# How to Detect Paper Material in Post Box

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#### Kintesh Patel

Our Office have total 70 PostBoxes in one Rack for Staff Members. I want to interface this PostBox Rack with LAN network to send E-mail to every staff member (once in a day) when there is a Post in there PostBox. Each PostBox is made of simple Cardboard and each postbox has approx 12cm height. I want to put sensor in each postbox and I want to interface all these sensor with Bit wise decoder ckt thru Swtching IC so I can read each sensor's output (0 or 1).

I don't want to use any costly sensor or any light or Infrared based sensor. (becos we have total 70 postbox means 70 inputs). I want to use Small piece of alluminium foils (plates) for each postbox as a Capacitive sensor so, when there will be a postal cover in the postbox It's capacitance will change.

Can anybody provide any useful information or any other idea for Cheapest Sensors for 70 postboxes ?
I am also searching for the ckt which can give me 0 or 1 digital output based on that postbox's Capcitance. Postal meterial may be only one post card.

Thanks.

(I am an Electronics Engineer and working as a Project Team Lead for Application Developers Team for the county in US.)

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#### Dean Reimer

I have a question: why? Are the staff at your office too lazy to walk to the mailbox once a day and check? This seems like a colossal waste of effort, though I suppose if a very large company could eliminate an in-house mail delivery service it may be worthwile.

I have tried using a capacitive prox to detect paper (corrugated board to be precise) and had absolutely no success. The dielectric of the paper is too low. I'm not sure you'll have much success with that route.

I can't think of any cheap method that does not involve photo based sensors. You can get some small, flat-pack, retroreflective sensors and use reflective tape for the reflectors. You could probably rig each box for about $15-$20.

Good luck,
Dean Reimer

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#### Johan Bengtsson

Well, making a circuit detecting changes in capacitance isn't that hard. But I seriously doubt it is easy done in this case - at least if you want any kind of reliability.

It is quite small changes in capacitance you want to detect and with that many sensors that close (as I figure it) would possibly make a lot of trouble.

I did not get the part of not using light based sensors because the number of postboxes and number of inputs. Solving that problem (less inputs than number of boxes) don't really have anything to do with selected type of sensor. Ie you will have to solve that anyway and it would
not be that much easier or harder because you have a capacitive sensror instead of an optical.

It can be done but it would be easier (and not neccesarily more expensive) with light.

/Johan Bengtsson

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