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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.

  • 0

    A simple 8 channel isolated relay board may do the trick if you have the power available. Or even a pair of 4 channel opto isolators if the volts and power managed is lower.

     

    Can you povide more details as to the schamatics and peraps a few pictures to help us help you

     

    the more info the better (I promise I wont come there and rob you image

     

    Peter

  • 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

  • 0 in reply to Robert Peter Oakes

    Hi Peter and Mark,

    Thanks for taking time to respond.  The original installation is shown below.  I hope you can enlarge it to read it.  I am based at the NPS Design Center in Harpers Ferry WV, but the installation is at the American Memorial in Saipan, so Peter, I am not worried about you showing up to rob me image.  One section of the exhibit is below.The web site is www.nps.gov/AMME.  I travelled there 2 weeks ago to do the preliminary exhibit upgrade, am going back in a few weeks to finish up. The alarms are only one part of the job.  Anyway, Zones 1,4,6 each have one PIR.  Zones 2,3,5 each have two PIRs in parallel.  The BBI Custom show controller on the left and the 6-channel Crown amplifier have been replaced with six individual Akman SR-8 players.  Each player feeds a separate alarm speaker located in the area that is alarmed. this is shown in WIRING DETAILS, top diagram, so a single relay would not work.  The other WIRING DETAILS are for other elements of the exhibit.  

    Yes, I am sure the existing PIRs are form A (NC) only.

    Is it correct to assume I would need 9 relays to convert the NC contacts to NO contacts?  Amy specifics regarding manufacturer and model and how to wire would be appreciated.  Again, 12VDC and 120VAC is readily available.

    Thanks,

    Eric

     

    image

    image

  • 0 in reply to Former Member

    OK, this looks relativly simple

    the sensors (PIR) are most lightly powerd from 12V (Either that or 120 so I picked the obvious one image ) and the switch simply grounds a third line

     

    so a simple inverter circuit should work, here is an example

     

    i was going to present 2 options but a relay on its own would be powered all the time and that would be more power consumption than needed so I came up with this

     

    your new players are LOGIC level inputs implying only 5V on them when not grounded through a switch (Typically) but either way will only close a circuit between the logic input to the controller and ground when the PIR opens its relay (Ignore the relay in the schematic, though it will work I could not find it on Newark/Element14 store so this is the equivilent if not the same one that they do have and it is like 2$ ish each so very cheap, on your local site look up the newark part numner 27K6708 for local price (Oh, the resistor is about 1K to give a 10mA ish Ibe and with a typical gain of  >100 results in a potential current ICE of 1A, so it will be in saturation and easily turn on the relay

     

    http://canada.newark.com/panasonic-electric-works/js1-12v-f/power-relay-spdt-12vdc-10a-pc/dp/27K6708?MER=PPSO_N_P_PowerGeneralPurpose_None

    image

    What you did not say is how secure you want this to be from tampering, with this circuit, if someone cut the wire then the sound would not play, now if you connected bith grounds (0V lines) together at the controller then it would still play if the wire was cut. As the controllers and the PIRs are all powered from 12V and if that is the same 12V this will not be an issue

  • 0 in reply to Robert Peter Oakes

    Hi Pete,

    I appreciate the answer, but it looks like I would need to buy a bunch of parts and build it in a project box or similar.  I was looking for a simple evice I could just connect a few wires to.

    While relatively expensive, the rdl ST-LCR1 will work- acording to their techsupport- and be easy to connect.

    ST-LCR1 ‐ Logic Controlled Relay - Momentary

    I  do need 24VDC for this

    Unless you know of something similar that runs off 12VDC and is easy to install, I'll probably go with the RDL but THANKS for you TIME!

    -eric

  • 0 in reply to Former Member

    looks like that unit is a 24V version of what I have drawn in a pretty box

     

    I would be supprised if the same company did not do a 12V version, either way, asside from needing a 24V supply, they should work just fine

     

    Note that the relays in them will be energized most of the time rather than off, I assume you would also be using the slave inputs, although if you used a pullup resistor from the 24V to the 24V Activation and used the PIR to ground it, you should be able to have the output relay de-energized normally if so desired. Yes you would need a resistor connected between the 24V and the 24V Activation but that should not present an issue I would have thought

     

    Peter