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magnetic switch activated alarm

Discussion in 'General Electronics Discussion' started by mikeymuss669, Jun 5, 2011.

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  1. mikeymuss669

    mikeymuss669

    5
    0
    Jun 5, 2011
    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:)
     

    Attached Files:

    Last edited: Jun 8, 2011
  2. davenn

    davenn Moderator

    13,673
    1,892
    Sep 5, 2009
    hi there
    Welcome to the forums :)

    ahhh there's a number of probs with the relay cct you have drawn
    I will do a redraw for you and you can see how it works for your use... ok :)
    and post it soon

    cheers
    Dave

    ok got a cct tested ... all ok :)

    [​IMG]


    ok The changes .....
    1) the diode should be across the relay to dampen the back EMF produced by the relay coil
    2) you dont need a voltage divider across the Base of the transistor

    I used a BC337 NPN, transistor, the relay I have used is only drawing 40mA, and that transistor can handle that easily
     

    Attached Files:

    Last edited: Jun 6, 2011
  3. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Mike; the circuit should work as such, though the 10k resistance is a bit high and may need a darlington to pull the relay fully.
    What voltage do you see across the relay coil?
    As opposed to the diagram I see you're also switching the negative supply to the wailer circuit.
    Can you explain better what you mean with "But no current passes through the negative part of the relay to the 1st part"?

    Dave; Mike's circuit uses normally closed switches and has a latching function. Your cct doesn't latch, and needs to use the NC contact on the relay.
     
  4. davenn

    davenn Moderator

    13,673
    1,892
    Sep 5, 2009
    ohh crap, I overlooked the normally closed condition of the switches :rolleyes:
    sorry

    put it down to seriously under the weather with the flu. I only went to work today cuz we are doing stocktake else I would have been home in bed :(

    Dave
     
  5. mikeymuss669

    mikeymuss669

    5
    0
    Jun 5, 2011
    thanks man but as Resqueline said I need the latching function so that the alarm stays on even when you put the magnet back on the reed-switch. Anyway I tried your solution and same thing happens the relay clicks but no wail.
     
  6. mikeymuss669

    mikeymuss669

    5
    0
    Jun 5, 2011
    well I get 10v (with magnet off) from the +/- ends of the relay coil.
    What I mean is even though the relay clicks I don't get any current out of the - end of the relay switch (that black cable going to the wailer part).
    I checked again and the wailer works, it's just that relay switch on the - end which is not working for some reason. Tried another relay... But no joy :(
     
  7. davenn

    davenn Moderator

    13,673
    1,892
    Sep 5, 2009
    in your cct diag you are NOT switching the negative rail to the wailer cct ONLY the positive

    BUT in your component layout drawing you are using the other set of relay contacts to switch the negative. do what you have done for the cct diag not the layout drawing

    Also ensure you are using the correct set of contacts on the relay else you would see volts when the relay switches

    Dave
     
  8. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    What Dave said.
    I take it that the positive supply contacts deliver juice to the wailer cct, but not the negative rail contacts?
    10V is in my experience enough to activate all 12V relays completely, just be aware that with insufficient voltage then all contact sets may not engage fully.
    It should be possible to make Dave's cct latching too, using the spare contact set.
     
  9. mikeymuss669

    mikeymuss669

    5
    0
    Jun 5, 2011
    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.
     
  10. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The "Common" pins are not in the center of the contact array as you seem to have drawn on your layout, but rather on the pin pair closer to the coil pins.
     
  11. mikeymuss669

    mikeymuss669

    5
    0
    Jun 5, 2011
    Thanks to both of you for your help.
    It's working now.
    Davenn got me to go back to the schematic (for using the positive only to switch to the wailer).
    And Resqueline made me understand that you REALLY have to look at the datasheet before wiring components.
    I attach the final breadboard layout with some indications for newbies.
    (the resistor at the far left is 1k)
    cheers
     

    Attached Files:

  12. conductor3

    conductor3

    84
    0
    Jun 7, 2011
    i can give you a good low voltage diagram i built one that worked very great before when i was 8 allthough you have one this is a good low voltage device
     
    Last edited: Jun 13, 2011
  13. conductor3

    conductor3

    84
    0
    Jun 7, 2011
    the remote buy a toy car with a remote take apart not remote wire battery leads to power on circuit wire one lead from motor to the mosfet one button on the remote will set off the alarm
    the other will turn it off open and close a switch it will still be on if you labeled the remote
    push the off button it will turn off the other is to test the buzzer
    hope you enjoy it :D




    please anyone check im only a kid
     

    Attached Files:

    Last edited: Jun 13, 2011
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