GB2369437A - An LED based temperature sensor - Google Patents

Abstract

The effect of forward voltage variation with temperature in a light emitting diode (LED) 10 is utilised to provide a temperature sensor. Thus, the existing power-on or light emitting LED (10) in a smoke or gas detector is used to provide a temperature response signal. An alarm (20) is raised when that temperature response signal from an LED (10) exceeds a threshold. Individual or several LEDs in an array may be used to provide respective temperature signals in order to trigger an alarm response. The alarm (20) may be to stimulate the sensor LED (10) to flash or sustain illumination.

Description

TEMPERATURE SENSORS The invention relates to temperature sensors.

A known characteristic of diodes, such a light emitting diode (LED), is that for a constant forward current, the forward voltage across the p-n junction varies linearly with temperature. However, this characteristic has not been applied to the field of temperature sensors.

According to the invention, there is provided a temperature sensing device, comprising an LED, means for driving a substantially constant forward current through the LED and means for sensing variation in the forward voltage across the LED.

Currently, many smoke and gas detectors contain LEDs as power-on indicators, to show that the detector is working. It is proposed to combine this function with that of a temperature sensor. Accordingly, by a second aspect of the invention there is provided a smoke or gas detector, comprising means for detecting either smoke particles or gas molecules, means for triggering an alarm in response to detection of a threshold limit of said particles or molecules, an LED, means operative when the LED is driven by a substantially constant forward current for detecting variance in the forward voltage across said LED which is dependent on temperature, and means for triggering a temperature alert in response to detected forward voltage. According to a third aspect of the invention, there is provided a temperature sensing device, comprising a spatially arranged array of LEDs, means for driving a substantially constant forward current through each LED, means for sensing variation of the voltage across each LED, and means for producing a temperature dependent output corresponding to the value of the sensed forward voltage.

The following is a more detailed description of temperature sensing devices embodying the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a block diagram illustrating the circuitry for a stand alone form of the devices; Figure 2 is a block diagram illustrating the integration of the device of figure 1 into a smoke or gas detector; and Figure 3 is a block diagram illustrating the circuitry of an array of the devices.

An LED 10 is connectable to a device circuit 12 for normal illumination of the LED 10, or to a sensor circuit 14, by means of switching means 16. When connected to the sensor circuit 14, the LED 10 acts as a sensor. This is described in more detail below. For a constant forward current, the forward voltage across the p-n junction of a diode varies linearly with temperature. This characteristic is therefore true for LEDs, and may be exploited for the production of inexpensive stand-alone temperature sensors. In its simplest form, a temperature sensor may comprise an LED 10 which is connected to a sensor circuit 14. The sensor circuit 14 supplies the LED 10 with a constant current so that the forward voltage across the LED varies linearly with temperature.

In addition, the sensor circuit 14 contains means for measuring the forward voltage across the LED 10. An alarm decision circuit 18 responds to a signal from the sensor circuit 14 to trigger an alarm 20. The alarm decision circuit 18 may simply trigger the alarm 20 when the voltage measured within the sensor circuit 14 passes a threshold value. This threshold could be calibrated to correspond to a predetermined temperature.

An alarm is shown by continuous illumination of the LED 10, and could be accompanied by an audible alarm. In non-alarm mode, the LED 10 can be periodically switched by switch 16 so that it is energised by the circuit 2, thus indicating that the sensor is functioning properly.

Figure 2 shown a temperature sensor like that of Figure 1, but which is integrated into a smoke or gas detector (such as a carbon monoxide detector). The circuit comprising the LED 10, the LED drive circuit 12, the LED sensor circuit 14, the switching means 16, the alarm decision circuit 18 and the alarm 20 is as described for Figure 1. However, an additional smoke or carbon monoxide sensor circuit 22 is also connected to the alarm decision circuit 18, so that the alarm 20 can be triggered by the alarm decision circuit 18 either in response to a signal from the LED sensor circuit 14 (indicating detection of a particular temperature) or in response to a signal from the smoke or carbon monoxide sensor circuit 22 (indicating detection of a threshold unit of smoke particles of carbon monoxide molecules). Alternatively, the alarm decision circuit 18 may only trigger the alarm 20 when signals from both the LED sensor circuit 14 and the smoke or carbon monoxide 10 sensor circuit 22 are received. For example, the alarm 20 would only be triggered if a high temperature and a large number of smoke particles were detected.

