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Trying to build ALARM !

Discussion in 'General Electronics Discussion' started by Kevin72, Jun 21, 2012.

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  1. Kevin72

    Kevin72

    3
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    Jun 21, 2012
    Hey everyone ! a few years ago I had seen a alarm that was used for gear bags,quads, and gas jugs. The way it looked was a box about the size of a cracker box. It had a wire (like a rca cable) that went from the box throu the bikes and bags then back to the box again. If someone comes along and cuts the wire The alarm will go off. The box has a siren like a 9volt smoke detector siren, a on and off key switch red and green l.e.d. light for on and off and female rca jacks oh and a 9 volt battery.my problem is how do you wire it so when the wire is CUT it makes the alarm go off? It may be simple for someone out there but i have no more hair left! Hope someone can help and thanks !!!
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,813
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    Nov 17, 2011
    A very basic principle is like this:
    [​IMG]

    If the alarm loop is closed, the transistor is off, no buzzer.
    If the alarm loop is broken/cut, the transistor is on -> alarm.

    Some more elaboration may be necessary, I hope you get the idea.

    Harald
     

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  3. Kevin72

    Kevin72

    3
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    Jun 21, 2012
    Alarm

    O.K. I got most of it. I think I understand that v1,v is battery u1 is buzzer, but the R1 is what a resistor? and I have no guess to Q1 npn. Electronics is something I no very little about so you are helping a dummy here. Thanks for the reply Harald I will get to work as soon as I can figure out the rest of this.
     
  4. CocaCola

    CocaCola

    3,635
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    Apr 7, 2012
    NPN transistor... And before you asked what one, try a 2N2222 or 2N3904
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Sorry, this morning I've been in a bit of a hurry.

    Here is an improved schematic and an explanation. I'll try to keep it simple:
    [​IMG]

    The resistor R (100kOhm) "generates" a current from the battery's plus contact. If the alarm loop is closed, this current flows directly to the battery's minus contact. The transistors therefore are in the OFF state and the buzzer is silent.

    If the alarm loop is broken, the current can no longer flow directly to the battery. Instead it flows into the base of the transistor Q1 (use one as CocaCola suggested). This current is amplified by the gain of Q1 (approx. 100, for this circuit the value is not too relevant). In this improved version there is a second transistor Q2 which amplifies the output current of Q1 a second time, thus giving an overall current amplification of 100*100=10000 (if you want to know more about this circuit google >darlington circuit<). Assuming 1 9 V battery and a base-emitter voltage drop for each transistor of 0.6 V, the current through the resistor is approx. (9 V - 2*0.6 V)/100 kOhm = 78µA. This current is amplified by a factor of approx. 10000 which gives an output current of Q2 of 10000*78µA = 780mA *). A typical buzzer should require a lot less current, so this circuit supplies enough power.

    End of simple explanation.

    Here a few more notes, in case you're interested:
    This is only a very general principle. This circuit, for example is notz tamper proof at all. A wood-be thief could just short circuit the alarm loop by a string of wire that bypasses the object to be protected and then cut the original wire. Thus circuit wouldn't notice. A more elaborate circuit could use special wire with a defined resistance for the alarm loop an measure the resistance. If the resistance is either higher than expected (cut wire) or lower than expected (bypassed wire) an alarm would sound.
    To save energy, the alarm loop could be operated in a pulsed manner, e.g. only be tested every 100 ms or so, thus requiring less current during the idle time.
    Also a practical alarm circuit could have a timer which would allow the owner of the alarm to disarm the alarm before it starts to wail (in case he forgot to disarm the alarm before opening the loop. A blinking LED could be an indicator allowing the owner to disarm the alarm within the first 5 to 10 seconds of opening the loop).
    Also you'd want to have a battery supervision so you can be sure the alarm will operate at least as long as your bike is partked (just an example). Because an empty battery makes the whole effort null.
    And last not least you need a tamper proof case for the circuit.
    Plus, plus, plus...

    Harald

    *) footnote: This doesn't mean that 780 mA will flow through the buzzer. Only that this is the upper limit. The circuit will not supply more current (at least not while maintaining the voltage across the buzzer), but will happily supply less current if the buzzer is satisfied with less.
     

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  6. Kevin72

    Kevin72

    3
    0
    Jun 21, 2012
    Alarm

    Wow! Thanks Harald! You just learned me something! Thanks for your time. Now I can get to work! Have a great day! Kevin
     
  7. naqionline

    naqionline

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    Jun 23, 2012
    I learnt something from this. thanks for sharing.
     
  8. jbelectric777

    jbelectric777

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    Nov 29, 2012
    Harald thanks for this basic logic, now I get it! 32 yr electrician the change on how you guys think and we think is the hardest thing because you use both pos and neg, we never ever switch a neutral, always the hot (un grounded conductor) im retired and love semi conducting electronics. I racked my brain for a closed loop trigger till I saw this where the open loop provides neg via Q1 and pos is already connected, bells went off and the secret formula made sense! many thanks! of course its simplified to cave man era but I know how to spruce it up. I was going to use a NOR LOGIC IC (lol) but I figured I already bought everything , ran all cables, ready to connect then I was at a halt, we electricians love normally open stuff! lol but this is awesome I hope more to come, I have questions. Jim Benson USA PA
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    11,813
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    Nov 17, 2011
    I'm glad it works for your understanding. You're welcome to ask, should more questions arise.

    Harald
     
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