EEVblog

[@alexbrown1050]

Here's me looking all around for an explanation of a modern Chinese chip and here's Dave, 10 years ago, answering questions I didn't even know I wanted answered. It's blowing my mind, of course the sensor is a capacitor and amplifier.

[@mon0railbredpig]

Cool. I've had a hallway pir in my junk box for a while now And been meaning to get around to reading up on it to use it in a project. Looks like you have done the leg work for me! Thanks for the awesome explanation.

[@zmast333]

No, some end of line resistors are also mounted on the connectors. This allows to detect if the wires are cutted or shorted by measuring the resistance so the alarm system is aware of the tampering.

[@headsplosive]

Security tech here. Most modern PIRs come with NC/NO relay, and most UL installations require EOL or double EOL resistor on NO contact. Being NC has nothing to do with power savings. Majority of installations are done with "quad" cable which is 4 conductor 22 gauge. Red and black for power and green and yellow for contacts. Most alarms (GE, DSC, Honeywell, Bosch, Verex) use negative as return, so you CAN run tamper and alarm using just 4 conductors.

[@SeanBZA]

Normally the NC contact is in parallel with a 2k2 resistor at the sensor. This allows discriminating between an open cable, a short or the loop being fine. If the alarm is armed either cutting or shorting the cable will trigger the alarm. If it is disarmed then cutting will not allow the alarm to be armed ( Fault detected in cable), while the short will show up on the alarm display. Tamper loop works the same, though often they are connected to an always armed input to prevent daytime disabling.

[@video2k007]

Hi Dave, you don't have to use four wires to use the tamper switch. you can run tamper switch and alarm switch in series. (quick and dirty but serves the purpose) greetings

[@mikeselectricstuff]

To screw the cover on if you don't want it to be unclippable

[@CampKohler]

There may be some administrative benefit of being able to distinguish between trouble and alarm conditions, say when the alarm is turned off during work hours, but you still want to monitor the tamper switch (which is called "supervision"). When the alarm is set, you don't care about the tamper switch, because someone shouldn't be able to sneak up on the PIR and set off the switch anyway if it was properly installed.

[@EEVblog]

You can if you want, they are both N/C, so just put them in series. If you only have 2 pair cable then this is your only choice if you want the tamper capability. If you have 3 pairs then you have the ability to monitor the tamper 24/7, so no one hacks the sensor during the day or whatever.

[@wirayoga2229]

EEVblog hello, can you send me the pdf file. Thanks

[@CampKohler]

First relays give total isolation between the PIR ckt and the long runs to the panel which may have God-knows-what riding on them (radio, TV, high voltage blips etc.). Second, relay contacts don't care what the levels of the panel signals are and so will work with all makes and models. Third, the near-zero-resistance of the contacts allow many sensors to be in series in an alarm loop. Fourth, the relay contacts are more robust than silicon and are not likely to blow due to spikes on the loop.

[@CampKohler]

That would work if you are taking one pix. The PIR contact will close for some short time dependant on the PIR's time constant, but will open if the motion stops. For a movie, you would probably need the contact to stay closed all during the video. You could add a following ckt that would close another relay for some suitable time period (say 30 seconds) for each click of the PIR. Some need one click to start and another to stop, which would require a bit more complexity. Buy a trail cam.

[@mikeselectricstuff]

All the PIRs I've seen use a NO relay and hold it closed until it detects - this allows a cheaper relay and means you get an alarm if power fails. Power draw isn't a significant issue.

[@PatheticComputing]

I bought some PIR sensors off ebay a while back, part# RE200B. I haven't done anything with them yet, but the technology has powerful applications. When put into a image sensor array, you get an FLIR camera, like the ones military and police use to find people in the dark. I recall a project where someone used one of these PIR sensors to make a "camera" by scanning a field of view using some stepper motors or servos. It has limited uses, but still pretty cool.

[@SamWalsh1]

Thankyou so much for this video, Helped me so much with my coursework! Also just watching a pro pick apart a circuit/datasheet is great for my learning as a trainee engineer..

[@Ts6451]

An alarm system may employ resistance monitoring to defeat such tampering.. Even without such features, you would still have to gain access to the area(bypass sensors on a door or window), then approach the PIR sensor without triggering it(or some other sensor), remove the insulation on the alarm cable without cutting the wires, figure out what combination of wires are used for what and tamper with the appropriate set. And after all this time and effort, you have access one sensor area...

