magnetic switch activated alarm

Discussion in 'The Projects Forum' started by mikeymuss669, Jun 5, 2011.

  1. mikeymuss669

    Thread Starter New Member

    Jun 5, 2011
    4
    0
    Hello,
    I assembled a circuit on a breadboard which is separated in 2 parts:
    the right part is for generating an alarm which wails.
    the left part is for starting the current flow to the first part when the magnet is detached from the reed switch.
    Apparently the 1st part works (I get the wailing sound).
    The second part almost works (the relay switches when I remove the magnet).
    But no current passes through the negative part of the relay to the 1st part.
    (I checked with the meter).
    I attach a photo of the breadboard and the schematic, the components are :
    three n2222 transistors
    two n6027 programmable unijunction transistor
    one 4001 diode
    a dpdt 12v relay
    a reed switch and a small magnet
    (plus various capacitors and resistors)
    The current is 12v dc
     
  2. #12

    Expert

    Nov 30, 2010
    16,250
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    The relay is wired wrong. You put the transistor circuit to power the relay and the noise maker until the relay clicks. If you want a latching relay, you need to use a double pole relay.
     
  3. mikeymuss669

    Thread Starter New Member

    Jun 5, 2011
    4
    0
    so you mean I should replace the actual relay with a latching one (10 pins)?
     
  4. #12

    Expert

    Nov 30, 2010
    16,250
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    No, I mean use a dpst relay wired so it latches. That keeps the transistor with only one job to do, start the relay.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I used a SPDT relay here, only because you're already using one in your original schematic. As you can see, only two contacts are used. This relay can be replaced with a SPST relay if you wish. The LED serves two purposes. It's primarily used to make this circuit less susceptible to loop resistance in long loops. The LED also serves as an indicator when the loop is open. The values of R1 and R2 were left blank because I don't know the specs on your relay coil. If you post this info we can fill in those values. Both will be very close to 1KΩ for 2N2222's or similar transistors though.
     
  6. mikeymuss669

    Thread Starter New Member

    Jun 5, 2011
    4
    0
    Hello,
    the relay I use is this one:
    http://www.rapidonline.com/netalogue/specs/60-4754e.pdf
    That's what I should be using according to the layout drawing.
    The schematic is an earlier version of that part of the cct. But I think the problem with that is that once the alarm goes of it can be deactivated by putting the magnet back on the reed switch. Whereas the layout drawing should prevent that because it stays latched. The only way one could then switch off the alarm would be by turning of the power.
    So I really want to get the layout drawing working.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    There are two 12V relay models shown in your pdf. A 1KΩ for R1 & R2 should work fine for the circuit I gave you... as is.

    These relays are DPDT relays, so if you want to use the second set of contacts for latching, instead of D2 and R2, you can. In this case the second set of N/O contacts will be connected between Q1's collector and GND. D2 and R2 would not be used.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Here's a redraw using your DPDT relay. Reset (in both circuits) is provided by the S4 (Arm/Reset) switch, which is also the power switch.
     
  9. mikeymuss669

    Thread Starter New Member

    Jun 5, 2011
    4
    0
    Thanks guys,
    it's working!!!
    I followed Cdrive's schematic. Now I have to study it a bit more with my (very) limited knowledge in electronics. But thanks to both of you for your help.

    I also put some values for newbies like me who would be interseted to do the whole project.
     
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