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Touch contacts help

Discussion in 'General Electronics Discussion' started by Nemesisdan, Nov 7, 2017.

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  1. Nemesisdan

    Nemesisdan

    2
    0
    Nov 7, 2017
    Hi all

    I have a wireless alarm sensor I am modifying for my new use case and need to add an externally attached magnetic reed switch.

    [​IMG]

    However, the alarm sensor has two parts to it. A back plate which attaches to the wall and a detachable main sensor unit which slides onto the back plate. The sensor unit contains the PCB circuit and battery.

    Since the battery will need to be changed periodically, I would need to detach the sensor unit from the back plate so don't want the external magnetic reed switch to be soldered directly to the PCB.

    Instead I was thinking of some metallic "touch" contacts on the back of the sensor and the front of the back plate, then soldering the external magnetic reed switch to the contacts on the back plate. When the senor unit is attached to the back plate, contact is made and completes the circuit.

    My question is, can anyone recommend some very small, almost flush contacts with a bit of spring, I can use for this purpose. I was kind of thinking along the lines of the battery contacts in a cell phone. The contacts on the battery are flat and nickel like, where as the contacts in the phone which "touch" the battery are almost "V" shape and have some spring.

    Any advise greatly appreciated!
     
  2. ChosunOne

    ChosunOne

    431
    132
    Jun 20, 2010
    As always, brand name and model number of the sensor would make it a lot easier to advise you. I am an experienced alarm systems technician, but I'm only familiar with wireless equipment used in the USA. All the wireless sensor/transmitters used here already have screw terminals built into the PCB for attaching external switches.

    You shouldn't have to remove the sensor from its back plate.to change the battery. The battery holder should be accessible when you remove the sensor cover. I am curious about the brand and model of your sensor/transmitter. An inexpensive Chinese import, perhaps?
    If you don't know the brand and/or model number, pictures of both sides of the PCB would help.

    EDIT: I recommend soldering a screw terminal block to the PCB, something like this one:
    https://www.ebay.com/itm/10-pcs-KF7...572876&hash=item35e87c0882:g:rDUAAOxyn~pR2nQY
    Historically, when spring-loaded connector contacts are used for sensors of any kind, there's a high risk of false alarms, even when gold-plated contact points are used. I speak from decades of experience.

    BTW, what part of Spain are you in? I lived near Sevilla for a few years, a long time ago.
     
    Last edited: Nov 9, 2017
  3. Nemesisdan

    Nemesisdan

    2
    0
    Nov 7, 2017
    Hi ChosunOne, thanks for your input. I'm based in Alicante, quite a drive from Sevilla :-(

    Yes, you are right. It's a Chinese import. Unfortunately, the sensor cover houses the PCB and battery so the whole sensor has to be removed from the wall mounting plate. The sensor itself has a back plate which has to be unscrewed, to access the battery and PCB. It would be simpler to have a front cover which could be removed separately without the need for a screwdriver, but it is what it is. I'm not sure I can modify the sensor cover in that way

    I thought about adding some screw terminals, but it would mean having a long wire always attached when changing the battery. Likewise, I was gonna add a toggle switch so the sensor would work with the internal OR external reed switch. Having both working at the same time would mean the sensor could be overridden by holding a magnet next to it when using the external reed switch.

    Everything came from hkvstar/Unifore Security

    They do have a sensor with external switch, however, the aesthetics were not appealing. It was more designed for industrial roller shutter doors.

    Sorry about the handwriting in the images, not easy to write with a mouse!!

    Any suggestions greatly appreciated!
     

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  4. ChosunOne

    ChosunOne

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    Jun 20, 2010
    Quite a drive, indeed. I don't think I ever got any farther east than Grenada. As far as I know, I never even heard Valencian spoken. I'm curious about whether I would have understood it, back when I was fluent in Spanish. That was quite awhile ago, más años de los que me gusta recordar.

    I see what you mean about having to remove the sensor to change the battery. I still have to warn you not to trust any kind of spring-loaded contact to connect a protective circuit. Contacts that depend on springs to make good contact have always been notoriously unreliable for keep a firm connection 60/60/24/7/365: They can seem to keep good contact for months, even years if you're lucky, but they tend to develop falsing problems sooner or later. I know it seems like they should work, but my experience has been that they don't.

    Having voiced my caveat, one of the electronics wizards on here may be able to find you something that fits; but I would personally recommend using a plug connection in the wire from the transmitter to the external sensor, perhaps something like those used in small plug-in batteries used in cordless phones.
     
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