HK1183371B - Optical smoke detector - Google Patents

Description

Optical smoke detector

The present invention relates to optical smoke detectors.

Optical smoke alarms use an infrared emitting source LED which is typically driven by a constant current source. The signal level generated by the infrared receptor from the light reflected from the smoke is compared to a fixed reference to determine whether an alarm threshold for smoke has been reached.

The present invention seeks to provide an improved optical smoke detector.

Accordingly, the present invention provides an optical smoke detector comprising: a light source; an optical receiver; and a control circuit for controlling operation of the detector; wherein the control circuit is made to: applying an unregulated voltage to the light source to cause it to emit light; monitoring a current through the light source so as to monitor light emitted by the light source; monitoring a current generated by light received through the light receiver to monitor the light received by the light receiver; generating a proportional signal indicative of a proportion of the monitored current; and comparing the proportional signal with a reference value and generating a smoke detection signal therefrom.

By using an unregulated power supply and monitoring the actual current through the light source and light receiver, and then determining the ratio of the two, the detection circuit can be greatly simplified and eliminate components, especially the need for a regulated power supply, as opposed to relying on a regulated power supply for constant light output and comparing the received light to a preset entity.

Preferably, the light source is an LED and preferably the current through the light source is in the linear range of the LED. In one arrangement, the light source may be unregulated and the current through the light source may be in the range of 200mA to 600 mA.

Preferably, the light source is driven by a high side semiconductor device and the control circuit is configured to switch on the high side semiconductor device for a preselected time period at preselected time intervals.

The preselected time period is typically 100 mus and the preselected time interval is typically 10 seconds.

Preferably, the light source is a light emitting diode and conveniently the light is infrared light.

The invention also provides a method of operating an optical smoke detector comprising a light source and a light receiver, the method comprising: energizing the light source with an unregulated voltage to cause the light source to emit light; monitoring a current through the light source so as to monitor light emitted by the light source; monitoring a current through the optical receiver to monitor light received by the optical receiver; determining a proportion of the monitored current to provide a proportion representative of the proportion of the received light and the emitted light; comparing the ratio to a reference value; and generating a smoke detection signal in dependence thereon.

Preferably, the current through the light source is. In one arrangement, the light source may be unregulated and the current through the light source may be in the range of 200mA to 600 mA.

Preferably, the current through the light source is in the linear range of the LED. In one arrangement, the light source may be unregulated and the current through the light source may be in the range of 200mA to 600 mA.

Conveniently, the light source is energized for a preselected time period at preselected time intervals.

Preferably, the light source is driven by a high side semiconductor device and the method comprises turning on the high side semiconductor device for a preselected time period at preselected time intervals.

Typically, the preselected time period is 100 μ s and the preselected time interval is 10 seconds.

Conveniently, the light source is a light emitting diode and the light is infrared light.

The invention is further described by way of example hereinafter with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view from below of an alarm according to a preferred form of the invention;

FIG. 2 is a side elevational view of the alarm of FIG. 1;

FIG. 3 is a circuit diagram of the control circuit portion of the alarm of FIG. 1; and

fig. 4a and 4b are diagrams illustrating the operation of the control circuit.

Referring to these figures, there is shown a preferred form of optical smoke alarm 110 having a housing 112, the housing 112 having a base 114 and a cover 116. The base can attach the alarm to the surface of the ceiling of the main room by any suitable means. The base generally has a flat bottom surface 118 for interfacing with a ceiling or intermediate mounting plate, and side surfaces 120. The latter has a plurality of openings 122 arranged along the circumference of the alarm for allowing the ingress of smoke and the like. The lid 116 is generally "cup" or "dish-shaped" having a side surface 124 and a bottom surface 126 to define the interior of the lid. The bottom surface 126 generally has an inner surface (not shown) facing the base 114.

The alarm has an optical sensor 131 preferably contained in the housing between the inner surface 127 and the base 114 and a control circuit 130 for controlling the operation of the detector. The alarm may also include a sound generator 132 (fig. 3) which emits an audible alarm when triggered by the control circuitry in response to signals received from the sensors. Alternatively or additionally, the sounder may be mounted remotely from the alarm and activated by radio or other wireless signal transmission.

