Which kind of relay I need for my project?

Discussion in 'The Projects Forum' started by GiovaGiov, Jan 17, 2016.

  1. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    I need a relay which would be able to act in this way:

    State 1: When there is no power supply on the coil, the COM contact of the relay is placed on the N/C contact (default state);

    State 2: When I apply a power supply (12 volt) on the coil, the COM contact of the relay immediately switch to N/O contact and must keep this state indefinitely;

    State 3: When the power supply is removed, the COM contact of the relay, switch again to the N/C contact (default state), and will remain in this position for a certain time (eg settable between 5 and 10 mins); when the power supply is applied again on the coil, the relay should keep the State 3 (as I've said for a certain settable time) and then back to State 2.

    I need this relay to trigger a buzzer; the power supply of the relay will be “controlled” by a magnetic reed switch (like this one: http://s14.postimg.org/ouxbb05sx/magnetic_switch.jpg): when a magnet is placed near to the magnetic switch, the current is free to flow on the coil of the relay (the magnetic contact will set the its COM to N/C) – State 2 for the relay -;
    when the magnet is far from the magnetic switch, the magnetic contact will set the its COM to N/O and the power supply will be interrupted – State 3 for the relay -;
    when the magnet will be near again to the magnetic switch (COM on N/C), the State 3 of the relay should be keep for the settable time (eg 5 mins) and then the relay will back to the State 2.

    I heard of Time-delay relays (http://www.allaboutcircuits.com/textbook/digital/chpt-5/time-delay-relays/) but I'm not sure of which kind of TDR I need for my purposes, if they have a large time settable (in terms of minutes), if they work with low voltages and if they are easy to find and if they are cheap to buy. I also heard of the possibility to implement a latching relay, but is not very clear to me the logic.

    Or maybe can I achieve my needs using a timer? I'd wish to keep everything as simple as possible, so would be fine to find something of assembled (eg on ebay).

    Thank you.
     
    Last edited: Jan 17, 2016
  2. thumb2

    Member

    Oct 4, 2015
    66
    4
    A relay for a buzzer ? What is the current rating of the buzzer ?

    If you remove the power you need to retain energy for some time.
    However, look also for latch relays, but for 5 or 10 minutes on when power supply is off .. I still have no idea for it.
     
  3. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    The buzzer is a simple piezo buzzer (which has a separated power supplies with batteries); however in future I could consider to implement a kind of alarm-siren. The application is not critical, is just to check the state of a door (and as I've think to wire everything, also to start the alarm when the household power grid is off, eg during a black out). Nothing of professional, however.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,571
    2,381
    Here are timer definitions/types.
    Max.
     
    GiovaGiov likes this.
  5. thumb2

    Member

    Oct 4, 2015
    66
    4
    You say that you will remove the power supply.

    But if you want to realize step 3, you may need some digital sequential logic circuitry, which may will need a power supply.
    It's a system with memory.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Don't know where you are located, so this may be just a language problem, but...

    Your understanding of N/C and N/O contacts is backwards. For a SPDT relay (or multiple poles), the N/C contact is Normally Closed. That is, this is the contact the armature is touching in the default state, when there is no energy to the relay coil. When the coil is off, the n/c contacts are closed (making an electrical connection) and the n/o contacts are open (no electrical connection). Energizing the coil reverses these conditions.

    Please re-write your description using these terms correctly. It will make it much easier to suggest a circuit.

    ak
     
    GiovaGiov likes this.
  7. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    I'm sorry. I edited my first message as you suggested me.
    Thank you.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Much better. Now we can discuss things.

    What is the power source for the system? A time delay relay (or an equivalent circuit) will need power during the 5 to 10 minutes that the relay is ignoring possible new commands. So something has to power the system continuously no matter which state the magnetic reed switch is in. In your description it sounds like you want toe switch to control the relay directly. this will not work. The switch will have to control a circuit that controls the relay. The relay uses the switch as one input and the 5 minute timer as the other input to decide when to activate the relay.

    Is the buzzer sounding only in state 2?

    Also, do you have to have a relay? If, for example you have a 12 V power source and a 12 V buzzer, the timing, decision logic, and buzzer switching can be done without relays. What is your skill set regarding building an electronic circuit? Do you want to use only pre-made modules that you simply wire together, or can you read a schematic and wire up a small circuit on perf board?

    ak
     
  9. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    As I've said in the message number 3 ( http://forum.allaboutcircuits.com/t...lay-i-need-for-my-project.119752/#post-952330 ) the buzzer itself is feed with a separate power source, which consists in a batteries pack (4,5 volt); the relay should be provided with a wall wart: when the power supply of the relay is interrupted by a magnetic switch (or totally absent due a black-out) this must trigger the buzzer and keep the alarm state for some minutes (would be also ok indefinitely) also when the wall wart will provide the current again.

    Nope: as I've specified above, the buzzer have to sounding only in state 1 (When there is no power supply on the coil of the relay) and consequently in state 3 (When the power supply is removed from the relay): the state 3 have to be keep also when the power supply come back: the state 3 would be ok if can go on for some minutes or also indefinitely (in this case I will have to manually interrupt the separated power source of the buzzer), If i wire a sort of latching relay, I can at least obtain the State 3 for an indefinite time?
    EDIT: to be more clear: I have a buzzer module mounted on a pre-made circuit when sounding only when two contacts are open: this is because the alarm have to be activated only in state 1 and state 3.

    I am able to build simple electronic circuits (eg using a NE555 timer and a bunch of components), I am not able to read complex schematics, would be fine use a pre-made modules.

    Many thanks!
     
    Last edited: Jan 18, 2016
  10. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    Maybe I need something like this, a bistable latching relay?
    http://shop.ciseco.co.uk/3v-to-5v-bistable-latching-relay-kit/

    I could feed this relay with the same power source of the buzzer (4,5 volt): so, to reset the alarm and the state of the relay, I can simply interrupt the 4,5 volt with a dedicated switch (a reset switch). I'm waiting for some expert to see if this would be a viable solution.
     
    Last edited: Jan 18, 2016
  11. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    Hi,

    I ended up with the following schematic: I will act on the power source of the buzzer/alarm, and I will use, as the sensor for the door, this kind of switch/magnetic sensor:
    [​IMG]

    The following is the schematic/wiring:
    [​IMG]
    However, although I implemented a latching relay, in the above schematic the relay will lost its state when the door will be closed again, nullifying the "latching" wiring. Maybe there is a way, inserting another relay, to block the latching relay on the energized state also when the latching relay will lost the power source from +12 V? (the energized state will mean that COM (2 and 7) will be connected to NO (1 and 8).

    Many thanks.
     
Loading...