How does the sensor on a cheap security lamp work?

[ericmark]

I had assumed it used the dopier effect, and sent out a microwave signal and measured the frequency of the bounced signal; however the one I have fitted has some characteristics which don’t seem to match that idea. It can detect items crossing the beam better than those coming directly towards the lamp. It also detects light. And I can reverse my car into the drive and take two steps before it turns on, but my wife will set it off in her car just as she enters the drive. I will assume something to do with heat of the radiator, so it seems it has something to do with heat as well as movement?

The unit has three adjustments, movement sensitivity, light sensitivity and time. It only cost around £5 so can’t be too much inside it.

 

[flameport]

http://www.st.com/web/en/resource/technical/document/application_note/DM00096551.pdf

 

[JohnW2]

I had assumed it used the dopier effect, and sent out a microwave signal and measured the frequency of the bounced signal; however the one I have fitted has some characteristics which don’t seem to match that idea. It can detect items crossing the beam better than those coming directly towards the lamp. It also detects light. And I can reverse my car into the drive and take two steps before it turns on, but my wife will set it off in her car just as she enters the drive. I will assume something to do with heat of the radiator, so it seems it has something to do with heat as well as movement? The unit has three adjustments, movement sensitivity, light sensitivity and time. It only cost around £5 so can’t be too much inside it.

You're surely just describing, exactly as they are, a bog-standard cheapo PIR detector, aren't you? Doppler microwave would presumably be appreciably more expensive.

Kind Regards, John

 

[ericmark]

So it detects a fast increase in the infer red energy within it's field of view, and is saturated in daylight so can't detect any change. Is that how it works? So reversing in will not trigger it as car is not changing the energy it can detect, but coming in forward will as radiator is hot. This also explains why it seems more sensitive at night than during the day.

So where there is an outside light which emits a narrow frequency of light say a blue LED then the device will not be triggered, but a tungsten lamp would trigger it?

Do sodium lamps give off infer red energy? As there is one quite close. Now with a LED 5.5W bulb not really a problem how many times it switches on, but the original tungsten lamp was very short lived, and seemed to be on far more than the new LED.

 

[ericmark]

I missed the link to start with, that really explains how it works. Thank you both.

 

[JohnW2]

So it detects a fast increase in the infer red energy within it's field of view...

More-or-less. It detects a sudden change in IR magnitrude OR a movement of a source of IR energy across its field of view (hence not very sensitive to IR sources moving directly towards or away from the sensor).

... and is saturated in daylight so can't detect any change. Is that how it works?

Not really. PIRs will usually work as well in bright daylight as in the dark, if one wants them to, since the IR sensor is minimally sensitive to visible light. However, in many/most applications, one does not want them to work during daylight - so there is a separate (visible light) photocell and an adjuster so that (if one wishes) one can set the PIR to only work when the (visible) light level is below the set level. Conversely, you can adjust it so that the PIR always works, regardless of the ambient visible light level.

So reversing in will not trigger it as car is not changing the energy it can detect, but coming in forward will as radiator is hot.

That will be part of it - although, as I have said, PIRs are at their least sensitive when a heat source is moving directly towards or away from the sensor.

This also explains why it seems more sensitive at night than during the day.

The ambient temp is going to be lower at night, so heat from radiator, bonnet or exhaust will have a greater differential from ambient, and therefore more likley to trigger the PIR.

So where there is an outside light which emits a narrow frequency of light say a blue LED then the device will not be triggered, but a tungsten lamp would trigger it?

As above, PIRs are not (or should not be) triggered by visible light. If the visible light is bright enough, it may inhibit the PIR action, depending upon how you have set the light sensitivity adjustment.

Do sodium lamps give off infer red energy?

I'm sure they will give off some (as does virtually any lamp) but I have no idea how much. However, since the light is (once on) of constant brightness and not moving, it should not trigger a PIR (except, conceivably, when the lamp first comes on).

Kind Regards, John

 

[333rocky333]

As you say, it is a fact that seems unknown to most people that pirs on the lights work better if you design so as to cross the beam rather than walk towards it, I think thats more to do with the way the lens is configured

 

[JohnW2]

As you say, it is a fact that seems unknown to most people that pirs on the lights work better if you design so as to cross the beam rather than walk towards it, I think thats more to do with the way the lens is configured

I think that they are usually configured into 'segments' (IIRC, often 16 or so) and it is movement of the peak IR signal between segments which is the primary triggering happening.

Kind Regards, John

 

[ericmark]

Thank you, daft as it seems never thought about what was inside the device.

 

[JohnW2]

Thank you, daft as it seems never thought about what was inside the device.

You're welcome. It's amazing how we sometimes "take something for granted" for years without really thinking about the "why/how" - and that happens to all of us!

Kind Regards, John