Occupancy Sensor - the basics

[ToughButterCup]

Can someone take us through the basics of these apparently useful switch systems, please?

 

I have a very simple use-case to sort out. You know the kind of thing: return  home from the weekly shop: both hands full of heavy bags, you open the back door. It would be good if a sensing switch automatically put the lights on.

And there's the first puzzlement for someone like me.

• Should the switch be triggered by heat (PIR, I assume) or
• by the movement of the door?

 

And then there's a whole host of considerations  that only @Onoff or @ProDave will be able to explain: Momentary Push Switches (whassat?), dwell times, dual switching, timer switched circuits to name just a few. Can you wire a normal (one gang) On /Off switch to a circuit which also has an occupancy sensing switch? If you can, which switch has precedence?

The more I read the tanglier the story gets.

 

Whats one of those switches called that you get in stairwells sometimes: all you can do is press the switch and then the light stays on for (say) 20 seconds while you fumble and fail to find your keys - and then it switches off?

 

It seems this time,  that the simple must be made complex before it becomes simple (again)

[ProDave]

I deal with this by an outside PIR triggered light that comes on as you approach the front door.  That illuminated the keyhole (you WILL have to put something down if carrying, to unlock the door)

 

Once unlocked you open the door, turn an inside light on and pick up your stuff.

 

Why make it more complicated?

 

Our unload shopping routine is usually carry it all to the front door and place it just inside the door.  Then when all there, go inside, wipe feet, change to indoor footwear and carry shopping to kitchen.  This avoids keep walking in with wet feet etc. 

[Jeremy Harris]

We have lots around the house, for much the reasons you've outline, @AnonymousBosch.  The one in the utility room above the back door is brilliant, as the lights come on when you walk in with arms loaded with shopping, washing etc.

 

There are two types, passive infra red (PIR) and microwave Doppler (sometimes called radar).  Initially I had a PIR one by the back door (one of these: https://www.tlc-direct.co.uk/Products/TLPIRFL.html but we had problems with it false triggering from shadows through the window in the back door, so I replaced it with one of these Doppler ones:  https://www.danlers.co.uk/microwave-presence-detection-switches/mwcefl-ceiling-flush-mounted

 

Elsewhere we have the PIR switches and they mainly work OK, with just a very occasional false trigger.  I fitted them in the WC, various cupboards, our under eaves loft space, the services room and our walk-in wardrobe.

 

The advantages of PIR are that it mainly responds to changes in warmth, so reliably detects people and animals.  The disadvantage is that they also detect warm air movement, so, for example, our walk-in wardrobe light will often come on when the bathroom door is opened, as the rush of warmer air coming in under the door to the wardrobe is detected.

 

The advantages of the microwave Doppler switches is that they only sense movement of solid objects, so they aren't triggered by warm air movement or shadows.  The snag is that they can sense through walls and doors, so movement outside a room, close to a wall or door, may be detected and turn the light on

[Gone West]

1 minute ago, JSHarris said:

The advantages of PIR are that it mainly responds to changes in warmth, so reliably detects people and animals.  The disadvantage is that they also detect warm air movement, so, for example, our walk-in wardrobe light will often come on when the bathroom door is opened, as the rush of warmer air coming in under the door to the wardrobe is detected.

We have a PIR operated front door light which annoyingly occasionally comes on when a tractor drives past with high level front lights switched on. No other road traffic does the same and I can't understand the reason why. So could it be reflections of the tractor light in the window switching the PIR which is above the front door and out of line of sight of the tractor.

[Jeremy Harris]

5 minutes ago, PeterStarck said:

We have a PIR operated front door light which annoyingly occasionally comes on when a tractor drives past with high level front lights switched on. No other road traffic does the same and I can't understand the reason why. So could it be reflections of the tractor light in the window switching the PIR which is above the front door and out of line of sight of the tractor.