[@MrDubje]

At my dads work thieves once came in trough a window and just put sticky tape over the sensors. The plastic of the fressnel lens is special because it lets pass ir light trough it. Place some transparent plastic over the sensor and see. About the relay: how about those (old) pir sensors for automatic doors? They click a lot too. Got 3 different models of them for hobby purposes. One of them has a read relay, not hermetic. Others do have a fairly common relay in them. Optex brand.

[@bgdwiepp]

Security tech here too, though we do more enterprise stuff with access control ect. NormallyOpen + double EOL is the way to go imho, can sense open circuit tamper, can sense closed circuit tamper, can sense alarm all on two wires and quicker sense time too (so better for door release push buttons ect). If you're wiring up something like an electric mortise lock you can the same sense ground for the inputs and run a single 6 core cable (usually 3 inputs; door reed, tongue/lock, handle)

[@CampKohler]

The connector screws have machine screw threads, whereas the "spare" screw is a sheet metal thread or plastic thread (which appear similar).

[@ajm114]

This is just my assumption, but correct me if I'm wrong. I think the top 2 reasons why they use relays instead of FETs. 1. They completely separate power from the input power and alarm lines, so if their was a short or something went horribly wrong the sensor is less likely to take a beating or the CP. 2. Sometimes it's cheaper then FETs. 3. They are easier to configure for alarm systems that may work with Normally Open Detectors then Normally Closed.

[@MrDubje]

Thanks, correct. When i typed that I was aiming on the metal can enclosure of the relay. They are in fact always from glass as far as my knowledge goes with these. About the PIRs, they were relativly old style/model. No IR antimask, bottom lens or all that modern jazz.

[@alphaprot2518]

The reason for using N/C contacts in security appliances rather has something to do with tamper protection. You want the alarm to go off not only if the motion sensor is activated by motion but also if some smart intruder decides to disable it by cutting the wire or if the cable just breaks and leaves an open connection. This is called "Drahtbruchsicherheit" - the appropiate English term would be something like "wire-breakage protection". This is mandatory e.g. for emegency shutdown buttons and other equipment which needs to be fail-safe.

[@Th3Su8]

The screw is used to secure the cover in place instead of relying on the plastic snap.

[@The1neo34]

Always wanted to know how these work !! thank you :D

[@CampKohler]

NO contacts can be supervised, say on a loop of simple (no electronics) fire detectors. They all go in parallel and the unit on the end of the line has an EOL resistor across it. Supervisory current runs down the loop, through the EOL resistor and back and the panel monitors the current, any break giving a trouble (vs. alarm) condition. You don't want a fire alarm to go off because a wire breaks. True, the switch contacts themselves aren't monitored, but they are dependable, UL-listed contacts.

[@CampKohler]

Obviously there has to be voltage to be useful. To improve on the definition of DRY, it means the voltage and current being switched by the contacts won't cause arcing, so that the switching cycles are limited only by the number of times the contacts can touch without physically wearing themselves out due to friction (vs. being chewed up by arcing). With regards to open (not enclosed) contacts, dry switching won't set off explosive vapors (although I wouldn't want to be there watching a test).

[@Kompost]

21:45 Intruder! It's got antlers! Laughed my ass off there :)

[@bgdwiepp]

Because alarms are generally the least important thing, if you can prevent the person getting into an area in the first place or see them getting in you don't "need" an alarm. The main things I was getting at is you can use the same ground for multiple alarm contacts, and with 2 resistors you can get four useful states, that is all.

[@CampKohler]

The contacts are sealed in a glass tube, so they free from corrupting air. They are very small, so take up less space, are lighter, etc. and inexpensive.

[@TheCrazyInventor]

Perhaps so you can detect/log different alarm trigger states? Don't know why you'd want that, but they give you the option anyway. And you can still do that, just hook them up in series yourself. You don't need to run 2 seperate wires for that.

[@sven3550]

I realy enjoy teardown tuesday. I'd be epic if you would make a episode about vacuum tubes.

[@jerzmacow]

Could you imagine if every datasheet was like that!? If only...

[@0xbenedikt]

NXP made pretty decent ones. But yes, this one is just lovely.

[@ssj3gohan456]

A FET cannot be made to be normally closed. That's really the only reason. For all other practical purposes you're right.

