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Continuity triggered circuit

Discussion in 'General Electronics Discussion' started by ericg, Jul 27, 2011.

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  1. ericg


    Jul 27, 2011
    Hello all,
    I am trying to come up with a solution to an increasingly problematic issue where i am. Vandals always steal transformer feeder cables (abt 50mm pure copper) and render whole neighbors without electricity.:eek:
    So far i am using a satellite modem to relay a text message to nearby recipients and power holdings officials in the event of such an occurence. The problem is the message delivery time is very erratic (sometimes within 2mins other times as long as 15mins).
    To tackle this i am considering incoorporating an alarm circuit into the existing solution such that once the tampering occurs, a siren goes off:cool:.
    The solution as it is right now is such that a monitoring cable runs along the feeder cables and is continous all through to the satellite modem such that in order to steal the feeder cables, one would have to break the continuity of the monitioring cable, this action is what triggers the satellite modem to send an sms.
    I would like to add a circuit such that the action of breaking the continuity would trigger the siren going off.
    The satellite modem contacts can not accept current i.e. they are dry contacts hence the use of only continuity.
  2. daddles


    Jun 10, 2011
    One solution would be to run a low voltage relay (e.g., 5 volts) with the protection wire. Connect the siren to the NC (normally closed) contacts of the relay. As long as the relay is powered through the wire, the siren is off; when it is cut, the siren turns on.

    It probably won't take thieves long to figure out what's going on -- this can be defeated with a simple jumper wire. A later revision can do more sophisticated things...
  3. poor mystic

    poor mystic

    Apr 8, 2011
    I would expect the "later version" to incorporate a Wheatstone Bridge. For non-electronics people, this is a simple detector which is capable of being 'tuned' to the exact impedance of the 'protection wire". If there is any change in the wire, the bridge can detect it.
  4. daddles


    Jun 10, 2011
    Another method would be a fiber optic cable with an encoded light signal. If the correct code doesn't arrive, the alarm squeals. It could easily detect sub-millisecond dropouts (as could a copper wire with a signal on it).
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