 It may well be that the tractor lights are higher up than a car, and as they are a source of moving heat the PIR senses them and switches on.  One fix might be to try and turn the sensitivity down on the PIR, so it only responds to the movement of nearby heat sources. 

 

Unfortunately a microwave Doppler sensor won't get around this problem, as I have one set up part way down our drive as a trigger to start the CCTV recording, and whilst that's generally a great deal more reliable than the built-in motion detection in the cameras, it will get triggered by a large truck driving along the adjacent lane (not something that happens very often, though)

[ToughButterCup]

22 minutes ago, JSHarris said:

[...]

so I replaced it with one of these Doppler ones:  https://www.danlers.co.uk/microwave-presence-detection-switches/mwcefl-ceiling-flush-mounted

[...]

 

I was very pleased to read this in the user manual and fitting instructions

Quote

MANUAL OVERRIDE [...] may be over-ridden with the help of a retractive (momentary) switch to switch on/off the lights manually

[...]

To turn load ON (if sensor has already switched load OFF), Short press (<1 second) the retractive switch. The load will switch On and goes into sensor mode. Please note: Load will switch ON even if the lux level exceeds the daylight threshold setting.

 

To turn load OFF (if sensor has already switched load ON), Short press (<1 second) the retractive switch. Please note: Once switched OFF the sensor cannot switch the load ON until the preset hold time has elapsed.

 

Now, that makes sense.

 

30 minutes ago, ProDave said:

I deal with this by an outside PIR triggered light that comes on as you approach the [...]door.  [...]

 

Yes Dave, I've thought a lot about that. But our back door is right next to our Great Crested Newt Highway - and while newts wont (or better put, shouldn't)  account for too many additional 'switch-ons' we get lots of nocturnal activity on the south side of our house (its a wild-life corridor).

Is it possible to 'tune' PIR to sense only (say) the heat coming from a car engine or human, but not a small rodent?

[ProDave]

I used to worry about PIR lights triggering falsely in the days of a 500W halogen tube.  Now with a 5W LED I don't mind if the cat sets it off.

 

I tend to buy a light fitting with the sensor built in. That gets you a light that looks right and gives the required light but usually you don't get the choice of what type of sensor it has.

[Jeremy Harris]

16 minutes ago, AnonymousBosch said:

Is it possible to 'tune' PIR to sense only (say) the heat coming from a car engine or human, but not a small rodent?

 

Yes, pretty much all of them have a sensitivity adjustment and turning this down will both reduce the range at which people trigger the light and increase the size of warm thing needed to trigger it.  GCNs shouldn't trigger one, as they are cold blooded.  The mammals hunting them (presumably with the appropriate licence to hunt a protected species) might trigger the sensor, though.

[TerryE]

My son-in-law uses the Google timeline API to track his and my daughter's mobiles, plus polling the router to see if they are Wifi connected.  He uses this for two reasons: (i) to detect if the valid  occupants are at home.  (He gets an alert if the movement detectors in house fires and one of them isn't at home), and (ii) to crank up the CH if either comes within a 10m radius of the house.

[ToughButterCup]

@TerryE, you'd be the first to tell me to KISS 

[Jeremy Harris]

Not sure I'd ever voluntarily connect anything to the Evil Empire, either.  It's becoming increasingly clear that Google is at least as evil as Facebook, Microsoft et al, in terms of abuse of personal data.

[SteamyTea]

There was a bit on the Radio last week where they discussed connected homes. They had some industry experts on it.

Can't remember what show it was, possibly You and Yours.

May be worth a search on the Radio 4 website.

I think the take home message was 'don't'

[Onoff]

2 hours ago, SteamyTea said:

There was a bit on the Radio last week where they discussed connected homes. They had some industry experts on it.

Can't remember what show it was, possibly You and Yours.

May be worth a search on the Radio 4 website.

I think the take home message was 'don't'

 

Was it this?

 

https://www.bbc.co.uk/programmes/w3csy7k0

[SteamyTea]

Yep that's it.

Thanks.