[@ssj3gohan456]

It seems like the high quality caps are the low-value ones (10µF) that act as an AC coupling between the opamp and comparators. Lower quality ones very possibly have such a high leakage current (often in the tens to a hundred µA) that the coupling would be bad. At least that seems like a much better explanation than reliability, because any third-rate 85C cap don't necessarily have worse reliability than any other third-rate 105C cap. Or maybe it's simply a cost thing.

[@CampKohler]

If you have two sensors with overlapping coverage, you can't fiddle with one without being caught by the other. Same with TV cameras; you always want one watching the other, else they steal your camera and you can't see who did it! AFAIK, reed relays are always hermetic, because they are built that way; the two reeds are held in physical relationship to each other by an enclosing glass tube which, by its very nature, seals out the air. I never saw an exposed read except when broken.

[@DjResR]

I have salvaged 2 Pyronix KX15DD, 1 Pyronix KK15ED and unnamed (with security company Falck logo) PIR sensors that all use solid state relays and exact same LHI968 sensor element. Edit: Quite robust devices, when I found two KX15DDs one was smashed and top lens stepped on, took the parts home, put it back together and it happily flashes it's blue light to the movement again._

[@PihkalTheTihkal]

You can but since they"re normally closed contacts,you'll have to wire them in series,in parallel wouldn't work. Correct me if i'm wrong,Dave.

[@Lachlant1984]

That bit at the end of the video is cute.

[@Knight8365]

Sagan's getting big!

[@ElectronicsAustralia]

Another great video Dave! Learnt so much about such a simple device :-)

[@ajm114]

Awesome video! I love working with PIR sensors so you made my day. =)

[@mikeselectricstuff]

So you can detect tampering when the system is not armed.

[@mkevlog3557]

because you need power to keep it turned on, so you would have to reverse the open, close stuff and cutting wire would disable alarm easily, reliability is quite bad in this case

[@robina.jensen6114]

You don't need 4 wire for alarm and tamper. 2 wires and resistors are enough. You use 1k in series with tamper and 1k parallel with alarmrelay. Resistor values depend on the system specs. :-)

[@RandyLott]

The little man is trying to break in! Watch out! Great video. DaveCAD now has autorouting?

[@mikeselectricstuff]

View counts are sometimes delayed

[@Matthias051]

Thank you so much again for your awesome video. I Enjoyed it very much as every time

[@syncmaster710n14]

On my alarm it gives you a read out of what sensors triggered it and weather it was tampered with or not.

[@MistahHeffo]

Why would they need to use a relay at all, why not just put a FET of some description across the alarm terminals? it would be far cheaper and more reliable than a special purpose relay

[@PihkalTheTihkal]

Good point,didn't think of that.

[@TheH0nk]

nicei just thought as well about the mythbuster experiemnt and that graph just explained that

[@jamiesonclements]

Extra screw for the blue connectors most likely

[@ssj3gohan456]

Even the cheapest old stock broker cannot sell you a sealed reed relay for less than a 2N700x... cost is definitely not an obvious guess. Also, one of the terminals of the alarm output line is already connected to the circuit, so the separation argument doesn't seem right either. I'm betting on the normally open nature of FETs.

[@Gameboygenius]

My guess would be it's just a spare, in case you ever need it.

[@mykmmc]

great one dave keep'em comin

[@NerdNordic]

Could it be that the high quality caps are assigned to the part of the circuit that continuously active whereas the cheap ones only are active while the sensor detects IR radiation ?

[@CampKohler]

It can be pronounced either way (just from long, sloppy usage). It was named after Frenchman Augustin-Jean Fresnel, a resident of Fresno, CA, who pronounced it Fre-no just to piss people off. :-)

[@123456789robbie]

the spare one? probably to replace a screw lost from the screw terminals

[@123Jeffdude]

More optical explanation teardowns please!

[@headsplosive]

PS: Oh yeah, and 99% of the time the relay is ON when the PIR is idle, so that you get an alarm state if power fails.

[@GiorgioCapocasa]

This is *so* cool! I was planning to experiment with a "motion sensing" night light as soon as I have time and this gave me a really nice introduction to the topic! Could you upload somewhere the diagram you made? My lamp has only one IC with a fairly scratched ID... maybe it's a quad op-amp but I'm not sure...

[@jerzmacow]

Exactly. It would be epic.

[@123456789robbie]

could you not wpire the alarm and tamper switches in series, so that if either is broken the alarm goes of?

[@tuva600]

Why can't you just hook the tamper switch up with the same wire as the alarm one?

