PIR circuit - Page 1
Forum section: electronics / projects

author: Greg J
author_rank: Regular Contributor
timestamp: May 09, 2021, 07:52:14 pm
content: Hi y'all,I've never designed anything with a Passive IR sensor. Doing a tear-down of a trinket I got from china that is suppose to scare cats away , etc - I was surprised at how complex it is and how much of it is dedicated to running the PIR sensor.The video is here (hopefully nobody minds) :https://youtu.be/pOECemP6lwQThe chip has designation: BISS0001 (rather odd one):http://sc-tech.cn/en/BISS0001.pdfI just cannot believe that this cannot be achieved with something as simple as examples shown here :https://circuitdiagramcentre.blogspot.com/2012/10/passive-infra-red-pir-sensor-pinouts.htmlWhat I can believe , is that they are using (possibly) a microcontroller to drive the piezo speaker. It has no designation, its not even rubbed off - so I can't tell you what it is.

author: Buriedcode
author_rank: Super Contributor
timestamp: May 09, 2021, 10:10:06 pm
content: I too have noticed that often PIR sensors have a dedicated IC and quite a few passives.But ask yourself, why that second link is so simple - specifically, is that a website that is likely to provide a schematic for a circuit that can perform adequately for security purposes?The internet is full of schematics, many are very simple, using very common/jellybean parts (like the 741, LM393 NE555...) and have things like using opamps as comparators (can be done, but with caveats).Whilst I'm sure many of these circuits iwll work, they are often simplified for convenience to make a ciruict that is as simple and cheap as possible, with little regard to any kind of performance (sensitivity, power consumption, noise performance etc...). As such, they can be hit-or-miss, but can be useful for a learning experience and trying out ideas.Edit: To add an example, take a IR proximity sensor. A quick google shows "modules" and schematics of a simple phototransistor and a comparator, with a simple IR emitter and resistor. An object reflects the IR to the phototransistor, until it can cause enough current to flow through it to reduce the voltage on a comparator node, below a threshold. Simple, and might work, but how would it behave in sunlight? Or in darkness? Or with LED lighting? Would the range stay the same? etc.. I'm not trying to shit on websites like the above, but keep it mind these are often just shared for content (or because people genuinely want to test the performance on blogs).I say, build the second circuit, see how it behaves in a room with the original. Whilst the datasheet for that deidicated PIR sensor doesn't go into much detail this one does a bit:https://www.onsemi.com/pdf/datasheet/ncs36000-d.pdfNote the heavy filtering.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 10, 2021, 03:15:31 pm
content: PIR sensors generate very small voltages, which need a lot of amplification to do something useful with it.If you look at an IC for a PIR sensor, (with example application schematic) then you see that most of it is a two or 3 stage amplifier, with a bit of AC coupling thrown in to prevent DC from being amplified (may (no: WILL) saturate the pre-amp, and some filtering to keep out unwanted signals.Those PIR IC's also have some kind of timer build in, which is usually adjustable via some pots, and there is a power supply section on the PCB, except for those hobby modules maybe, which expect to be powered from a clean supply.The more of this circuitry you take out and make short cuts, the worse the performance will be.The best PIR sensors feed the amplified signal into a microcontroller which further filters and analyses the signal, and can then for example distinguish between a person walking by or temperature changes which are caused by a gush of cold (or warm) air when a door is opened, or other disturbances and ignore those.

author: Skashkash
author_rank: Regular Contributor
timestamp: May 10, 2021, 03:54:32 pm
content: They also make PIR sensors that have all the signal conditioning and processing built into the can.Still does all the same amplification, filtering, timing etc.Easier to use, less configurable, more money (maybe), and more power consumption (maybe).https://www.senbasensor.com/products/digital-pir-sensor/

author: HwAoRrDk
author_rank: Super Contributor
timestamp: May 10, 2021, 04:37:29 pm
content: Quote from: Greg J on May 09, 2021, 07:52:14 pmYes, I've seen that done in these kind of devices. Using a micro as a glorified 555 timer, and the PIR chip just turns the entire microcontroller on/off. These OTP no-name MCUs are just as cheap (or cheaper!) than a timer chip.You'll probably find they're taking advantage of the micro's flexibility and cycling the ultrasonic frequency through several different values in a loop. The one I looked at missed a trick though by not randomising the cycling.

author: Greg J
author_rank: Regular Contributor
timestamp: May 11, 2021, 02:38:48 pm
content: Thank you everyone. Joys of taking apart Chinese products - and discovering new stuff!As it turned out - the circuit is quite complex for something that detects movement and produces 24-66kHz noise! Bonkers!