Mail Delivered Notifier - RF Antenna Question - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: April 18, 2021, 01:26:44 pm
content: I am modifying a wireless door bell to notify me when the mail delivery person has placed mail in my box. The mail box is the typical painted stamped metal type. The wireless button will be configured to trigger when the door is opened.I am considering placing the button circuit board inside the metal box for weather protection. This would be easier than designing a weather-proof container to house it outside of the box. But, the antenna for the button is the folded trace type that is typically integrated into the PCB and I am concerned that the metal box would greatly attenuate the RF signal. So, I have a couple of questions.- Could the metal box be used as an extension of the antenna by coupling the antenna trace to the box with a length of wire? If so, what special considerations are needed.- Or, could a length of wire be soldered to the base of the antenna trace and, then, run outside of the box? If so, what special considerations are needed.Thank you for your help.

author: highpower
author_rank: Regular Contributor
timestamp: April 18, 2021, 06:40:47 pm
content: How far away is your mailbox? Front of the house or out at the road? Most wireless doorbells are pretty much useless beyond 15 -20 feet under even the best conditions in my experience. I solved a similar problem by using a Dakota Alert motion sensor instead. That way I'm alerted to ANY deliveries like USPS, FedEx, UPS etc. I use one that transmits on a MURS frequency and I can use a $25 Baofeng HT radio as the receiver. That way I can have it with me anywhere on my property.

author: edavid
author_rank: Super Contributor
timestamp: April 18, 2021, 07:28:16 pm
content: Quote from: blueskull on April 18, 2021, 01:38:10 pmThat's silly, modifying your own mailbox is 100% legal as long as it still meets USPS standards.To the OP, when I used an old fashioned "Signamail", I drilled a hole in the bottom of the mailbox and ran a short wire antenna down the post. It was very reliable.If you don't get enough range with your doorbell, search Amazon or eBay for "wireless mail alert", and you will find many options.

author: ElizatronicWarfare
author_rank: Contributor
timestamp: April 19, 2021, 05:12:58 am
content: We need more information - How far away is the mailbox from your reciever? What frequency are you using? How much broadcast power is available? Why not simply run an outdoors-rated cable from the mailbox to your house?

author: Berni
author_rank: Super Contributor
timestamp: April 19, 2021, 05:59:37 am
content: First make sure your doorbell can do the distance you need it to on its own, they don't always have the best rangeAs for the antenna the easiest way is to drill a hole in the bottom of your mailbox and hang a wire out of it. This solves problems with rain getting in and makes a good monopole antenna with the metal mailbox serving as a ground plane (So it actually helps the antenna work better rather than disturbing it). Just check what frequency the doorbell transmits on and make that wire hanging out 1/4 the wavelength of that frequency. Since doorbells are typically pretty compact and use <1GHz means that this antenna will likely work quite a bit better than what is originally on the doorbell.To make it look nice you can go find a thin 2mm metal rod and pass it trough a small plastic cable gland (Those that screw down to clamp down on the cable)Quote from: blueskull on April 18, 2021, 01:38:10 pmIt probably is illegal to tamper with a public mailbox for sending mail, but not your own mailbox in front of your house. You own that piece of cheep bent sheet metal and you probably bought it yourself at a hardware store. As long as it functions as a mailbox its fine, it could be a bucket nailed to a wooden post with MAIL written on it.

author: edavid
author_rank: Super Contributor
timestamp: April 19, 2021, 02:31:23 pm
content: Quote from: Berni on April 19, 2021, 05:59:37 amIn the US, a bucket is not OK. Mailboxes have to meet USPS standards:https://www.usps.com/manage/mailboxes.htm

author: t1d
author_rank: Super Contributor
timestamp: April 19, 2021, 04:32:06 pm
content: Thanks to everyone for the great answers and your efforts to help me. I appreciate you very much!

author: Peabody
author_rank: Super Contributor
timestamp: April 20, 2021, 01:47:49 am
content: I built a mailbox notifier using the Lolin D1 Mini that connected to my wireless router at 2.4GHz. Surprisingly, it worked from inside the mailbox even though the D2 Mini has a trace antenna. I don't think it has anything to do with coupling to the metal mailbox and converting that into an antenna. I think it's just that a metal mailbox is probably pretty leaky RF wise. Mine has some holes in the bottom.In the end I used a D1 Mini Pro, which has a connector for an external antenna. You can buy coax cable with the proper connector on it, and strip back about 30mm of the braid at the end. That's the antenna for 2.4GHz. But of course your antenna will depend on what frequency the doorbell uses.I don't know about soldering coax to the trace antenna. Antennas are pretty mysterious, so it might not work at all. And there's no way to tell where it should be soldered. But I'd strongly suggest you test it as is inside the mailbox first. It might work fine.By the way, the biggest challenge may well be the cold when next winter rolls around. Your battery may not work so well.