Referring to fig. 3, a light emitting circuit 150 of the control circuit 130 is shown, wherein a high side driver gate 152 is used to switch current into a light source 154 of the optical sensor 131. In the illustrated embodiment, the high side driver gates are transistors, but any suitable semiconductor device may be used. Preferably the light source is a Light Emitting Diode (LED) and the emitted light is preferably Infrared (IR) light. Conventional approaches typically use low side driver transistors (e.g., NPN transistors) for regulating current. However, this requires a higher minimum supply voltage to ensure regulation. In the preferred embodiment of fig. 3, transistor 152 is fully turned on to drive LED 154 and the current is regulated.

A current limiting device is used to limit the current through the light source 154. In the illustrated embodiment, the current limiting device is formed by a voltage divider resistor chain including resistors 156, 158. The emitter of the transistor 152 is connected to a power supply line 162, typically +3v, and a charging capacitor 160 is connected between the emitter and the power supply line. The capacitor is charged when the transistor is in the off state and discharges charge through the transistor 152 and the LED 154 when the transistor 152 is on, so as to periodically provide high current pulses to the LED 154 without excessive current drain from the battery. A resistor 164 connecting the emitter and the capacitance 160 to the supply line allows the capacitor to recharge when the transistor is in the off state.

The value of the current through light source 154 can be determined by measuring the voltage across resistor 158 and this is applied to the input of microprocessor 136. Resistors 156, 156 act as a voltage divider and reduce the voltage to an acceptable level for microprocessor 136 to ensure that the voltage input to microprocessor 136 does not exceed a specified range.

The control circuit 130 also has a sensing circuit 170 for monitoring light received by a light receiver 172 of the optical sensor 131. The optical receiver is in the form of a receive diode coupled to the input (inverting input) of an operational amplifier 174 of the circuit 170. When the output of the operational amplifier is further amplified by a second operational amplifier 176 and applied to the input of the microcontroller 136, the other input of the operational amplifier 174 is connected to a voltage reference level formed by resistors 178, 180 in the form of a voltage divider.

Resistors 178, 180 and capacitance 182 provide a bias voltage for sensing circuit 170. All operational amplifier voltages stabilize the voltage rise, so the settling time for the rise (since the capacitor is charging) is very short. When the circuit is powered by a battery, the circuit will typically be powered in as short a time as possible to minimize current loss.

Typically the control circuit 130 will be in a sleep mode, waking up at preselected time intervals to check for the presence or absence of smoke. When the control circuit switches to the wake mode, it applies an opening pulse (in this embodiment an inversion pulse) to the base of transistor 152, turning on the transistor and partially discharging the capacitor 160 through the LED 154. The current through the LED produces a voltage drop across resistor 158, which is monitored by microprocessor 136. Typically, transistor 52 is turned on continuously for about 100 μ s every 10 seconds.

When the LED 154 is powered to emit light, the photoreceiving diode 172 produces a current proportional to the received infrared radiation. Which is amplified to produce a signal on the output of amplifier 174. The signal is further amplified by amplifier 176. A level of infrared radiation will always be received because reflections from inside the surface to the smoke sensing cavity of the sensor 131 are established around the LED 154 and receiving diode 172. As smoke enters the cavity, more radiation will be reflected from the smoke and the amount of radiation incident on the receiving diode 172 will increase. Thus, if other operating conditions remain the same, the output of amplifier 176 will increase.

Referring now to FIG. 4a, the response of the sensing circuit 170 is shown in clean air. The current through the infrared light emitting diode 154 is indirectly measured using a series resistor 158. The change in current through the diode with varying supply voltage and thus the change in light output of the LED 154 is shown by curve 150. The change in current generated by the receive diode 172 with incident light and measured by the sensing circuit 170 is also shown as curve 152.

Because of the very low supply voltage, there is not enough voltage to drive the current through the light emitting diode 154. When the threshold voltage of the diode is reached, the current increases. In a rather wide range of emitting diode currents, the ratio between this diode current (i.e. the emitted light) and the current generated by the receiving diode in response to the incident radiation is relatively stable. A typical range of usage for the LED current is 200mA to 600mA and the values of the components and supply voltage are selected to ensure that the current through the LED 154 is always within the above range when the transistor 54 is on.

If smoke enters the optical sensor cavity 131, then the amount of reflected light incident on the receiving diode 172 increases, so the current through the diode 172 increases. Figure 4b shows the response of the diode when the cavity is partially or fully filled with smoke. The LED (emission) current shown as curve 154 is unaffected. However, the current generated by the receive diode 172 increases as shown by the curve 156 on the curve 152.

