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Converting Normally Closed contacts to Normally Open?

Former Member

Hi Pete,

 

I am a National Park Service audiovisual technician, and currently assigned to update the sound and lighting for a 10 year old museum exhibit in a National Park Visitor Center. This exhibit is not monitored by staff. There nine precisely aimed Normally Closed (NC), Passive Infra-Red (PIR) motion detectors that trigger mp3 audio files of the sound of an alarm when a visitor intrudes into parts that they are not allowed to enter (people try and steal parts of the exhibit) and the circuit opens.  I didn't do the design, but assume mp3 players are used because the audio outputs can go to speakers in the exhibit via amplifiers, as well as line out through a mixer to a separate speaker at the ranger desk.  

 

The NC PIRs were connected to a proprietary device that recognized the change from NC to NO, and played the warning mp3 audio files.  That device has been replaced with NO input devices that only recognize contact closures: Akman SR8 players (www.akman.com.)   An obvious remedy would be to replace the PIRs with Normally Open (NO) versions, but I want to avoid that because no one makes NO PIRs that have the same characteristics of the existing NC PIRs, and setting up new NO PIRs would take days of adjusting, if even possible.

 

I am looking for a device that will convert the NC contacts to NO contacts, so that the Akman players normally see an open circuit 99%+, and a closed circuit when the PIR is triggered.  Ideally, this device would be powered with the 12VDC available from the Akman players, which also power the PIRs, and be reliable for many years.

 

Thank you,

Eric Epstein.

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  • 0

    nps-av

    Are you certain the PIR output is only NC.?

     

    While this is the normal 'Fail to Safety' scenario (which is good), when used with intruder alarms they often have a resistor in the line to detect short or open circuit wiring.

     

    It seems that ANY of the PIR's will cause the alarm to go off, so you could simply connect them in series, then put them into a single relay to change it from Normally closed (with the relay energised) to normally open to simulate the pushbutton.

     

    Relay contacts are marked according to their unenergised state, so your pushbutton becomes the normally closed relay contact.image

    This will detect any PIR tripping, a break in the wiring to any PIR, or the relay dropping out (power, broken coil), but obviously will not detect a break in the wiring between the relay and the SR8.

     

     

    It is shame these guys don't allow for NC in software, since you can change the characteristics of the inputs by entering text in the file ... maybe you should raise it with them.

     

    Mark

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  • 0

    nps-av

    Are you certain the PIR output is only NC.?

     

    While this is the normal 'Fail to Safety' scenario (which is good), when used with intruder alarms they often have a resistor in the line to detect short or open circuit wiring.

     

    It seems that ANY of the PIR's will cause the alarm to go off, so you could simply connect them in series, then put them into a single relay to change it from Normally closed (with the relay energised) to normally open to simulate the pushbutton.

     

    Relay contacts are marked according to their unenergised state, so your pushbutton becomes the normally closed relay contact.image

    This will detect any PIR tripping, a break in the wiring to any PIR, or the relay dropping out (power, broken coil), but obviously will not detect a break in the wiring between the relay and the SR8.

     

     

    It is shame these guys don't allow for NC in software, since you can change the characteristics of the inputs by entering text in the file ... maybe you should raise it with them.

     

    Mark

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