Using NO magnetic switches instead of NC



  • Just thinking out loud, but the window switches linked to in the build section are NC type, meaning that when the magnet is close the circuit is complete. This means you're at best using a sensor that sleeps most of the time and is drawing some power, even if you get fancy.

    Wouldn't it make more sense to use a normally open switch if you are running a battery powered sensor? That way it could be off most of the time. Put the switch in line with the power so when a window opens, the switch closes and the sensor is powered so it sends a window open status. Using a couple transistors you could create a latching circuit that keeps power on after the window closes (which would open the power circuit). By also running that switch power to a pin on the arduino it can detect the switch closing and send "window closed" at that time. Once it has made the report it just opens the latch and you are drawing zero power again. Even better than sleeping.

    Thoughts?


  • Admin

    @pete1450

    Yeah somewhat correct. But for my part I would like to have the node wake-up periodically, sending an "yes I'm alive" message. Perhaps with a battery status indicator as well.

    And if you only wake up once every 30 minutes a standard coin cell could last for years.



  • This certainly wouldn't work in that case. I'm a very lazy(and cheap) person, and if I can't easily make batteries last a year, I'm going to wall power it. I can open a window quickly every few months to make sure it still works. I meant to put something in there about battery level too. It could report that at each opening.


  • Contest Winner

    @pete1450

    If you are going to power it with AC power, why not have it report every few minutes, even if it is not modifying the state. That way you have a little heartbeat indicator and know at any time wether the node is in communication with the gateway.


  • Admin

    Still, with an cr2032 battery, which should have around 200mAh, you could get a theoretical lifetime of 3 years, if you only power it up every 30 minutes.

    So I don't see any problems with having the atmega powered all the time :)



  • @BulldogLowell
    No, you're right. I meant I'm not interested in a hearbeat so that wouldn't be a problem to me. If I wanted a regular heartbeat I would wall power.

    @tbowmo I agree 3 years is a good amount of time, but I have serious doubts about real world performance. I'm not trying to say the current implementation is the wrong way; it has it's uses. Just trying to come up with a way to maximize battery life and it doesn't get any better that 0mA. Just need to be aware of the tradeoffs.


  • Contest Winner

    @pete1450 said:

    No, you're right. I meant I'm not interested in a hearbeat so that wouldn't be a problem to me. If I wanted a regular heartbeat I would wall power.

    even once a day on battery would still be a long battery life...

    Like you, I hate buying batteries. I also don't like not knowing if my sensor is operational.... a little tradeoff, I guess.


  • Hero Member

    I think the main reason why most door/window switches are usually NC is so that should a burglar gain access to the wires to the sensor and cup/snip them the alarm would be tripped. Of course in this type of use batteries are not a consideration....

    Back to mysensors.... i too like the status sent periodically so i know if a sensor is alive or dead.



  • @gregl said:

    I think the main reason why most door/window switches are usually NC is so that should a burglar gain access to the wires to the sensor and cup/snip them the alarm would be tripped. Of course in this type of use batteries are not a consideration....

    Back to mysensors.... i too like the status sent periodically so i know if a sensor is alive or dead.

    Fair point. When it really comes down to it, I got started thinking about this in relation to a mailbox sensor. I want to put it there and not touch it for a year. It's flippin cold in Tennessee right now.


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