Digital Electronics Computers & Video Security

A forum for recording my findings and discussion of matters related to electronics, computers, security and other technical subjects.

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Location: Queensland, Australia

Automation and Integration Architect

Tuesday, October 03, 2006

Premises Security (part 8) Non-video surveillance

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In this session I'll finish off the devices which cause a monitored circuit to be broken by detection of a change in the environment, rather than through physical operation of a switch.

Just to re-iterate, there are a huge variety of devices in this category and a multitude of variations on design, features, and connectivity. I have simply picked-out the most common ones which I have dealt with over the years.
Once again, if there is something I have skimmed-over or not covered, raise it in the comments and I'll do my best to cover it for you.

(As an aside, it's been interesting to note that as I checked for web resources in this area of the discussion, there is plenty of info on PIR devices but not much on IR barrier beam technology, despite the fact beams were in use before reliable PIR products came on the market)

Security is More Than Just Intruder Detection
In providing premises security, we not only want to think of intruders, but also things like keeping stock cool (or warm) and ensuring security lighting turns on at night etc.

So, lets take a slightly closer look at Temperature sensors, and Light/Dark detectors.

Description: Thermal Fuses
Operation: When a pre-set temperature is exceeded, the thermal fuse goes to an OC state. Depending on the design of the fuse, this may self reset when the temperature re-crosses the threshold, or in some cases the fuse has to be manually replaced to restore the circuit. This latter type is quite rare apart from their use in power supply circuits. Even there, resetable fuses and 'crowbar ' current limiting circuits are rapidly replacing the humble 'fusible link'
Common Use: Motor over-temperature cut-outs, Appliance Fire Safety cut-outs, Thermal fire sensors
Connectivity: Wired in-series with the monitoring circuit. Often placed in thermal contact with the equipment which they protect.

Description: Thermistor or semiconductor based devices
Operation: As temperature changes, differential-monitoring digital circuitry monitors for a pre-set threshold to be reached, at which point the output circuitry will toggle the NO/C/NC contacts.
Depending on the circuit design the output may reset as temperature returns back across the threshold.
One benefit of this type of temperature surveillance is it's ability to monitor either too-high or too-low temperature thresholds.
Common Use: Freezer alarms, plant-rooms, thermal fire sensors
Connectivity: The sensor itself (thermistor or semiconductor) may either be integrated into the same housing as the digital detection circuitry, or may be at the end of a calibrated 'fly-lead '. The surveillance system is most commonly connected to the NO/C/NC contacts of the device output.

Description: LDR(Light Dependant Resister) or Semiconductor sensors (Photo diode/transistor)
Operation: As the level of light falling on the LDR or photo-conductor changes, the associated digital circuitry detects this and compares it to the pre-selected trigger level. When that trigger point is reached, the circuit toggles the output NO/C/NC contacts
Common Use: Monitor inside of safe-rooms for door opening, Controlling security lighting, Watch for car headlights, Watch for torchlight
Connectivity: By wiring the NO/C/NC contacts into the monitored surveillance circuit.

And so to Part #9 of the series where we return to finish considering the two points raised in Part #1 of this series.
The first point was the trade-off of cost & size vs functionality, risk and aesthetics which led into the discussion of the various types of security system and the devices used (Parts #2 thru #8)
The second point was the possibility of having the "best of both" in a small easily installed surveillance system AND extended power coverage by use of a "UPS" as a power backup.

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