Some smoke or gas sensors use LEDs as a part of the detecting means. Where this is the case, as in, for example, an optical scattering smoke or particle detector, it can be seen that a single LED may be used to perform both the function of smoke or particle sensor and temperature sensor. The LED 10 is connected to the LED drive circuit 12 to be illuminated. In this state, the LED 10 can be used as part of the smoke or particle sensor, that is, to provide the source of light for the smoke or particle detecting. Periodically, switching means 16 will switch to connect the LED 10 to the LED sensor circuit 14, to act as a temperature sensor.

The performance of some smoke or gas sensors varies in accordance to the temperature. In some detectors, temperature compensation circuitry has been added to give as accurate a performance as possible. By using a single LED 10 to form part of the smoke or gas sensor and as a temperature sensor, it will be seen that the signal from the LED in temperature sensing mode may be used to compensate for the variation in performance of the smoke or gas sensor comprising that LED. In this way, the smoke or gas detector comprising the LED 10, the LED sensor circuit 14, the smoke or gas sensor circuit 22 and the alarm decision circuit 18, may be selftemperature compensating.

A third embodiment of the present invention is shown in Figure 3. An array 30 of individual LEDs is provided. Each LED is individually and subsequently interrogated in order to provide a temperature response in a similar fashion of the single LED arrangements described with reference to Figures 1 and 2. Thus, each LED in the array 30 acts as a temperature sensor in accordance with the present invention but also acts in concert with other LEDs of the array 30 as described below.

Each LED of the LED array 30 is coupled to a bi-directional communication bus 30 so that signals can be exchanged with both a normal LED display control circuit 40 and an interrogation processor 33. The normal LED display control circuit 40 checks each LED of the array 30 for normal operation periodically by energising that LED to cause illumination as described previously with regard to Figure 1. The interrogation processor 33 comprises an LED interrogation circuit 34, an alarm decision circuit 36 and LED alarm display circuit 38. Each LED of the array 30 acts as a temperature sensor. Thus, the LED interrogation circuit 34 checks each LED of the array 30 in turn to determine a voltage signal response which is an indication of the temperature of that LED. Clearly, the bidirectional bus 32 must be able to determine each individual LED, either through hard wiring or appropriate"hand shaking"identifications between the processor 33 and LEDs, and then provide an identified voltage signal for that LED to the interrogation circuit 34.

The interrogation circuit 34 upon receipt of the identified voltage signal from an individual LED of the array 30 will determine a temperature quotient for that LED.

This temperature quotient will be used by subsequent circuits 36,38 of the interrogation processor 33.

The function of the alarm decision circuit 36 is to interpret the temperature quotient from each individual LED and, possibly sets of individual LEDs in the array 30. Thus, the circuit 36 could provide a wide range of interpretations including:a) simple comparison of the actual temperature quotients received from the interrogation circuit 34 against either an acceptable temperature quotient response for that individual LED or across the whole LED population of the LED array 30; or b) compare sequential actual temperature quotient responses received over time from the circuit 34 for that LED or a set ofLEDs to determine response trends, or c) compare individual, or sequential, actual temperature quotient responses from the circuit 34 for a specified set or all of the individual LEDs of the array 30; or d) a combination of the functions a), b) and c) above.

Essentially, the decision circuit 36 is a comparator array to compare the actual LED response with a"trigger"response level. If the actual LED response is above, below or the same as the response level, dependent upon the configaration of circuit 36, then an alarm signal is raised.

In any event, the decision circuit 36, dependent upon the temperature quotient signals received, provides an alarm signal to the LED alarm display circuit 38. This alarm signal acts again through the communication bus 32 in order to activate, if necessary, a local alarm to the individual LED of the array 30. This alarm may be to energise the LED of the array 30 so that it is persistently illuminated or flashes in an alarm sequence. Furthermore, the display circuit 38 could communicate an alarm condition to a control circuit (not shown) which may control further ancillary features and modes of illumination ofLEDs on the array 30.

For example, a monitoring control circuit (not shown) could (1) arrange for the individual thresholds within the decision circuit 36 to provide a higher sensitivity to temperature changes over time or simply reduce the trigger temperature level at which an alarm is activated through the alarm display circuit 38.

