Thnks for the thought provoking questions Dippy - comments below
Do you mean a 30m to 50m range?
>Yes – at least 25 metre point to point range
What 'beam' angle?
>None just a straight line, in principle it is really as simple as triggering a broken beam
You realise if you start going for long ranges or widths that it won't be able to resolve a single person versus 5 good friends holding hands?
>Yes this is a problem with virtually all counters (typically a PIR has a 0.3 sec delay); it can be partially resolved by undertaking a site sample survey and creating a correction factor. Many counters are on ‘shared’ paths, where you find problems with mothers and prams, people walking dogs and pairs of people using bicycles. Depending on the site the correction factor can be as high as 1.5. It is generally acceptable that such systems are only around 75-80% accurate (even most road traffic counters struggle to get high precision)
And a single person can 'shade' everything else if walking side-by-side.
>Yes - no easy solution - all counter systems suffer the same problem unless you can forced people through a narrow space (and sometime you can).
>The long range is generally only required for open low use areas where little control exists with access points.
Indoors or outdoors?
>Sorry should have said - outdoors
Is it to detect a moving target or just the 'presence'?
> It really just needs to detect a person moving through a single point – a laser or infra-red transmitter-receiver was my first thought, but a single station is easier to setup and in theory less likely to cause problems.
But, on the face of it, I can't think of a solution for a single station device, long range, probably narrow beam, using low power and no doubt a tight budget? Unless you have a source of special polythene lenses in conjunction with PIR.
> Budget is not really a problem – commercial short-range PIR counters are >A$1000
>I had not considered the idea of using special lenses (I have been using narrow beam PIR Fresnel lenses and can in ideal conditions get around 20 metres outdoors) I know some of the PIR activated security cameras are claiming ranges up to 150m.
> Perhaps it just has to be a transmitter and receiver set-up – that is liveable just a few more restrictions on installation locations
I tried a powerful target/retro reflection single-station design a while ago. Very precise but never got anywhere near those ranges. And it used 4 D cells and lasted 6 months.
If outdoors/long range/PIR then you're asking for false triggers unless you do a lot of procesing yourself?
>This is a problem with short-range PIR – finding a suitable set up point helps, accepting a level of false trigger is tolerable, equally post-processing data is not really a major problem, currently all data is processed in Excel. I do not how you would differentiate between real and false data
I take it you were hoping that this device would be matchbox sized for easy disguise?
> Not really the current PIR devices are around 125x75x30mm, and can be easily installed in a bollard, similar landscape hardware or buried, far larger devices can be ‘hidden’ provided the ‘beam’ generator is relatively small (the PIR sensors can be connected to the datalogger via a simple shielded 3 wire cable several metres long – the PIR sensors I have made are 19mm wide and 60mm long). If hidden in a bollard I suppose a lantern battery could be used.
