Function of photocell on operation of PIR?

I have a PIR motion detector designed to sense if someone is walking
towards my house.

In addition to a dial for adjusting PIR sensitivity, this motion
detector unit has a photocell mounted to face downwards. The motion
detector unit is designed for 24 hour operation so clearly the
photocell is NOT for sensing darkness for "night time only"
operation.

So what does the photocell do?

Unfortunately I can't test it reliably under light and dark
conditions because it is a wirefree detector unit which transmits
pulses every few seconds and gives very sporadic responses.

I figure the photocell probably alters the PIR's sensitivity
charactersistics in some way. Is this right?

But would the unit be made more sensitive or less sensitive in
brighter daylight?

 

Its more than likely that the motion sensor and the photocell are wired
in series, and the output taken from the centre of them.

The resistance varies WILDLY with ambient light, and this arrangement
allows a basic comparison between them.

Then AC couple that to detect the d/dt of the lot and you are in business.

 

All of the fixtures that I have seen that are motion only, do not have a
photo cell. Sort of dumb turning on a light in the day time. My porch
fixture has a photo cell and motion detector. I do not have a adjustment on
the photocell. I try to keep the sensitivity down so that anyone on the
sidewalk does not trigger it. A couple of steps into the yard and it will
turn on. I leave the fixture on all of the time so in the evenings when it
starts getting dark the motion sensor takes over and will turn the light on
according to my settings.

 

On Tue 26 Apr 2005 17:12:29, AlanBown wrote:

I too thought a daytime lamp was silly but it works rather well:

When the PIR is tripped, the unit makes a loud sound. If you plug
a lamp into the mains socket on the unit then power is sent to
that socket. You might have a red bulb in your study and get a
visual alarm that someone is approaching. Could be useful if you
didn't want them to hear the audio alarm.

Sounds a bit like mine. What do you find the photocell does to
the PIR's ability to sense people in different conditions of
ambient light?

The unit I describe sounds more of an alarm.

Your unit lights a lamp and seems more as a warning to intruders
or a courtesy to visitors.

 

The photocell is nothing to do with varying the sensitivity of the device -
it is there to detect 'darkness'.
Commonly these things are arranged so that the pir triggers a lamp of some
for some fixed time, to illuminate a path for example, but such illumination
is not required in daylight.
The position and direction of view the photocell (most frequently actually
either a photo-resistor or a photo-transistor) depends on the combined
optical characteristics of the housing, lensing, and photo device. Pointing
down is the most common way of getting the best average ambient illumination
level. Pointing up almost never is.

 

On Tue 26 Apr 2005 21:55:12, R.Lewis wrote:

I think you may have skipped too lightly over where I say that the
motion detector unit is designed for 24 hour operation so clearly
the photocell is not for sensing darkness for "night time only"
operation.

 

He never said it was a PIR operated light.

In fact I assumed it was part of an alarm system.


are for

 

This sounds like a unit that I have. It can be set for motion activation
and/or darkness. For some strange reason, the manufacturer didn't see
the need to incorporate a daytime inhibit on the motion detector.

 

Why would you need a pir that did not detect in the daylight?
You may need a pir that does not turn on a light in daylight hours but why
inhibit the pir function?