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2021, 05:15:19 am
content: I built mine using a Moteino clone I build with a RFM69B, it's mounted to the inside of the mailbox door so when the mailbox is opened the antenna is exposed outside. It's been working reliably for a couple years now.

author: Berni
author_rank: Super Contributor
timestamp: April 20, 2021, 05:56:03 am
content: Quote from: edavid on April 19, 2021, 02:31:23 pmAh didn't know that. So that's why all the mailboxes in the US look the same.Here in Europe the mailboxes tend to only have to function as a mailbox and be placed in a publicly accessible area (As in not behind your front fence). Typically mailboxes here are designed in a way that lets anyone drop mail into it, but make it difficult to get the mail back out without the mailbox key to open it, rather than just being a box with a door. I guess over here people just value privacy of there mail more, since its not actually a requirement, but everyone uses such type of mailboxes. As for being required by law is to have the house number clearly visible from the public road. Tho it is still typical to label the mailbox itself with the house number and family last name just to be sure.Just one of these cultural differences i suppose, much like our windows open differently than US windows.

author: t1d
author_rank: Super Contributor
timestamp: April 20, 2021, 01:04:59 pm
content: Quote from: Peabody on April 20, 2021, 01:47:49 amI tried the button, today. It did not ring, even though the distance to the box is something less than 100 feet/30 meters. So, an antenna is needed.The manufacturer did not state the working frequency anywhere... Not in the advertisement. Not in the manual. Is there any way to catch it with my 100MHz oscilloscope. I fashioned a near-field probe from a loop of copper wire, but I have never used it. How about a frequency counter. My frequency counter has more bandwidth than my scope, IIRC. If either will do the job, how do I take the reading?Quote from: Peabody on April 20, 2021, 01:47:49 amI did not mean to say that I would tap into the trace antenna, itself. There are components tied to the bottom of the trace antenna, IIRC. I should be able to tap in there.Quote from: Peabody on April 20, 2021, 01:47:49 amI live on the Gulf coast of Alabama. Cold is not a problem. Come to think of it, heat might be, if the battery is inside the box. I would think that the box could get as hot as a car (140*F/60*C.) I could add vent holes to the box.

author: t1d
author_rank: Super Contributor
timestamp: April 20, 2021, 01:11:17 pm
content: Quote from: james_s on April 20, 2021, 05:15:19 amI take it that you mean an MCU. I don't code. I tried to learn, but I finally decided that I just do not enjoy coding (I rather dislike it considerably.) So, that is not an option. But, I understand how that might be the best way to build this project. There are so many options that you could add in.

author: themadhippy
author_rank: Super Contributor
timestamp: April 20, 2021, 01:29:01 pm
content: cant ya just cut a hole in the front door and fit a letter box?

author: Peabody
author_rank: Super Contributor
timestamp: April 20, 2021, 01:42:12 pm
content: Could you post a link to the doorbell? Maybe someone will have an idea about the frequency.

author: edavid
author_rank: Super Contributor
timestamp: April 20, 2021, 02:13:30 pm
content: Quote from: t1d on April 20, 2021, 01:04:59 pmIt's either 315MHz or 433MHz, but don't worry about it. A random length antenna should be fine.

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2021, 05:23:21 pm
content: Quote from: t1d on April 20, 2021, 01:11:17 pmWell that's not really the point. What I meant was if you mount the device on the inside of the door and use a mercury switch to trigger it when the door is open, the antenna will be exposed when it triggers.Even if you don't code yourself, you can use code written by other people to duplicate projects, and once something is working it's fairly easy to make small changes.

author: highpower
author_rank: Regular Contributor
timestamp: April 20, 2021, 06:10:39 pm
content: Quote from: james_s on April 20, 2021, 05:23:21 pmThese days, if a US postal carrier were to spot a mercury switch on a mailbox door - prepare to be SWATted and receive an extended stay at club Fed.

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2021, 07:19:27 pm
content: Quote from: highpower on April 20, 2021, 06:10:39 pmI have one on mine, and there's one on my mom's mailbox, been there for a couple years now, nobody has complained. It's just a little plastic project box with an antenna sticking out one end. The mercury switch is a tiny glass capsule inside the box.

author: t1d
author_rank: Super Contributor
timestamp: April 20, 2021, 07:41:30 pm
content: Thanks to everyone for the great ideas and support. I sort of answered everyone, below.In my area, the postal worker will not get out of their vehicle to come to a slot in the door.Link =https://www.amazon.com/gp/product/B08G73NT8V/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1"A random length antenna should be fine." = Excellent!"What I meant was if you mount the device on the inside of the door and use a mercury switch to trigger it when the door is open, the antenna will be exposed when it triggers. "- Okay, now I understand about the door being opened. Thanks for the extra explanation. If radio waves are directional, the box is pointing away from the house. Might that be an issue, for having the antenna only in the box? Sorry, I just don't know RF.- Mercury Switch = One of the challenges of this build is that the button transmits continuously, as long as the switch is closed. That means a mercury switch would ring the bell, until the post-person finishes flipping through the mail stack, puts my mail in the box and closes the door. That would not be good for my nerves, or the battery. So, I am having fun sussing out how to make a switch that will ring the bell (press the button) only one time. I don't want to do that with a MCU, for the sake of the battery and , again, I don't code. Making a mechanical solution requires some tricks, but I have a few ideas...Project box mounted on the door - That was a good solution.