Gate sensor

Discussion in 'The Projects Forum' started by hatchie, Aug 21, 2008.

  1. hatchie

    Thread Starter New Member

    Aug 21, 2008
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    0
    Hi guys,

    I'm planning to make a gate sensor for some friends and whilst I have an EE background I haven't done any circuit design since university, so I thought I'd pick your minds here.

    It is a pretty basic setup. Their house is quite a distance from their front gate, and they have a dog so it has two purposes: the first is letting them know when the gate is being opened, and the second is knowing whether it has been left open or not. (The dog got out last w/e and got hit by a car, fortunately it is ok though).

    The basic design was to include a volume controlled bell/chime to indicate when the the gate was opened, and a LED to indicate if it was either open or closed. An additional option would be to have a optional alarm to ring when the gate has been left open, and an override button to turn this off.

    Any suggestions would be much appreciated!

    Thanks in advance,

    -hatchie
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    You can get a magnetic door bug very cheaply. The magnet mounts on the gate and the magnetic switch mounts on the gate post. A resistor goes across the switch contacts. When the gate is closed, the switch is closed. When the gate is open, the resistor can be sensed. When the cable from the house to the gate is broken, no current flows.

    A constant current source can easily be thrown together to power the bug. A comparator will let you know if the gate is closed (low voltage from constant current source to ground) open (medium voltage from current source to ground) or the cable is damaged (maximum voltage from current source to ground.)

    The rest is either logic gates or a microprocessor, and any needed drivers for the LED and chime.
     
  3. kammenos

    Active Member

    Aug 3, 2008
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  4. hatchie

    Thread Starter New Member

    Aug 21, 2008
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    Thanks for the replies. I had a bit of a search but couldn't find anything suitable, so I've decided to crunch this one from the ground up. Here is my first (vague) attempt at a schematic:

    http://i34.tinypic.com/6ynuba.jpg

    I'm just kinda talking out loud here, so to speak, so if anyone feels like correcting me, or adding general comments, I'd be very appreciative.

    Anyone had any experience with magnetic sensors or can suggest a good one to use? Something suitable for outdoors. My assumptions below are based on the fact that the sensor is similar to a switch and flips between high impedance and low if the sensor is open or closed.

    So there are three possible voltages at the "+" input of the two comparators (VCM1, VCM2):
    -gate open: magnetic sensor high impedance so the voltage will "low" (~I1*R2)
    -gate closed: magnetic sensor is low impedance, so voltage will be "high"
    -sensor cable damaged: no current flows at all, so voltage will be "highest"

    In order to provide a reference signals for the two comparators, I'm guessing a simple voltage divider circuit will do. Resistances are chosen between the ones described above so the comparators switch on or off at relevant times. Seeing I only need two, will something like the LM393 will be ok?

    This brings up a query I have about suitable current and voltage sources. Can I just run a the voltage from a simple DC source (converted from mains), and rely on a basic voltage divider setup, or do I need something more reliable? Likewise, what about the current source? Can I put together an equivalent circuit using the same DC source, or....? If so, what are my options?

    Chime circuit: something that will activate a chime only once when the voltage from the output of the 2nd comparator (VCOM2) goes high. Any ideas or suggestions?

    Thanks in advance for anyone taking time to comment!

    -hatchie
     
  5. kammenos

    Active Member

    Aug 3, 2008
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    schematic not available :(
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Close... if you are taking voltage across the sensor, and using a constant current source, voltage will be directly proportional to impedance. It's the old "Ohm's Law" thing.

    Try a monostable multivibrator.
     
  7. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    "Anyone had any experience with magnetic sensors or can suggest a good one to use?"

    Look around in Radio Shack, Lowe's or Home Depot in the security section -- they should have magnetic switches there. If you really want to roll your own, pick up a magnet and a magnetic reed switch, but you won't have the pretty plastic cases.

    --Rich
     
  8. Externet

    AAC Fanatic!

    Nov 29, 2005
    760
    57
    Geta wireless doorbell gadget; install the button at the gate in such way that gets pressed when door is not closed.
    Add a light to the chime if wanted, and an override switch too.
    Miguel
     
  9. Externet

    AAC Fanatic!

    Nov 29, 2005
    760
    57
    Get a wireless doorbell gadget, install in such way that presses the button when the gate opens.
    A spring loaded gate to close will help too.
    Add a light and override switch to the chime if preferred.
    Miguel

    Sorry, double post as first did not appear.
     
  10. hatchie

    Thread Starter New Member

    Aug 21, 2008
    4
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    Odd that that link isn't working for you. Here is another:

    http://imagesocket.com/view/gatesensorcf4.jpg

    >Close... if you are taking voltage across the sensor, and using a constant >current source, voltage will be directly proportional to impedance. It's the >old "Ohm's Law" thing.

    Sure - but with the sensor in parallel with the resistor (isn't that how they normally operate?) they the voltage predictions should be correct....
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Second image asks me to verify your date of birth.

    Many free image hosting sites are worth what one pays for them.

    Try uploading your schematic as an attachment here - use the paper clip icon in the reply window.
     
  12. hatchie

    Thread Starter New Member

    Aug 21, 2008
    4
    0
    Third time lucky.... (oddly both links worked for me from different computers...strange)
     
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