Proximity Sensor

A "How To..."

                            by Chuck Thurston

The basic idea with this project is to create a sensor that feels things coming near it without any physical contact.

The answer is a cheap wall stud finder.  The really cheap ones use a capacitive "antenna" as their sensor.  You set it against the wall and hold the button, the finder sets itself based on the density of everything near it to be its reference point.  Then as you move the finder over the wall it will pass over a stud inside the wall, which of course increases the local density.  The LED display and a audible beep tell you you're on the stud.

So what if it wasn't against a wall, and it's just sitting there stationary.  It'll do the same thing when something comes near it as well.

The range it senses is dependent on the local density around it when it's turned on, and the amount of interference the antenna is seeing.  But in a location with little interference and not much metal, the range can be several feet.  So anything with either an electrical charge or density will trigger the device.  Which pretty much means anything and everything.

Don't waste money buying a more fancy stud finder.  It actually won't work as well, if at all.  The nicer ones have a side sensing feature that triggers if the thing doesn't sense a wall, or anything near the side of the finder, which is of no help here at all.

So, OK.  Let's get started...


Straight out of the package. This stud finder is $12, or so, from Orchard Supply Hardware stores.


Pull three screws to take the case apart, then a couple screws to remove the circuit board.
Carefully pry the antenna board and piezo-speaker (a little brass disc) out as well.


So, here it is out of the case.


The momentary push button switch needs to be replaced with an On/Off switch.


Remove the original switch.


Connect another On/Off switch with maybe 4" of wire.


View of the other side.


Replace that red antenna wire with a longer wire from the circuit board. You'll connect it to something later.


Drill holes in a project box for the LEDs and a hole for the piezo-speaker. Also try to line the circuit board to the box's side so you can punch a hole to be used for tuning the potentiometer (the little blue thing on the board).


In this case, I also had to remove and attach the battery connections to fit into the compartment and then to the board.  But this will of course depend on the project box you use.  The next step is to hot glue the board in place, and mount the switch.


Now about that antenna wire...Connect the antenna wire to the screw of a heavy brass drawer pull.  Any metal object will probably work, but the drawer pull seems to be a nice mix of size and practicality.


Screw the project box together.  To tune the thing, you'll have to turn it on and off a bunch of times, moving your hand away fast as the circuit sets itself.  Turning the potentiometer each time a little one way or the other until you find a setting that goes to a solid green light and senses your hand coming near the drawer pull.

   And you're done!


Here's another cheap version I've used. This one is $9 from Lowes.

 

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