CLAIMS

1.

A detector such as an intruder detector providing first and second detection means having first and second outputs, respectively and wherein an alarm signal is produced by the detector if, in use, a signal is produced in response to an intruder by either of the first or second detection means at the respective output.

2.	A detector as claimed in claim 1, wherein the first and second detection means comprise at least one detection element.

3.	A detector as claimed in claim 2, wherein the at least one detection element is an infrared pyroelectric detection element.

4.	A detector as claimed in claim 3, wherein the infrared pyroelectric detection element is passive.

5.	A detector as claimed in claim 2, wherein the at least one detection element is a bolometer type which records a temperature change.

6.	A detector as claimed in claim 1, wherein a detection method which produces a signal in response to a temperature change is used.

7.	A detector as claimed in claim 3, wherein each infrared detection element operates in the wavelength region 5 to 14 urn.

8.	A detector as claimed in any preceding claims wherein each of the first and second detection means comprise a pair of detection elements.

9.	A detector as claimed in claim 8, wherein the pair of detection elements is connected in series.

10.	A detector as claimed in claim 8, wherein the pair of detection elements is connected in parallel.

11.	A detector as claimed in any of claims 2 to 10, wherein each detection element is selected from a lithium tantalate or ceramic type element.

12.	A detector as claimed in claim 5, wherein the bolometer type detection element comprises silicon.

13.

A detector as claimed in any of claims 8 to 11, wherein a first detection element of a pair comprises a positive element which produces a positive going voltage signal in response to a positive temperature change and a second detection element of a pair comprises a negative element which produces a negative going voltage signal in response to a positive temperature change.

14.	A detector as claimed in any of claims 8 to 11, wherein the first detection element is a negative element and the second detection element is a positive element.

15.	A detector as claimed in any of claims 2 to 14, wherein each detection element of the detector monitors a distinct zone.

16.

A detector as claimed in any of the preceding claims, wherein the detector comprises means for switching between a first mode of operation wherein an alarm signal is produced by the detector if, in use, a signal is produced in response to an intruder by either of the first or second detection means at the respective output, and a second mode of operation wherein an alarm signal is produced by the detector if, in use, signals are produced in response to an intruder by both of the first and second detection means at the respective outputs.

17.	A detector as claimed in any of the preceding claims, wherein the detector includes monitoring means and signal processing means.

18.	A detector as claimed in claim 17, wherein the monitoring means is a recorder which records the outputs from the detection means in time.

19.	A detector as claimed in claim 17 or 18 wherein the signal processing means is means to compare the recorded outputs with stored output values.

20.	A detector as claimed in claim 19, wherein the stored output values are indicative of output values previously recorded for known intruder situations.

21.	A detector as claimed in claims 19 or 20, wherein the signal processing means compares the recorded output values and the stored input values and chooses between producing an alarm signal, if the two valves do not match and suppressing an alarm signal, if a match is found.

22.

An alarm system including at least one intruder detector providing first and second detection means having first and second outputs, respectively, and wherein an alarm signal is produced the detector, if, in use, a signal is produced in response to an intruder by either of the first or second detection means at the respective output.

23.	An alarm system of claim 22, wherein one or more of the intruder detectors includes monitoring means and signal processing means.

24.	An alarm system as claimed in claim 23, wherein the monitoring means may be a recorder which records the outputs from the detection means in time.

25.	An alarm system as claimed in claim 23 or 24, wherein the signal processing means is means to compare the recorded outputs with stored output values.

26.	An alarm system as claimed in claim 25, wherein the stored output values are indicative of output values previously recorded for known intruder situations.

27.	An alarm system as claimed in claim 25 or 26 wherein the signal processing means from the comparison of the recorded output values and the stored values chooses between producing an alarm signal, if the two values do not match and suppressing an alarm signal if a match is found.

28.	An alarm system as claimed in any of claims 23 to 27, wherein the monitoring means and signal processing means are provided in a Control and Indicating Equipment referred to as a central control unit, and be interactive with any number of detectors in the alarm system.

29.

A method of operating an intruder detector comprising : providing an intruder detector having first and second detection means having first and second outputs, respectively, and operating the intruder detector such that an alarm signal is produced by the detector if, in use, a signal is produced in response to an intruder by either of the first or second detection means at the respective output.

30.

A method as claimed in claim 29, wherein the method may further contain the steps of : monitoring the outputs in time ; comparing the monitored outputs with stored outputs; and producing the alarm signal if a match is not found between the monitored outputs and the stored outputs and suppressing the alarm signal if a match is found.

31.	A method as claimed in claim 30, wherein the method further includes the step of storing output values indicative of known intruders.

32.

A method of operating an alarm system comprising: providing and alarm system including at least one detector having first and second detection means having first and second outputs, respectively, and operating the alarm system such that an alarm signal is produced by the detector if, in use, a signal is produced in response to an intruder by either of the first or second detection means at the respective output.

33.

A method as claimed in claim 32, wherein the method further includes the steps of: monitoring the outputs from the at least one detector in time; comparing the monitored outputs with stored outputs; and producing the alarm signal if a match is not found between the monitored outputs and the stored outputs and suppressing the alarm signal if a match is found.

34.	A method as claimed in claim 33, wherein the method further includes the step of storing output values indicative of known intruders;.