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"Mechanical On/electronic off" switch component

Discussion in 'General Electronics Discussion' started by ragnard, Feb 2, 2013.

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  1. ragnard


    Feb 2, 2013
    I'm working on a project where users unlock lockers programmatically (with a password), however in case of power outage or equipment failure keys need to be available, but are only supposed to be used in those 2 situations.

    I'm trying to setup an "Alert circuit" which is triggered whenever a key is used (even when unpowered), but can only be turned off by the proper user entering their password and turning the alert off. While not high security, I think this will be sufficient. Does anyone have any ideas for components to accomplish this in a simpler way than what I outline below?

    The one way that I've come up with would be tricky mechanically and probably pretty finicky. I would have a switch that is spring loaded to hold a gap closed, and a solenoid that is spring loaded to open between the gap in the switch whenever a key is used (which depresses the switch and opens the gap. When the solenoid is expanded into the gap the switch is unable to close and the "Alert" is set, even without power (as soon as repowered the alert light is blinking). In order to reset and turn off the alert, the user must enter their password, and press a reset button which pulls the solenoid back out of the way momentarily, and the system is ready to detect the next key usage.
    Last edited: Feb 2, 2013
  2. shrtrnd


    Jan 15, 2010
    Look into a relay, not a solenoid.
  3. ragnard


    Feb 2, 2013

    A relay was my initial thought, but I haven't found anything with nearly enough throw to envision it working. I'm currently thinking some surge suppressors have a function that mimic what I'm attempting (i.e. a reset button closes the circuit while power breaks/opens it). Which now gets me thinking of push button circuit breakers in general just used in a "normally off" state.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I don't understand your description of your idea. Perhaps you could draw it up and scan the drawing and post it here.

    I'm also not clear on what you want to do.

    So you have lockers that can be unlocked electronically (through a solenoid of some kind) but you also need to be able to unlock them with a physical key if the password is lost or power has failed. And you want some method to detect when the key has been used, and raise an alert after the key has been used until the correct password is entered. Is that right?

    So the system that detects the use of the mechanical key needs to operate without power? So it needs to be mechanical? I guess it could be mechanically activated by the key barrel, and mechanically reset by the solenoid? And the system wouldn't be able to raise an alarm until the power was restored.

    What happens if the password is lost permanently? How would you be able to reset the alarm condition and set a new password? These are important functions of the electronic lock controller, and I think you need to describe that in a lot more detail and/or give a link to a URL for it, if it's a commercial product.

    I'm just thinking out loud and trying to clarify your requirements. I think you need to get into a lot more detail in your posts. Also remember that we're mainly electronics people - at least I am, so we may not be familiar with the mechanical options available.
    Last edited: Feb 5, 2013
  5. sirch


    Dec 6, 2012
    How long is your longest power outage? Could you arrange a capacitor such that it is charged when the power is on and discharged only when the key is used? Then on power-up of the lock controller you could check for charge in the capacitor.
  6. gorgon


    Jun 6, 2011
    If this is a new design and you are making the hardware too, it could be very simple.

    If you use a bolt type lock, the key could release a lock flap on the door, and the door can be opened. It can not be closed again since it will be stopped by the bolt and stay open to the lock bolt is activated to open again. The alarm is just a magnetic door switch, which state is combined into the logic of the lock controller.
    Last edited: Feb 5, 2013
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