The current level through the LED 154 and the corresponding current generated in the receive diode 172 is monitored by the microprocessor 136, which generates a proportional signal indicative of the proportion of received light and emitted light. The microprocessor then compares the proportional signal to a reference value and triggers an alarm signal if the proportional signal exceeds a preselected reference value.

The response of the infrared LED 154 and the detection diode 172 is effectively linear over a wide operating range. Thus, the ratio of these two signals is constant for a given level of incident light. The calculated ratio is compared to a calibration reference value to determine whether a critical concentration of smoke has been reached.

The ratio will increase with increasing smoke concentration and when in the case of 'clean air' the ratio is not dependent on emitted light, so the LED 154 current covers a wide range.

The current ratio is therefore independent of the supply voltage (within design limits) and an increase in this ratio indicates an increase in smoke concentration.

The alarm described and illustrated above does not use a constant current source. Instead, the light source is driven using an unregulated power supply. The LED current is measured and the ratio of the received signal to the LED current is compared to a reference value.

Therefore, low voltage consumption is required to drive the LEDs (no linear regulator is required) and a lower voltage supply such as a 3v battery can be used without a boost circuit.

Accuracy is also improved. In conventional circuits, an ASIC (application specific integrated circuit) provides a regulated output voltage that drives individual transistor/emitter resistor combinations to provide a nominally constant current. The current varies with this temperature change.

The control circuit uses fewer components than conventional alarm circuits, thereby achieving higher reliability and reduced cost.

Claims (17)

1. An optical smoke detector, the detector comprising:

a light emitting diode;

an optical receiver; and

a control circuit for controlling operation of the detector;

wherein the control circuit is configured to

Applying an unregulated voltage to the light emitting diode to cause the light emitting diode to emit light;

monitoring the current through the light emitting diode to monitor the light emitted by the light emitting diode;

monitoring a current generated by light received by the light receiver to monitor the light received by the light receiver;

generating a proportional signal representative of a proportion of the monitored current; and

comparing the proportional signal with a reference value and generating a smoke detection signal therefrom.

2. The detector of claim 1, wherein the current through the light emitting diode is within a linear range of the light emitting diode.

3. The detector of claim 2, wherein the current through the light emitting diode is in the range of 200mA to 600 mA.

4. The detector of claim 1, wherein the current through the light emitting diode is in the range of 200mA to 600mA, and the proportion of the monitored current is substantially constant for a given level of incident light, and therefore independent of supply voltage.

5. The detector of claim 1, wherein the light emitting diode is driven by a high side semiconductor device and the control circuit is configured to turn on the high side semiconductor device for a preselected time period at preselected time intervals.

6. The detector of claim 5, wherein the preselected time period is 100 μ β.

7. The detector of claim 5, wherein the preselected time interval is 10 seconds.

8. The detector of claim 1, wherein the light is infrared light.

9. A method of operating an optical smoke detector comprising a light emitting diode and a light receiver, the method comprising:

energizing the light emitting diode with an unregulated voltage to cause the light emitting diode to emit light;

monitoring a current through the light emitting diode to detect light emitted by the light emitting diode;

monitoring a current through the optical receiver to monitor light received by the optical receiver;

determining a ratio of the monitored currents to provide a ratio indicative of the received and emitted light;

comparing the ratio to a reference value; and

a smoke detection signal is generated accordingly.

10. The method of claim 9, wherein the current through the light emitting diode is within a linear range of the light emitting diode.

11. The method of claim 10, wherein the current through the light emitting diode is in the range of 200mA to 600 mA.

12. The method of claim 10, wherein the current through the light emitting diode is in the range of 200mA to 600mA, and the proportion of the monitored current is substantially constant for a given level of incident light, and therefore independent of supply voltage.

13. The method of claim 9 wherein the light emitting diodes are energized for a preselected time period at preselected time intervals.

14. The method of claim 13, wherein the light emitting diode is driven by a high side semiconductor device, and the method comprises turning on the high side semiconductor device for a preselected time period at preselected time intervals.

15. The method of claim 13, wherein the preselected time period is 100 μ β.

16. The method of claim 13, wherein the preselected time interval is 10 seconds.

17. The method of claim 9, wherein the light is infrared light.