Sensors vs. the mailman - Page 1
Forum section: electronics / projects

author: rea5245
author_rank: Frequent Contributor
timestamp: April 14, 2018, 05:29:40 pm
content: Hi,Years ago, I wired up a mailbox to send me a signal when the mailman delivered my mail. Opening the door caused a tilt switch to turn my circuit on. The circuit used a MOSFET to keep power coming even after the mailbox door was closed; once the door was closed, I used an LED on the top and a phototransistor on the bottom to detect whether there was mail sitting on the bottom of the mailbox, and a reed switch on the mailbox flag to distinguish between incoming and outgoing mail. I sent a signal back to the house via ZigBee, where an adorable little mailbox toy would raise its flag. It worked very nicely. (You can see a write-up athttp://www.galacticstudios.org/mailbox/.)Now I've moved to a new place and want to build a new mail detector. The problem is that my new mailman is very creative in how he leaves the mail (it's the old adage about making a foolproof system). Sometimes he lays it flat on the floor of the box, but sometimes he likes to prop up letters against the side. Sometimes he pushes the mail all the way to the back, and sometimes he leaves it near the front.The most obvious solution is to extend my current scheme and have LEDs and phototransistors positioned in multiple places. But before I do that, I thought I'd solicit alternatives here. What's a good way to detect if an object is in an area? It shouldn't require any moving parts, since water can get into the mailbox and freeze.Any ideas?Thank you,Bob

author: iwtommo
author_rank: Contributor
timestamp: April 14, 2018, 05:53:31 pm
content: Write a note on the mailbox asking the postie to lay the post down?Perhaps a motor vibrator that shakes the box a certain time after the lid is lifted to try and get the vertical post to fall horizontal?Get a different mailbox where letters can only be put in one wayI love problems like this. Ha

author: CopperCone
author_rank: Super Contributor
timestamp: April 14, 2018, 05:57:06 pm
content: man that would be some funny shit if the control system messes up and the mail box starts vibrating on the mailman

author: rea5245
author_rank: Frequent Contributor
timestamp: April 14, 2018, 06:06:45 pm
content: Quote from: CopperCone on April 14, 2018, 05:57:06 pmOne of my concerns has been to conceal all the electronics. I don't want anyone thinking there's a bomb.- Bob

author: RoGeorge
author_rank: Super Contributor
timestamp: April 14, 2018, 06:13:12 pm
content: Try putting mirrors (or adhesive mirror-like foil) on the inside walls of the mailbox, then one light beam (maybe a laser pointer) reflecting multiple times on those inner mirror walls before touching the photo detector.Any paper inside the mailbox should now stop the multiple reflected light beam, and trigger the mail detector.

author: CopperCone
author_rank: Super Contributor
timestamp: April 14, 2018, 06:27:44 pm
content: they leave it open sometimes lol

author: BrianHG
author_rank: Super Contributor
timestamp: April 14, 2018, 07:04:46 pm
content: You just need a sensor which sets off your indicator lamp, which stays on, whenever the mail box is opened to put the mail in it or take mail out. When you pick up the mail, you then reset the indicator lamp.In other words, don't try to detect the presence of mail, just detect when the mail box is opened.

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2018, 07:12:13 pm
content: Fails to recognize when mail was picked up, but nothing was delivered.Could make a false bottom with a load-cell to detect the presence of mail by weight.

author: ebastler
author_rank: Super Contributor
timestamp: April 14, 2018, 07:17:00 pm
content: Quote from: BrianHG on April 14, 2018, 07:04:46 pm+1. That's what I was about to write after reading the original post.

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2018, 07:26:49 pm
content: We don't have pickup+delivery mailboxes with flags, OP does. So he has to take into account if mail was picked up, but nothing was delivered.

author: ebastler
author_rank: Super Contributor
timestamp: April 14, 2018, 07:42:27 pm
content: Quote from: Marco on April 14, 2018, 07:26:49 pmAhh, right. I see that simple "the door was opened" sensors actually are already mentioned on the OP's web page, linked in the initial post.I can think of some exotic sensing mechanisms to cover all mail orientations, but they probably won't be robust. How about using the different sound absorption properties of mail vs. metallic mailbox walls? Send a sonar "ping" into the cavity and measure the duration until it decays...Either using multiple light sensors, or (very primitive) installing a horizontal crossbar inside the mailbox which prevents the insertion of vertically oriented envelopes, is probably the pragmatic way to go.

author: BrianHG
author_rank: Super Contributor
timestamp: April 14, 2018, 07:49:02 pm
content: How about mounting the mailbox on a load cell and measuring it's weight?Or, a flat load cell inside the mailbox along the bottom. It should be easier notice the weight of 1 letter that way.Another way is to have the inside of the mail box painted black and optically check for any reflections at any angle with 1 white LED and a photocell at the other end. Anything inserted other than black at any angle will change the luminosity at the other end of the mail box.Note, the 2 LEDs and 2 receivers would go at the top of the mail box at each end pointing down. Even letters stuffed on 1 side will give more reflective light back compared to an all black box.Don't forget to use a 20-60Khz TX carrier and tuned AM analog level receiver to negate the effect of daylight.

