Security Lighting
Security lighting is mainly used to automatically illuminate the approach to a home or garage entrance. This has the advantage of allowing people to gain access without fumbling in the dark. It can be provided by a simple timeswitch. This is only suitable if the lighting is maintained during the hours of darkness. Seasonal changes would of course have to be taken into account.
A PIR ( passive infra red ) sensor will provide the same function more economically. The lighting load will only be switched on when required. It does not require seasonal adjustment. It also provides added security by deterring intruders.
Security light sensors contain a “passive infra red” sensor. When exposed to infra-red light its electrical resistance changes. This resistance change is amplified by an electronic circuit. The output of this circuit operates a small relay. The relay in turn switches power to any lamp or lamps connected to the sensor. The curved white plastic screen on the front acts as an optical collector of infra-red light. The screen angles available vary from 90° to 360°.
The sensor head can be adjusted downwards to reduce the distance scanned. It can also be adjusted from left to right to avoid scanning unwanted areas. Further areas can be avoided by blanking off the plastic screen as required.
These units have a number of features.
1. They only react to moving heat.
2. A built in photocell deactivates the unit during daylight hours. This means that the light will only be in operation during hours of darkness. The level of light at which it activates is also adjustable.
3. The time for which the lamp(s) are “on” is adjustable from approximately 5 seconds to 12 minutes.
4. Some models have a sensitivity adjustment. It helps to minimise nuisance operation by for example cats or dogs.
5. Adjustment of the unit during daylight hours is often facilitated via the control switch ( See Figure 28 ) as follows:- Turn off the supply for approx. 10 seconds and then turn it on again. Adjustment is now possible.
6. The lamp(s) can be locked “on” as follows:- Turn off the supply for approx.1 second and then turn it on again. This may explain why the lamp(s) are sometimes locked “on” accidentally. Most units return to normal operation after 8 hours.
7. Return to normal operation is achieved as follows:- Turn off the supply for approx. 5 seconds and then turn it on again.
Typical Specifications