(2) Once the decision circuit 36 has determined that at least one LED has sensed a temperature above its acceptable level, indicative of a fire or heat zone, then that LED and/or others in the array 30 are energised to flash in synchronisation in order to indicate a safe escape route away from the heat zone or fire or, if fire location is required, towards the fire or heat zone.

(3) Individual LEDs of the array 30 may be associated with a specific room or cabinet. Thus, the control circuit energises and sustains through the alarm display 38 those individual LEDs of the array 30 associated with rooms or cabinets which currently are at a temperature above acceptable for that room or have exceeded that temperature for a time period but have now been returned to an acceptable temperature.

It will also be understood that the control circuit could act with the LED normal display circuit 40 to confirm correct operation of each LED, of the array 30. The control circuit could also alter the alarm circuit 36 to establish different alarm thresholds for each LED between night and day and/or respective locations.

The alarm display circuit 38 may also itself send signals to the LED array 30 to illuminate the LEDs in sequence, in order to direct people either towards or away from an area where an abnormal temperature has been sensed by an individual LED of the array 30. Thus, for example, firefighters may be directed towards a"hot"area by means of the LEDs along the array 30 flashing one by one towards the LED or LEDs which have sensed the threshold temperature.

It will, of course, be appreciated that any suitable means of indicating an alarm may be used, including audible means, and that direction towards or away from the affected area may be indicated by means other than sequentially flashing LEDs.

Claims (18)

1. CLAIMS 1. A temperature sensing device, comprising an LED, means for driving a substantially constant forward current through the LED, and means for sensing variation in the forward voltage across the LED.
2. 2. A device according to claim 1, comprising indicating means for indicating when the forward voltage reaches a predetermined threshold value.
3. 3. A device according to claim 2, in which the indicating means comprises means for causing continuous illumination of the LED when the forward voltage reaches the threshold value.
4. 4. A device according to any preceding claim, including indicating means for periodically illuminating the LED to confirm its operational condition.
5. 5. A smoke or gas detector, comprising means for detecting smoke particles or gas molecules, means for triggering an alarm in response to detection of a threshold limit of said particles or molecules, an LED, means operative when the LED is driven by a substantially constant forward current for detecting variance in the forward voltage across said LED which is dependent on temperature, and means for triggering a temperature alert in response to detected forward voltage.
6. 6. A smoke detector according to claim 5, including means for triggering an alert in response to both the smoke or gas detection and to a detected forward voltage indicative of a predetermined temperature.
7. 7. A smoke detector according to claim 5, including means for triggering an alert in response to smoke or gas detection.
8. 8. A detector according to any of claims 5 to 7, including indicating means responsive to a signal from the LED for compensating for temperature dependant performance of the smoke or gas detecting means.
9. 9. A detector according to any of claims 5 to 8, in which the LED is also used as a power-on indicator.
10. 10. A detector according to any of claims 5 to 9, in which the indicating means comprises means for causing continuous illumination of the LED when the forward voltage reaches the threshold value, and including means for periodically illuminating the LED to confirm its operational condition.
11. 11. A temperature sensing device, comprising a spatially arranged array of LEDs, means for driving a substantially constant forward current through each LED, means for sensing variation of the voltage across each LED, and means for producing a temperature dependent output corresponding to the value of the sensed forward voltage.
12. 12. A device according to claim 11, including comparing means for comparing the temperature-dependent output of each LED with the temperature-dependent output of one or more other ones of the LEDs.
13. 13. A device according to claim 11 or 12, in which the indicating means comprises means for causing continuous illumination of each LED when the forward voltage reaches a predetermined threshold value, and including means for periodically illuminating each LED to confirm its operational condition.
14. 14. A device according to claim 13, in which the means for causing illumination of each LED when the forward voltage reaches the threshold value comprises means for causing flashing of the LED.
15. 15 A device according to claims 11 or 12, including means for illuminating a plurality of the LEDs when the temperature-dependent output of at least one of the LEDs reaches a threshold value, the plurality of LEDs being illuminated sequentially in a direction progressing with reference to the spatial array towards or away from the said one LED.
16. 16. A temperature sensing device substantially as described with reference to Figure 1.
17. 17. A smoke or gas detector substantially as described with reference to Figure 2.
18. 18. A temperature sensing device substantially as described with reference to Figure 3.