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I've attached the schematic of a Passive Infra-Red detector I've been
tinkering with. I'm using it to wake the PIC up from SLEEP by a port
change interrupt. As shown, it works well. Current draw is about 50uA
(TLC271 in LF, LP mode) and with a proper plastic Fresnel lens, as
found on security lights, will detect movement at 10m indoors. If you
use this without a lens the distance is a lot shorter. To increase the
angle of view I use two of these at 60 degrees, output of a each
through a diode to b.4.
The PSU is 5V from F84 pin a.2, as once the movement is detected
I shut the PIR down for 3 minutes. When turned back on, I ignore
any IRQ from it for about 15 seconds while it stabilises. This isn't a
security application, so picking up every trigger is not important. BC548,
if you aren't familiar with it, is a general purpose npn signal transistor.
The PIR chip must be orientated to detect movement effectively, the two
piezo elements inside should be perpendicular to the axis of movement
so they can make a difference signal. (I think I remembered that
correctly, my notes are somewhere else).
The question I have is - is there some feedback or auto-regulation
I could add to this circuit to eliminate the 10M preset or make it
"tighter" but without significantly increasing the current drain ? It's
just a 0.5 - 4Hz amplifier and comparator - can it be made "smarter"
or less hands-on ?
The PSU for the whole circuit is a 6V gel cell + LP2951, and the only
two parts alive most of the time are PIC and the PIR. Everything else
is powered off by the PIC switching the LP2951. The PIR circuit is
stable, no problems in the 6 weeks it's been on so far, but if I could
make it self-settling that would be a big advantage when it comes
to installing a number of them. ie set and forget, no need to worry
about any drift, if it happens. Putting a smaller preset between two
resistors does make it easier to set, but it does still need to be set
Attachment converted: wonderland:pirgif.gif (GIFf/JVWR) (0001660E)
>The PIR circuit is stable, no problems in the 6 weeks it's been on so far,
> but if I could make it self-settling that would be a big advantage when it comes
> to installing a number of them. ie set and forget, no need to worry
> about any drift, if it happens. Putting a smaller preset between two
> resistors does make it easier to set, but it does still need to be set
At a quick glance I would say the answer would be to remove the
10M pot and feed the non inverting input through a low pass of the
inverting input. The low pass being of a value such as to enable
detection over drift.
If the input rises fast enough.... the inverting terminal voltage rises
but the non inverting isnt there yet.... trigger!
I remember doing this sometime in my distant past.
Uni of Auckland
This is the most recommended way to do it. I would suggest it too. The
low pass filter is the most effective way to detect changes in the
signal without worrying about calibration or presets.
The only issue is that the PIC should wait some time after power on to
then consider the output of the TLC271 valid, in some way, you can see
this delay (60 seconds) in several motion detectors, including the
A resistor of 4M7 connecting both inputs of the TLC, with a 4.7µF from
pin + to ground, will give you a delay of aprox 22 seconds to load,
plenty for fast triggering at the - input.
Lance Allen wrote:
I have two related questions:
#1) Where can one get these PIR detectors?
#2) Are these similar to the sensors used in the non-contact thermometers?
If not, what kind of sensors are usually used there?
At 10:11 PM 2/1/00 -0500, you wrote:
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
cornell.edu ICQ #: 3329174 shb7
The PIRs found inside of the cheap motion detectors (*) are tuned to
human heat signature, so most sensible for human movement than other
The Sensors found at the Non-Contact-Pyrometers embrace a wide range of
infra-red and they are more "linear" then the cheap from motion
detectors, and more expensive too. Non contact pyrometers use a set of
lenses to focus a narrow angle up front.
(*) $8.00 at Home Depot, Lowes, Wal-Mart, and others, with all the
electronics, lamp sockets and more... I can't believe how can they do
Sean Breheny wrote:
> I have two related questions:
> #1) Where can one get these PIR detectors?
> #2) Are these similar to the sensors used in the non-contact thermometers?
> If not, what kind of sensors are usually used there?