author: rea5245
author_rank: Frequent Contributor
timestamp: April 14, 2018, 08:20:01 pm
content: Quote from: BrianHG on April 14, 2018, 07:04:46 pmMy family can not be counted on to do that. In fact, they can be counted on not to do that.Plus, it doesn't cover the situation where the mailman picked up outgoing mail but didn't deliver anything to us.

author: rea5245
author_rank: Frequent Contributor
timestamp: April 14, 2018, 08:22:12 pm
content: Quote from: Marco on April 14, 2018, 07:12:13 pmI've thought about that, but the bottom plate might freeze into position in the winter, forcing me to go out to check the mailbox at the worst time of year (and yes, it would be barefoot in the snow, uphill both ways).

author: rea5245
author_rank: Frequent Contributor
timestamp: April 14, 2018, 08:24:26 pm
content: Quote from: ebastler on April 14, 2018, 07:42:27 pmThat would also block all my packages from Amazon and Digi-Key. I can't live that way!

author: rea5245
author_rank: Frequent Contributor
timestamp: April 14, 2018, 08:26:16 pm
content: Quote from: BrianHG on April 14, 2018, 07:49:02 pmI'm worried they'd freeze up in the winter.

author: james_s
author_rank: Super Contributor
timestamp: April 14, 2018, 08:31:17 pm
content: What's wrong with just detecting the box has been opened? At least for me some mail is delivered almost every day, frequently it's nothing but junk but there's no reasonable way to detect the quality of the mail. I suppose you could install a camera with IR illumination inside the box to take a look at the mail.

author: rea5245
author_rank: Frequent Contributor
timestamp: April 14, 2018, 08:34:40 pm
content: Quote from: james_s on April 14, 2018, 08:31:17 pmFirst, because it will be triggered when we put outgoing mail in the box. Second, it's a long enough walk to the mailbox - especially in the winter - that I want to be sure something's been delivered to me.

author: james_s
author_rank: Super Contributor
timestamp: April 14, 2018, 08:40:38 pm
content: But you know when you've put mail in the box, so you can reset it as soon as you get back to the house, or you could have a button to reset it on the side of the mailbox.Is it worth walking down to the box if it's only junk mail? That doesn't seem much better to me than an empty box. The only way I can see to predict that is to use a camera to see a picture of the mail.

author: rea5245
author_rank: Frequent Contributor
timestamp: April 14, 2018, 08:44:42 pm
content: Quote from: james_s on April 14, 2018, 08:40:38 pmMy family can be counted on not to do that. This has to be automatic.

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2018, 08:59:01 pm
content: Quote from: rea5245 on April 14, 2018, 08:26:16 pmIf you put in a false bottom and seal with silicone, the silicone might be flexible enough for the load cell to still detect the weight. Water proof and won't get fooled by snow on top of the box.

author: texaspyro
author_rank: Super Contributor
timestamp: April 15, 2018, 12:39:28 am
content: Fool proof way:When door is closed, wait a couple of minutes.Trigger a servo to release butane or propane into the mail box.Trigger a piezo sparker.Wait a few seconds.Enable a smoke detector module.If it triggers... "You had mail".There are very few problems that can't be solved with the suitable application of pyrotechnics...

author: Nominal Animal
author_rank: Super Contributor
timestamp: April 15, 2018, 01:27:12 am
content: Paint the inside of the mailbox matte black, with a paint that absorbs IR. (Many do.)At the far back upper corner, put a small IR proximity sensor, or IR LED+receiver pair, perhaps hidden by a triangular piece of IR-transparent but optically dark plastic. A small (ATtiny or similar) microcontroller stays asleep for most of the time, but periodically checks if the IR reflectance has changed. Better yet, add a microswitch to detect the opening of the box, so that the microcontroller can compare the IR reflectance state before and after the mailbox was opened and closed.

author: jmelson
author_rank: Super Contributor
timestamp: April 15, 2018, 02:50:33 am
content: Quote from: Marco on April 14, 2018, 07:12:13 pmWith all the junk mail in the US, this has happened only ONCE in 30 years!So, may not be a real issue.Jon

author: texaspyro
author_rank: Super Contributor
timestamp: April 15, 2018, 03:12:23 am
content: How about just using a small camera in the mailbox (with an LED light source) and a little image processing to detect changes? I've seen camera modules that can do a lot of the processing on board and send images over a serial port.

author: BrianHG
author_rank: Super Contributor
timestamp: April 15, 2018, 04:18:10 am
content: You don't even need to go that far. A remotely triggered wireless camera, or, one that takes a snapshot a few times a day in the mailbox with led flash would do the trick. Just check your cell-phone for the last image inside the mailbox.

author: texaspyro
author_rank: Super Contributor
timestamp: April 15, 2018, 04:28:37 am
content: Quote from: BrianHG on April 15, 2018, 04:18:10 amBut then you would need to get off your lazy ass and check the images. Think of all the effort a smarter camera would save you...