[@CampKohler]

When you reach 302 views, YT stops and does some kind of validity checking before counting higher. (Don't know what the process is.) That's why you will see that number on many videos.

[@tymitunie]

Yes, Dave get the proper sensor and then do a review

[@SakariNy]

Well, if you want to know wich one triggered the alram, of course. But yes, does it really matter?

[@ChrisTheGregory]

So let's say (hypothetically) that a company producing PIR sensors uses a subcontractor who, for cost reasons, uses a below-spec white-light filter. The PIR sensor is installed in a home, but due to false positives at specific times of the day, the owner deactivates the alarm. If an intruder breaks in and kills the owner, how culpable would the sensor company and contractor actually be in terms of lawsuits? Not that this has ever happened, I just thought it was an interesting question.

[@byusven88]

In terms of a lawsuit the sensor company would be 0% culpable. The owner chose.

[@gnagyusa]

Sagan's adorable at the end. Oh no, a spoiler!

[@ObiTrev]

If half of the PIR sensor were covered with a metal plate (constant temperature heatsink), would it output a signal that is comparable to the amount of IR light reaching the remaining exposed element? So it would act a a highly directional long range photodiode, right? I'm trying to make a long range IR Thermometer that uses analog circuitry.

[@CampKohler]

Dave, whenever you say "dual" it sounds like "jewel." It takes me a while to remember that in each video. What is its sensitivity to flames (as viewed by the sensor, not Sagan playing with matches underneath it)?

[@Gameboygenius]

27:37 A wild Sagan appears!

[@hyperhektor7733]

I can not understand how a cheap plastic lens can hanlde FIR Wavelenghts of 9µm? Are they made of a special plastic or coated? I thought you need special expensive lenses for this spectrum.

[@kennybode]

Have you seen a 2 pin sensor ?

[@mechadrake]

Dave, you did not tell us young players for what where you probing when you reverse engineered the circuit. I now have a gaping hole in my knowledge :)

[@geraldellis1177]

do one on countermeasures and iable material to trick em

[@bcsupport]

Dear Dave, Can you please can record your voice saying "Call Geico now to save a bunch of money on car insurance"

[@HighTechCountryBoy]

Sagan is so big now

[@byusven88]

How are some PIR sensors pet aware?

[@landonrivers]

pets give off heat. there's a pyroelectric sensor inside the PIR sensor, meaning it's sensitive to heat

[@FrankSandqvist]

But Dave, cats are so cute~ :D

[@MarlosZappa]

YES, I'm also a dog person. I might still be good at electronics someday after all.

[@CampKohler]

Can't get any "silenter" than that. :-)

[@CampKohler]

That's no Photoshop! That's a MS Paint!

[@flodins]

D2 is wrong polarity (:

[@Whisper6911]

how does this video have 302 views, and yet 318 total votes?

[@NerdNordic]

Nice! :D

[@skonkfactory]

It's pronounced "frehnel"- the s is silent, like in French.

[@CampKohler]

If you want to see a neat application for IR very-low-power cameras, look at coppertheft.info. Absolutely astounding what they can do with a few AA cells nowadays.

[@KaPH33n]

25:35 to 26:06 was some hot ee pr0n

[@EEVblog]

LOL!

[@headsplosive]

>though we do more enterprise stuff with access control ect. LOL how is that even relevant? I do alarms, access, CCTV, ip, radio, networking, netsec, programming and electronics repair. Most of our customers are fortune 500 companies. Double EOL can be fooled with 3 MOSFETs that break the green and yellow and switch it to a local resistor network under 1 msec so the panel doesn't pick up the "switcharoo". Modern alarms are wankery.

[@CampKohler]

A slightly different tack on the Jones philosophy: tinyurlDOTcom/saganj

[@jtsiomb]

Stop saying fresno god damn it :) Freh-nell

[@MrDubje]

Nice photoshop!

[@tymitunie]

Bullshit. You can do the wiring whit only 3 wires (not pairs) and you will gain the ability for separate TMP and alarm. TMP and an alarm are based on the resistor (Of course depending on the system and its programming). Dave if you want to do a review of PIR sensors do it like whit the multimeter's and DO NOT compare shitty brand 5$ cheppy to a fluke!!! Get a Pytonix or Siemens and then do a review. & I bet the analog you have it "clicks" when it "see" movement 20y-old tech. blah... PKI is Grt