EEVblog #275 - PIR Sensor Teardown & Tutorial

[Fox]

Hi Dave,

interesting tutorial part of the Video.
The reason they use a NC relay is not for energy saving.
Any decent Alarm system drives a current (couple of mA) trough every of its alarmlines to detect if there is a broken or cut cable.
So one can't simply rip the detector of the wall or cut the cables.

And keep up the Teardown Tuesday i like it.

[SeanB]

A standard alarm input circuit discriminates 3 conditions:

OK - a resistor of 2k2 is present across the terminals. Ideally it should be mounted in series ( some parallel) with the alarm sensor NC contact, and the sensor normally uses a relay that is always powered, so that the inactive state implies the sensor has power.

Short circuit - the cable has been shorted, either by a fault or by deliberate action.
Open circuit - The sensor has detected action that triggers it , either motion, fire or smoke, in the most common sensors used.

The alarm will generally be triggered when armed if the cable goes short or open circuit. When it is disarmed going short will show as a fault, possibly triggering a alarm of system fault if programmed in to the controller. Going open will be displayed as a triggered circuit with no other action.

The tamper switch is used in the same way to add extra redundancy against cable faulting or tampering, and will generally be run in series to all the sensors ( done to save inputs on the panel) with the resistor somewhere in series, generally in the panel in the common lead. It will trigger an always armed tamper signal, and is often not used in residential installs, as it adds extra cores in the cabling.

Alarm controllers are complex systems, generally using a PIC micro ( you can blow the program fuses so you cannot read the eeprom and they are rugged and cheap) with a input using 2 comparators to drive 2 input pins. The system parameters are stored generally in a I2C eeprom that is read at power on, or when programming ( thus there are 2 jumpers to short that allows you to do a factory restore by simply shorting SDA to ground during initial power up) mode is exited. Most allow add on boards and displays to provide extra inputs and a LED or LCD display for programming or operator use ( Those are 100 page programming manuals, with bits you set for options, and can be a little hard to understand the first time round, especially if you are using a set of 11 3mm LED indicators to both set an address and write a bit on or off with the keypad) and for connecting a radio module. Built in modem as well, with auto answer, line cut detection and it disconnects all phones in the house. Built in battery charger, with newer units using PTC fuses instead of glass units. All are designed to operate for 20 years reliably, though the battery will need replacing every 3 years or so.

[G7PSK]

It is possible to cut the wires to a pir without tripping the alarm. I had what I thought was a faulty pir,the led's had stopped working but the rest of the alarm circuits all worked and so did the system it could be armed and disarmed. I called out the alarm engineer (the whole system is on contract) when he investigated he found that no power was going to the pir in question. on further investigation it turned out that a mouse had chewed through the wire in exactly the right sequence not to trip the alarm and shorted the correct pair to trick the system into believing the pir and that branch of the system was intact and fully working.
If I had caught the mouse I would have got it to pick the lottery numbers for me.

[SeanB]

Yes, the wiring must be protected against damage. Here the thieves will break into the roof, then rip out the radio and phone wiring from the alarm before breaking into the rooms to steal. My solution to that was to install 2 PIR units in the roof, back to back so that each provided the other with a part of the pattern. Does not work well if you have birds or bats nesting there though.

[PA0PBZ]

The reason they use a NC relay is not for energy saving.
Any decent Alarm system drives a current (couple of mA) trough every of its alarmlines to detect if there is a broken or cut cable.
So one can't simply rip the detector of the wall or cut the cables.

And that is why they use a NC relay, otherwise (NO) you'd have to power it all the time 

[SeanB]

The reed relay is normally open. The PIR closes the contact to show that it is not triggered, deenergising the coil and turning on the LED when triggered. Both use around 10mA, so the power is about the same in both cases. The reason for energising the relay all the time is to increase reliability, as if the voltage to the PIR is too low or is broken the relay drops out, opening the circuit. If the relay energised to show the alarm condition then a power loss or loose connection will stop it from working with no other indication to the panel.

[Neilm]

If we are being picky about the temperature scale, then scientists work temperature in Kelvin, not degrees Kelvin.  As the Kelvin scale is referenced to absolute zero rather than a point picked at random the degree is omitted. (Also true of the Rankin scale).

It should be noted that if you learned about the temperature scales prior to 1970 you were taught degrees Kelvin as that was the accepted notation at the time.

BTW - Did you know that when Anders Celsius first proposed his scale, he assigned 0 as the boiling point of water with 100 being the freezing point?

Neil