Alarm

1. An optical smoke detector comprising:

a light emitting diode;

a light receiver;

and a control circuit for controlling operation of the detector;

wherein said control circuit is configured to

apply an unregulated voltage to the light emitting diode to cause it to emit light;

monitor the current through said light emitting diode so as to monitor the light emitted by said light emitting diode;

monitor the current generated by light received by said light receiver so as to monitor the light received by said light receiver;

generate a ratio signal representative of the ratio of the monitored currents; and

compare said ratio signal with a reference value and generate a smoke detection signal in dependence thereon.

2. A detector as claimed in claim 1 wherein the current through said light emitting diode is in the linear current range of the light emitting diode.

3. A detector as claimed in claim 2 wherein the current through said light emitting diode is in the range 200 mA to 600 mA.

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

5. A detector as claimed in claim 1 wherein said light emitting diode is driven by a high-side semiconductor device and said control circuit is configured to switch said high-side semiconductor device ON for a preselected time period at preselected time intervals.

6. A detector as claimed in claim 5 wherein said preselected time period is 100 μs.

7. A detector as claimed in claim 5 wherein said preselected time interval is 10 seconds.

8. A detector as claimed in claim 1 wherein said light is infra-red light.

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

energising said light emitting diode with an unregulated voltage to cause said light emitting diode to emit light;

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

monitoring current through said light receiver so as to monitor the light received by said light receiver;

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

comparing said ratio with a reference value;

and generate a smoke detection signal in dependence thereon.

10. A method as claimed in claim 9 wherein the current through said light emitting diode is in the linear range of the light emitting diode.

11. A method as claimed in claim 10 wherein the current through said light emitting diode is in the range 200 mA to 600 mA.

12. A method as claimed in claim 9 wherein the current through said light emitting diode is in the range 200 mA to 600 mA, and the ratio of the monitored currents is substantially constant for a given level of incident light and is therefore independent of supply voltage.

13. A method as claimed in claim 9 wherein said light emitting diode is energised for a preselected time period at preselected time intervals.

14. A method as claimed in claim 13 wherein said light emitting diode is driven by a high-side semiconductor device and the method comprises switching said high-side semiconductor device ON for a preselected time period at preselected time intervals.

15. A method as claimed in claim 13 wherein said preselected time period is 100 μs.

16. A method as claimed in claim 13 wherein said preselected time interval is 10 seconds.

17. A method as claimed in claim 9 wherein said light is infra-red light.