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SOME EXPERIENCES WITH SECURITY LIGHTING
by
Francis Townsend: 
francis.townsend@bigfoot.com
In common with a lot of other people I have installed some outside security lights. As I have always been curious about how things worked, and their relation to computers, I had reason to open up the unit and examine the innards. As I have more than one type installed this 'opening up' was extended to all types and the following observations are common to all types examined.
Internally there are one or two circuit boards. These boards cover: Power Supply; Motion Detection; Lamp Control.
The Power Supply board is concerned with supplying a +5 volt operating supply. Point one is that +5v is the standard voltage used in computer LSI and therefore easily interfaced to them. The board construction involves no transformer, the reactive property of a condenser being used to drop the mains voltage down to +5v. This voltage is maintained by a simple regulating IC.
Motion detection is done with a PIR (Passive Infra-Red) detector. The actual detection is the PIR in conjunction with Op-Amp devices. The decision, as to whether it is dark enough to operate the lamp, is done by a photo cell. Some devices allow setting the darkness threshold. Others have a fixed threshold level. When a movement is detected a control allows setting the duration for the lamp to remain on.
The Lamp Control board consists of a transistor driving a relay whose contacts operate the lamp.
The point of interest from a computing aspect is the input to the lamp control transistor. As stated the logic voltage is +5v and when the device is activated this point is high, going low at in-activation. If this connection is broken and the two lines connected to a computer, (via opto-isolators), you have a method of controlling the lamp externally. The line from the motion detector being input to the computer while the line to the transistor being output from the computer. Apart from using a computer a collection of AND & OR gates allow controlling one lamp from many PIRs or many lamps from one PIR ad. infinitum. Another use is to control a surveillance video camera and VCR event recorder.
When using a computer the only requirements are some form of logic detection and control. Bi-directional printer ports and parallel ports satisfy these requirements. Most micro-controllers, IBM compatible PCs and even the old faithful Commodore 64 satisfy these requirements. It is possible to pick the latter up for only a few pounds.
I favour the latter as the Commodore 64 has all the requirements; a VDU output to a standard TV, a Data-Cassette to record events and even the ability to print. It is even possible to arrange an off-line power supply for the Commodore 64 using rechargeable batteries but the standard TOD circuitry, (which controls the onboard ticker), would not work because it relies on an AC frequency. Of course it would be ideal if a machine code program was used which could be blown onto a ROM and used in the cartridge port.
I hope the above gives readers food for thought. Any would-be prowlers around my property will be in for a shock. (Ed: I hope that is not electrically or you might get prosecuted!)
