Wirecut alarm system

Discussion in 'The Projects Forum' started by szabikka, Oct 13, 2014.

  1. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
    1
    Hi everyone!

    I'm currently working on an alarm system consisting of a PIR detector and a wire cut detector. I drew a circuit that seems to be working in simulator, but I would like to ask you to take a look at it. I'm concerned about missing resistors around the transistors (I don't want to fry them) but I would appreciate any other tips too. It works as follows: 1) if any of the wires are cut at the places marked by an SPST switch the buzzers should sound; 2) the 10 mF capacitor ensures that if both batteries are cut off at the same time the buzzers will sound (for a short amout of time); 3) The 555 is connected as a bistable so it remains on indefinitely. It's a bit difficult to understand at first, but as an amateur I couldn't come up with anything better (and if the burglar doesn't understand the circuit it's even better :) ). The diode in the lower left is going to the PIR detector, it doesn't come into account in the wirecut part.

    Thanks for your help in advance!
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    You are right to worry about your transistors - with no base resistors, they'll be toast. Think of the base-emitter path as a diode or even a short. You may also need to limit current through the collector-emitter paths, depending on those buzzers. And Q27 will pop like a fuse if it is turned on. [oops, I mis-traced where it went]

    What is D19 for?

    A 10µF capacitor won't run that circuit for more than a few tenths of a second. I haven't done the math, but I'm sure it's very short.
     
  3. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
    1
    Thanks for your reply wayneh!

    I was worried about the base emitter path myself. I will come up with something around them.

    If I take out D19, then the buzzers will immediately sound as electricity coming from the 12 V battery will flow through the emitter and base of Q23 towards the + pole of the 9V one.

    The capacitor is not a 10 uF but a 10 mF = 10000 uF. I chose this because it's relatively cheap (a 47000uF is very expensive) and should sound the buzzer for a few seconds (if i put in a current limiting resistor, maybe even longer).
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    OK, I had missed the 2 voltage sources. A 10 mF capacitor is huge and not seen often. Traditional capacitor marking uses "m" to indicate micro, so there is ambiguity and I made the wrong assumption.

    I don't have an alternative but this feels like an overly complicated solution.
     
  5. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
    1
    Yes, I know it's complicated and I tried to break it down into something simpler (using two 555 instead of the mess on the right side , or leaving out the 555 and building the left side like the right one, but something always went awry (buzzers always sounding, buzzers not sounding when negative poles' wires were cut, etc)
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    If you re-list the functions one by one, maybe there are some simple solutions.
     
  7. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
    1
    Hello again!

    I've spent the last few hours designing another wirecut alarm circuit. It is symmetrical now and easier to comprehend. It worked in the simulator, I got very happy and constructed it on a breadboard... it didn't work as it was supposed to. In the simulator if you connect the battery poles in the following order (turn on their respective switches), the buzzers won't turn on: 1.) 9V negative pole (SW33); 2.) 12V positive pole (SW28); 3.) 12V negative pole (SW29); 4.) 9V positive pole (SW32). However if I connect them in this order on the breadboard, the buzzers will sound as soon as 4.) is connected. Any other order of connection has also proven to be wrong as the buzzers started beeping. Any idea what could the problem be?
     
  8. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
    1
    Nevermind, I left in a flying wire that shorted part of the circuit. Works fine, as it is supposed to be.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    A success? Woohoo!
     
  10. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
    1
    Kind of. Sometimes it works, sometimes not. I think it has something to do with the capacitors. If any charge is left in them (I measured the voltage between the capacitors legs) the buzzers will sound when I'm still connecting the batteries.
     
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