input to amplifier A, the output of which is fed to the set terminal of a bi-stable switching device such as a flipflop F. 43081 - 5 The term amplifier is meant to include any required circuitry for transforming a signal from the cell C into a signal usable by the flip-flop including any necessary stages of pre-amplification, and any means allowing an output therefrom only when the output signal reaches a predetermined level, such as a level detector. The flip-flop output is fed to an integrator I and to an electronic switch Si, which closes in response to the flip-flop output, for a purpose to appear hereinafter. The integrator I has a time constant such that a predetermined number or pulses greater than one are required into the integrator to provide an output therefrom to the alarm K.

To provide a pulse of current to the LED and for other purposes to be described, a pulse generator P is provided, which connects to a power supply through a resistor Rl.

The electronic switch SI and a resistor R2 are connected in parallel with the resistor Rl. With the switch Si open, the current to the pulse generator P has a value such that the pulse rate is, for example 1 pulse every 5 seconds.

When the switch Si is closed, so that resistor R2 is in parallel with resistor Rl, the increased current increases the pulse rate to 1 pulse every 2 seconds.

In addition to providing a pulse to the LED, the pulse generator also applies substantially simultaneously a pulse of substantially the same duration to a normally closed switch S2 to pulse it to the open condition for the duration of the pulse and a pulse to the re-set terminal of the flip-flop through a discriminator D which converts the pulse to a spike at the beginning of the pulse cycle.

The switch E2 is connected between the output of the amplifier and ground, sc that the amplifier output is shorted to ground except during the time that the switch 6 0 81 - 6 S2 is pulsed open by the pulse generator.

The operation of the device can best be described by reference to Figure 2 which is a graph of the response of the-various components of the circuit during a pulse with a predetermined level of smoke present in the light beam.

The horizontal scale represents time and the vertical scale represents response. The vertical scale units are arbitrary and the heights on the vertical scale of the various curves have no relation to each other except as described hereinafter .

Each cycle begins with the application of a pulse from the pulse generator to the LED, the amplifier output clamp switch S2, and the re-set terminal of the flip-flop.

The pulse to the LED and the switch S2 are both represented on the diagram by Pl, since they are of the same duration. They may, of course be of different magnitudes and different polarities.

The pulse appearing at the re-set terminal of the flip-flop after passing through the discriminator is represented by PD1, and ensures that the flip-flop is reset or turned off at the beginning of each pulse cycle.

The application of the pulse to the LED produces a light output having a duration and relative intensity represented by curve LI.

If there is no smoke in the portion of the beam viewed by the cell C there will be no pulse of voltage generated by the cell and hence no output from the amplifier, and at the end of the pulse Pl the LED is deenergised and the switch S2 again closes to clamp the amplifier output to ground.

However, if there is smoke present in the light beam, a pulse of voltage will be produced by the cell, represented by curve VI of Figure 2 which will be amplified by 6 0 8 1 - 7 the amplifier to produce a signal at the set terminal of the flip-flop, provided that the amount of smoke is great enough to produce an output signal of the predetermined level. For example, it is common to allow an output signal, and hence an alarm, only when there is a predetermined concentration of smoke, such as 1 or 2%.

The percent smoke is usually defined as the amount of smoke that absorbs that percent of a light beam 1 foot long.

As illustrated in Figure 2, the amplifier signal level necessary to allow an output to the flip-flop is represented by dashed horizontal line L. Adjustment means (not shown) may be provided in the amplifier to adjust the calibration of the system so that the alarm point will be at the desired smoke percentage.

If the amount of smoke viewed by the cell has reached the specified concentration, the amplifier output will be as shown in curve A reaching line L at point Y, thereby applying a signal to the flip-flop set terminal, thereby turning on the flip-flop output (illustrated by curve FF) and closing switch Si to increase the current to the pulse generator (illustrated by curve C ).