William Chops Westfield
The PIRs found inside of the cheap motion detectors (*)
(*) $8.00 at Home Depot, Lowes, Wal-Mart, and others, with all the
electronics, lamp sockets and more...
So does anyone have enough data on cheap motion detector internals so that
they can be taken apart and used in low voltage circuits?
The PIR sensor in the posted circuit IS the motion detector from
a security light. The one in front of me is a Murata part, others I
sourced through an alarm company (who make their own PIR
sensor units) are Heimann, possibly part number LHI 958. Both
types are in a TO5 can. Specs are in a RadioSpares tech sheet,
they must have got that info from somewhere. Whilst on a search
for Murata or Heimann data I came across a cloud detector, if
anybody wants one (?!?!?!?)
> So does anyone have enough data on cheap motion detector
> internals so that they can be taken apart and used in low voltage
Reading the link posted by Jinx made me remember that years ago I
developped a device to collect solar irradiation, the prototype was
installed inside a PVC pipe. Sent to the customer he rejected the PVC
and asked for anodized Aluminum housing. According to this customer, PVC
in open weather and under the sun deteriorates in few years. It doesn't
happen to PVC pipes burried or fill with water (temperature?).
> Hi all, a circuit and a question
My question: What on earth is the 10k resistor doing in there? It
appears to be providing the PIR device with a tapped-off signal from the
collector of the first stage which is I suppose some bizarre sort of
negative feedback. But why?
> The question I have is - is there some feedback or auto-regulation
> I could add to this circuit to eliminate the 10M preset or make it
> "tighter" but without significantly increasing the current drain?
Lance Allen wrote:
> At a quick glance I would say the answer would be to remove the 10M
> pot and feed the non inverting input through a low pass of the
> inverting input. The low pass being of a value such as to enable
> detection over drift.
And Wagner Lipnharski wrote:
> A resistor of 4M7 connecting both inputs of the TLC, with a 4.7µF from
> pin + to ground, will give you a delay of aprox 22 seconds to load,
> plenty for fast triggering at the - input.
OK, but you now have the threshold set at *exactly* the average value
of the signal. The op-amp will switch rail-to-rail at the slightest
fluctuation, spending on average half the time at each output state. I
don't think this is what you wanted.
You need to introduce a threshold again. I suggest putting a couple
of hundred ohms (but you may need more) in series with the 27k collector
resistor and taking the abovementioned 4M7 or whatever resistor from
that. It will now be a few millivolts positive and the op-amp/
comparator will idle with output high. If you want otherwise, just swap
the op-amp inputs.
And I think your suggested time-constant is probably excessively long.
"Paul B. Webster VK2BZC" wrote:
> My question: What on earth is the 10k resistor doing in there? It
> appears to be providing the PIR device with a tapped-off signal from the
> collector of the first stage which is I suppose some bizarre sort of
> negative feedback. But why?
I think it is to act as a sensor threshold. Small positive pulses from
the sensor will make the first BC548 conduct, and this reduces the
voltage to the sensor (Vdrop on 10k), cutting off the sensor output
"glitch". When the sensor output is big enough, it will stay big even
with the voltage reduced by the 10k resistor and BC548 conducting. As
you said, it is a bizarre negative feedback, but "looks like" to be
effective, mostly to be able to generate a strong output even with a
small IR change.
> You need to introduce a threshold again. I suggest putting a couple
> of hundred ohms (but you may need more) in series with the 27k collector
> resistor and taking the abovementioned 4M7 or whatever resistor from
> that. It will now be a few millivolts positive and the op-amp/
> comparator will idle with output high. If you want otherwise, just swap
> the op-amp inputs.
BC548A at 100µA hfe is ~ 130,
BC548B ~ 230,
BC548C ~ 400.
In any case a 300 Ohms resistor would do it.
