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Quick question (hopefully) on PiFace inputs

scuffster

I would like to connect an Activ8 PIR sensor (http://www.responseelectronics.com/downloads/13649193548IR110_Activ8_DT_eng.pdf) to my PiFace inputs.

 

I'm powering the PIR sesnor from an externalk 12v supply. This powers the sesnor and the relay coil. The PIR has a normally closed relay, which is opened when motion is sensed. I managed to get this working (without the piFace board) just checking for an input_value on GPIO pin 12 and using the 3v output directly from the Pi to power the circuit. I'd like to move this to the piFace using the inputs on the piFace.

 

How would I do this? I assume this is a similar pattern to this one https://www.element14.com/community/people/ge0rgej/blog/2013/09/08/5--piface-digital-emulator . But it's not clear to me how to power the switched realy circuit ito the sensor and how that circuit should be cxonnected to the input ports inorder to detect when the circuit is open using digital_read(0) in my python code.

 

Any help appreciated.

 

Many thanks

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  • in reply to Problemchild

    Probably OK to do so but to be honest you could use an optoisolator then as Peter says you are less likely to get horrid noise/voltages in to the RPI. For a single channel they can be only £0.20-.30 so a good investment. If this PIR is in a house on a long cable run then some indirect method will certainly better.

  • in reply to Problemchild

    Yes I agree with your assessment John

     

    Alarm systems can end up with allot of very long cable runs and in my experience not always routed in the best fashion (Away from mains wiring etc)

     

    so yes, if this is going to be more than a LAB exersize and is to be used in the real world, I would also follow Johns advice re the Opto isolator

  • in reply to Robert Peter Oakes

    I've been thinking... As I have multiple PIRs...It may be better if I just use PIFace.  If I use PiFace, do you still need to use opto isolator and the pull up resistors?

  • in reply to Former Member

    The whole reasoning behind using the Opto couplers etc still holds true as the PI face does not isolate its inputs, there 5V rather than 3v3 but that's it. It does have a pair of relays that will provide control of devices and keep that isolated so score one there.

     

    It is not essential you use opto isolation but there is an increased risk of noise affecting the system and or damaging the PI or other connected devices. Not a high risk but a risk nevertheless so at the end of the day, you roll the dice and take your chances. If you cant afford a new PI or PI Face then go for the OPTO Isolator, If you can then take a chance, its up to you

     

    Peter