The output pulse from the flip-flop is stored in the integrator '1'. The increased current through the pulse generator P increases the pulse rate to a predetermined value, such as one pulse every 2 seconds or 5 pulses per second. As illustrated in Figure 2, the pulse repetition rate during stand-by operation (Case A) is 5 seconds, if smoke has not been detected. However, if smoke of the specified amount has been detected, as illustrated in Figure 2, the time to the next pulse (P2) is reduced to 2 seconds (Case B). 6 Ο β 1· - 8 At the beginning of pulse P2, the LED is again energised, the switch S2 opened, and a spike pulse applied to the flip-flop re-set terminal. Hence at the beginning of the second pulse, the flip-flop output is turned off, so that the switch SI opens, returning the pulse generator to its previous rate of one pulse per 5 seconds. Hence if the second pulse does not detect sufficient smoke to produce an output from the amplifier to the flip-flop, the pulse rate remains at the stand-by rate.

However, if the second pulse also detects smoke, the various components will react in the same manner as illustrated in Figure 2 resulting from pulse Pl, the pulse generator will again return to the faster rate, by reason of the second flip-flop output and a second pulse will be stored in the integrator.

Although the switch SI opens at the beginning of each pulse, if smoke is detected during that pulse, the switch SI closes again after about 10 micro-seconds (depending on the smoke concentration and the resulting rate of rise of the amplifier output, which determines the times at which curve A reaches level L).

Hence so long as each pulse detects smoke, the pulse generator will continue to run at the faster rate, since the open time of switch Si is only about 10 micro-seconds out of the total time between pulses of 200,000 microseconds (.2 seconds).

If the integrator 111 is designed to actuate the alarm K when the integrator has received 5 consecutive pulses, the alarm will be sounded in less than one second after the first pulse is received, even though the standby pulse rate is one every 5 seconds. - 9 In one form of this embodiment of the invention the pulses will continue to run at the faster rate until the smoke has cleared from the detector, at which time the alarm will shut off and the pulses will return to the standby rate.

However, in some systems, utilizing the detector it may be desirable to lock the alarm into the energized condition when the required number of pulses are received by the integrator.

To prevent the pulses from continuing to run the LED at the increased rate and to lock on the alarm until deenergised, means may be provided to de-energise the pulser when the integrator produces an alarm actuation signal.

For example, a switch S3 may be provided in the pulser circuit, which is opened by the output signal from the integrator when the integrator has received the required number of pulses. Hence when the alarm sounds, the pulser is de-energised at a time when a set signal has been applied to the flip-flop, providing a continuous signal from the flip-flop to the integrator, causing a continuous output to the alarm. The pulses remain deenergised until the signal from the integrator to the alarm is manually terminated by opening switch S4 in the integrator power supply line. If the alarm K is not of the lock-on type, it will turn off when the output from the integrator I terminates.

Although in the illustrated embodiment the normally closed switch S2 clamps the amplifier output signal to ground to prevent an output signal from the amplifier during the time that the LED is not energised, it will be understood that this switch may be positioned with equal effectiveness at other points in the system. The means 6 0 81 - 10 for preventing the passage of a signal when the LED is not energised may be a normally open switch disposed in series in the amplifier output line which is pulsed closed when the LED is energised.

Although -in Figure 2 the vertical line representing the flip-flop output and the vertical line representing the increase in current CP through the pulse generator are separated by a small horizontal distance, it will be understood that this is for clarity, since these two events occur substantially simultaneously.

In a modification of Figure 1, the output from the flip-flop F is coupled to the switch through a timer shown at 2? which keeps switch closed for a time equal to at least that of five higher rate pulses, e.g. for 1 second at 5 pulses per second.

If each of the following pulses does not produce a flip-flop output, the requirements of the integrator 1 are not satisfied, and at the end of the 5th pulse P^, the timer T opens switch and the pulse generator returns to the stand-by rate of 1 pulse each 5 seconds. This is illustrated in Figure 3.

However, as illustrated in Figure 4, if each of the subsequent 4 pulses produces a flip-flop output due to the continuing presence of smoke, the alarm is sounded on the fifth pulse, and each pulse from the flip-flop to the timer restarts the timer, so that the pulsing continues at the fast rate so long as smoke is present, plus 4 pulses. That is, if the smoke clears and pulse Px and subsequent pulses do not detect smoke, at the end of pulse Px+3, the pulse generator will return to the stand-by rate.