I would discuss the possibility to eliminate completely both BC548, but
what a hell, other than paint the house Jinx needs to solder something
on those boards.... :)
+5V +5V +5V
| | |
| | +++
| 47k R --- 1µF
| | |
| | 1M
| 4k7 R Out---o------------->
| | 100k |
| | |
| | 6M |
4k7 resistor should be adjusted for desired PIR sensitivity.
6M (or around) could be necessary to create histeresys.
Question: That particular PIR needs the 47k pull-down resistor at its
output as in Jinx's drawing?
Some details (not much) for Murata pyroelectric sensor IRA-E700ST0
It shows the 47k resistor in a test circuit, no apps circuits unfortunately
I've tried a range of resistors in there, and 47k seems to be the best
compromise between current consumption and decent output voltage.
The circuit does work much better with the 10k in circuit. I don't know
enough about EE to say exactly why. The problem I had most trouble
with was the rejection/non-amplification of small output fluctuations
due to changes in ambient temperature, such as when the sun came
out or there was a sudden draught of cold/warm air across the room.
If you look in a commercial/domestic security light unit you'll
probably find a quad amp with a lot of feedback and filter circuitry.
I don't have one to hand to compare Wagner's front end to one
of those. It could be that it's similar to a domestic unit. I can only
go by the original app note I followed all those months ago. Now
that it's out of the cupboard and back on the bench I'll be having a
look, with second sight, at the ideas put up. The intention is of
course to get the best product for the least $ and uA.
The app I have for this is not as critical as a security device, so
a much simpler circuit will do. My experience has been that the use
of a proper polyethylene Fresnel lens has far more effect on range
performance (about 500%), than trying to increase the gain, and all
the problems that entails. As long as unwanted transients are not
It'll be interesting to see how low the component count can get while
keeping the performance I've already got.
Can't wait for these days of chief cook and bottlewasher on three
jobs to be done with. I remember when I used to have time to think.
|>Date: Tue, 1 Feb 2000 22:57:08 PST
>From: William Chops Westfield <CISCO.COM> billw
>Subject: Re: [OT] PIR
> The PIRs found inside of the cheap motion detectors (*)
> (*) $8.00 at Home Depot, Lowes, Wal-Mart, and others, with all the
> electronics, lamp sockets and more...
>So does anyone have enough data on cheap motion detector internals so that
>they can be taken apart and used in low voltage circuits?
1. Most motion sensors now use a dedicated IC (about $US1.70 in volume)
plus a RE200B type of motion sensor (about $US0.90 in volume) plus a
My kit 30 has all the bells and whistles you can attach to a commercial
motion sensor IC, the KC778B in DIP form. The data sheet is on my website
under Kit 30. The data sheet for the RE200B is also there.
2. My Kit 76 is a module which uses the SM version of the same chip plus
sensor plus fresnel lens - PCB mounted.
All of these items are now made in China.
I can sell any of them to you at very good prices. For small quantities
sending me $US cash GBP or DM notes is the best.
DIY Electronics (HK) Ltd
PO Box 88458, Sham Shui Po, Hong Kong
Voice: 852-2720 0255 Fax: 852-2725 0610 Email: kitsrus.competer
No joy with your circuit suggestion unfortunately Wagner. I think
you may have under-estimated the gain required to get a usable
signal from the PIR sensors. Although there is some voltage
fluctuation evident, it still needs substantial amplification. There's
just not enough to operate a simple comparator reliably. Which
was why I added the second transistor stage in the original. Still,
worth a try.
Not the worst disappointment the week - I turned the TV on a few
days ago to what I thought was going to be a nature documentary,
"Superb Owl", only to find there was a gridiron game on instead.
>From what little I have read about the amplifiers for PIR circuits, they
are not very simple. Some amplify the sense signal in an op amp with lots
of negative feed back so as to obtain a nearly constant output voltage no
matter what the ambient level being sensed might be -- this "self
corrects" for normal environment changes. Then they incorporate one fast
acting and one slow acting amplifier, and compare the difference in the
fast output and the slow output. That difference is the signal used to
detect the presence (motion, actually) of the target. Hope this makes
sense !! Good luck...
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