Where to put a diode?

Discussion in 'The Projects Forum' started by LaneH, Jan 28, 2015.

  1. LaneH

    Thread Starter New Member

    Jan 28, 2015
    Hi all,
    I am considering building my own at home alarm system and as of now I have only started to think about how I plan to wire it. I have drawn up and labeled a circuit diagram to make it easier to understand.

    I noticed as I drew it that when the system is operating regularly I think all is well, but when power is lost and the relay switches to the backup battery that battery will be in parallel with the converter. Will this damage the converter, or is it already rectified internally?

    Do I need diodes? If so, would anyone be so kind as to draw them in or tell me where?

    Are there any other obvious issues with this arrangement?
  2. MaxHeadRoom


    Jul 18, 2013
    Alarm systems are usually wired with all detect switches in series, in the parallel arrangement if a conductor or connection would open, that alarm position would cease to work.
  3. LaneH

    Thread Starter New Member

    Jan 28, 2015
    I'm a little confused by that. The switches are closed when a door or window is opened. When no doors or windows are opened all switches are opened. Any one door or window being opened provides a path to the siren.

    In series, wouldn't all doors and windows need to be opened in order for there to be a path to the siren?

    I considered series at first to save on wire but decided against it for this very reason.
  4. DickCappels


    Aug 21, 2008
    Yes you need a diode or you need to make the relay disconnect from the power supply and then connect to the battery (single pole double throw action).

    Listen to MaxHeadRoom. It is more secure to wire them in series; for the reason Max stated and because a paralled-connected system could be disabled by breaking the circuit anywhere. In wired systems (RF systems are now very popular) the switches are normally closed and in series when the circuit opens the alarm is sounded, the dialer dials, etc. With a normally closed system you can even use and LED (for example) to indicate whether the circuit is intact before turning on the alarm.
  5. LaneH

    Thread Starter New Member

    Jan 28, 2015
    Alright, that changes things up quite a bit. Thanks for the quick responses. I'll draw up another version and see if it's an improvement.

    I'll need to use a relay to provide power to the siren when power from the detect switch path is lost?

    Edit: I also didn't consider the fact the in either arrangement the siren is turned off simply by closing whatever door was opened. What component would allow the siren to continue to sound after it has been engaged even if the door is closed after opening?
    Last edited: Jan 28, 2015
  6. crutschow


    Mar 14, 2008