A further modification is similar to the modification described above in that the timer T comprises a pulse 6 0 81 - 11 counter which is responsive to a first pulse from the flipflop to close switch to increase the pulse rate.

However, the pulse counter is not responsive to subsequent pulses from the flip-flop to extend the time during which switch is closed, but holds switch closed for a predetermined time whether or not there is any further flipflop output. The predetermined time may be established in any convenient manner, such as by an RC circuit, or by pulses from the pulse generator P.

If smoke is not detected on each of the subsequent pulses, the requirements of the integrator I are not satisfied, and the alarm is not sounded. However as illustrated in Figure 5, if smoke is detected on all of the subsequent pulses, the alarm is sounded and the pulse generator returns to the slow rate.

Since the flip-flop output to the integrator continues after the end of any pulse by which smoke was detected, the alarm will continue to be energised until the beginning of the next pulse at which the spike pulse to the re-set terminal of the flip-flop at the beginning of pulse Ρθ turns off the flip-flop output. The integrator output then decays and the alarm turns off unless it is of the lock-on-type, when the switch is used.

If smoke is still present, the pulse Ρθ will initiate a fresh signal on the set terminal of the flip-flop which will again start the pulse counter, closing switch to again increase the pulse rate. If smoke continues to be present on the subsequent 4 pulses, the alarm will again be energised on pulse Ρ^θ.

Hence during the presence of smoke, the alarm will be energised only between pulses when the pulse generator is running at the slow rate, and is off during the period ,46081 that the pulse generator is running at the fast rate.

This not only provides an intermittent alarm signal, which is more effective than a steady signal, it also prevents line transients caused by the energised alarm from affect5 ing the amplifier output.

In the illustrated embodiments a standby pulse rate of 1 pulse every 5 seconds, and a detection pulse rate of .2 seconds and a requirement of 5 consecutive pulses to energize the alarm are quoted by way of example only.

Any embodiment may utilise the system disclosed and claimed in U.S. Patent Specification No. 3,917,956 wherein the detector is isolated from the power supply during the time the light-emitting diode is energised and during this period is powered by a charge stored in a capacitor.

Claims (11)

1. CLAIMS: 1. A smoke detector, comprising a light-producing device arranged normally to pulse at a first predetermined interval, a light detector arranged to produce a signal pulse in response to a light pulse when smoke is present, a circuit responsive to a predetermined number greater than one of signal pulses to provide an output signal, and means responsive to a first signal pulse to decrease the interval to at least the next light pulse to less than that of the first predetermined interval.

2. A detector as claimed in Claim 1 in which the responsive means decreases the interval to the next pulse only.

3. A detector as claimed in any preceding claim including a bi-stable switch responsive to a signal pulse to shift from a normal condition to a second condition to produce an output signal.

4. A detector as claimed in Claim 3 including an integrator receiving the bi-stable switch output signals, said integrator being responsive to a predetermined number of output signals in a specified time to produce an output signal.

5. A detector as claimed in Claim 3 or Claim 4 including means returning the bi-stable switch to the normal condition after each signal pulse.

6. A detector as claimed in any of Claims 3-5 in which the responsive means is responsive to an output signal from the bi-stable switch to increase substantially the pulse rate of the light-producing device.

7. A detector as claimed in any preceding Claim in which the pulse rate of the light-producing device increases for a predetermined time sufficient to produce - 14 at the increased rate at least said predetermined number of signal pulses less one.

8. A detector as claimed in Claim 7 in which the pulse rate is returned to the rate corresponding to the 5 predetermined interval after said predetermined time whether or not subsequent light pulses have produced a signal pulse.

9. A detector as claimed in Claim 7 in which means is provided for causing the pulse rate to return to the 10. Rate corresponding to the predetermined interval when the output signal is produced.

10. A detector as claimed in Claim 8 or Claim 9 including means for stopping the output signal prior to the next following light pulse, whereby when smoke is 15 continuously present, the output signal is produced intermittently.

11. A smoke detector constructed and arranged substantially as herein specifically described with reference to the accompanying drawings.