I want to know if there is mail in my mailbox. For reasons which are immaterial, I cannot use a sensor to see if the door is opened (there is no door). The ONLY way to determine if there is mail is to sense whether there is a piece of mail within the confines of the box. The box is: Height 5" x Width 3" x Depth 12". I have thought about using a laser and a PIR sensor on opposite sides of the box (or from top to bottom) -- when a piece of mail breaks the beam, it will indicate the presence of mail; but, the piece of mail might be lying flat in the box (so a laser-sensor on opposite sides would not trigger a broken beam) or the piece of mail might be standing up leaning against the side of the box (so a top-to-bottom beam in the center of the box might not be broken). I would need to assess if there is any piece of mail within the entire confines of the box. I've thought of a pressure sensor on the bottom of the box; however, there might be just one postcard -- which might not trigger a pressure sensor (or, maybe, a pressure sensor could be made to be that sensitive -- I just don't know). I am extremely new to microprocessing; in fact, I have an Arduino, but I have not, yet, done anything with it -- I'm, just now, going to be ordering some peripherals (wires, sensors, etc.). Any help, thoughts, ideas will be appreciated. Thankx.
I would use an IR beam but two of them. One across the width of the box and the other top to bottom. Then any piece of mail gets detected.
Thanks Grumpy_Mike for your feedback and input.
I was thinking about using two laser-PIR sensors (one top-to-bottom, and one side-to-side); however, there might be a postcard in the front half of the box or a postcard might be in the back half of the box. I'm just thinking -- maybe I can put the laser-PIR sensors right in the front of the box -- so the beam would be broken when the mail goes into the box, but the beam would not be broken while the mail is sitting in the box. So, I would just have to register that the beam had been broken (when the mail was placed in the box) and not, necessarily, have to detect whether there was any mail sitting in the box (after it was put into the box).
+1 on GM's answer, sense in two axes.
I see you just posted and caught both of these next two ideas but I'll leave them there anyway. The new idea you just added, the only prob with the one sensor at the front is that you have to continuously-forever run the sensor, and catch one event day that lasts only a second or two. The multiple sensors can glance casually only once in a while.
Sensors at the center of the box, at 6" depth from the front both horizontal and vertical, might not see a piece of mail that is too short, and placed at either the very front or the very back of the box. I'd place sensors at the 4" and 8" depths, again in both horizontal and vertical dimensions.
That's 4 rather than 2 sensors which is already twice as many as 1 sensor. IR receivers are extravagantly expensive. Plain LED and photocell are dirt cheap. I this outdoors? Probably not if there's no door. Sun can interfere with both photocell and IR, but if not directly facing prevailing sun shouldn't be a problem.
You could also teach Schrodinger's cat to fetch the mail. Until the mail is fetched it can be thought of as both real mail and junk mail at the same time, and it's not worth the effort of getting the junk mail till there's also real mail in the box.
laser-PIR sensors
Seem to be mixing things up here.
PIR stands for Passive Infra Red - nothing to do with lasers. It is meant for detecting the IR from living things that they emit due to their temperature. You can not detect mail with a PIR.
and it's not worth the effort of getting the junk mail till there's also real mail in the box.
In the UK the mail man can't delver junk mail unless there is some real mail to delver to you. However that only means mail with your address on it, it could contain junk as well.
Thanks for the input -- this is giving me some ideas. The mailbox is in an office suite with many businesses. There is a large door which gives the secretary access to forty (?) mailboxes. Once the large door is closed, each mailbox has its own little door -- thus, the secretary does not open the little door; and opening the large door (which opens all of the little doors) does not, necessarily, indicate that there is any mail in my particular mailbox. Taking that into consideration, I could use the opening of the large door (which, also, opens my little door) to trigger a reed switch (or, maybe, a Hall Effect switch). The reed switch could, then, trigger the laser in the opening of my particular mailbox to activate and, thus, detect the passage of mail into my box. The laser would, then, automatically turn off after a period of time or until the Reed switch or Hall Effect switch detects that the large door is closed. So, everything would only be on for a short period of time. I understand the whole thing could be put into sleep mode; I would only need to activate the Hall Effect and/or reed switch during business hours. If I use a reed switch, I wouldn't even need to put everything into a sleep mode; everything would just turn on when the reed switch closes the circuit. I'm just thinking outloud here; your inputs are very helpful! Do you think that would work???
Apparently, instead of a laser and a PIR sensor, I would need a laser and a photocell (thanks for that clarification).