RFM69HW temp-humidity node


  • Hero Member

    Not sure if others would have interest in this, but I'm designing a PCB for an easy-to-solder TH node. As presently conceived, it would have 3 surface mount components (an LED on the front and a resistor and capacitor on the back), plus a DIP atmega328p, a header to accept an inexpensive si7021 TH breakout board, an FTDI header, and an RFM69HW. The idea is that it would run at 8Mhz and be powered by two AA batteries, so it's sized to be compact but still easy to solder. The same thing could be achieved with wires and some of the other boards out there, but this might be a little tidier if a TH mote is what you want as either the starting point or the end-point.

    0_1474419735865_th_node.png
    PCB dimensions are 0.65x2.55 inches. i.e. it is narrower than a typical AA battery holder, but roughly the same length.

    The LED and resistor are optional, and you could forego the capacitor as well if you wanted a truly bare bones TH solution. However, the pads are there if you wanted to utilize them. Also, you could skip the si7021 BoB if you wanted just a generic mote.

    I had hoped that someone would make an easy to solder board like this for the RFM69HW, but I got tired of waiting and finally decided to just make my own.


  • Hero Member

    Here's the si7021 TH board I have in mind:
    alt text
    You can find them on aliexpress for less than $3.


  • Hero Member

    For those who can handle soldering an SMD atmega328p, the PCB can be reduced in size to less than 1 square inch in area. The usual recommendation is to use a reflow oven, though I've managed t do one by hand. I wouldn't call it easy though. Hence, the motivation for this more easily soldered design.

    If you're more interested in using an NRF24L01+, then there already exist boards on this forum that are optimized for thatradio module (e.g. Gert Sander's has a number of them).


  • Hero Member

    As you can see here, the smaller version--which is less easily soldered--is barely much larger than the RFM69HW module:

    0_1474436035648_small_TH_node_side2_v001.png
    0_1474436045714_small_TH_node_side1_v001.png

    If powered by 2x AA batteries, though, the small size is overkill.


  • Hero Member

    For slightly easier assembly I made it a thru-hole LED and sent it off to the FAB last night. I'll clean up the silk screen on the next iteration if this first attempt proves out.
    0_1474472740145_thruholeLED.png



  • Yesssss! A RFM "slim node". Way better than using an adapter board for the radio. Will you be sharing the designs for both the TH and SMD versions?


  • Hero Member

    @DavidZH

    Sure, if there's interest. The one caveat is that they are Diptrace files, not the more common Eagle files. Why? I found a brief tutorial on Diptrace here: https://www.youtube.com/playlist?list=PLWy-YwxbAu8EkNv6iMsfLeH6Yahcwejwx
    and that was sufficient for me to learn enough of Diptrace to actually use it. I've never done anything like this before, so that tells you how easy it is. It's liberating to quickly bang out your own PCB's, that's for sure.


  • Hero Member

    I'm also having fabbed a little FTDI adapter board:
    0_1474558869350_ftdiAdapter.png
    That's because a lot of the parts that you find on a mote, such as a power indicator LED, power and RF noise filtering capacitors, and pullup capacitor on the reset line needed for DTR, are common among nodes, and so repeating them on each node just to have them there for those occasions when you are FTDI connected to a PC seems redundant. This way you need just one adapter, and each node can be more simple. If you have more than one node (and who doesn't?), that translates into less parts and assembly time overall.

    So, you simply plug your FTDI connector on the left when you want to upload code or monitor your serial debug, and then connect your mote in on the right. The Rx and Tx lines are just passthroughs, and I did a copper ground pour on the back because it seemed fitting. I spread the parts on this board out a bit so that there'd be space for a finger to fit to hit the reset button. I was just eyeballing it, so you could probably pack them more closely if you wanted to, or even make a fully integrated FTDI board that has all this built into it. I was just aiming to make it as simple as possible without spending much time on it.


  • Hero Member

    I also just now designed this pro mini interstitial board and sent it off to be fabbed:
    0_1474666049854_interstitial.png
    Just attach an RFM69HW and the si7021 to the board, and then plug it into a pro mini. Voila! You now have a temp-RH node. You will have to remove the LED from pin 13 on the pro mini, but that's no big deal. Also, it's obviously intended for a 3.3V pro mini, not a 5V pro mini.

    I suspect that for most people, this would be the simplest to put together. The board is less than 1 square inch in area.


  • Hero Member

    BTW, you can now buy RFM69HW modules on ebay at just $2.70 each. I purchased 10x from Alice, and they arrived today. I wish LoRa modules were that cheap....


  • Hero Member

    I received a tiny test board today from the fab so I could see how well an smd atmega328p fits on the pads:
    0_1475193241277_atmega328_smd.png

    The alignment is right, but I think I can probably narrow them slightly. Is there a rule of thumb for how thin to make them, or conversely, how much spacing to put between them? My only concern is that without enough spacing between them (and I don't have any idea how much spacing is enough), there might be a risk of solder bridging. The more space, the lower the odds of that.


  • Hardware Contributor

    @NeverDie

    Check the datasheet for the atmega328p-au. It has a section on PCB layout.

    I would go for same width as the actual pins, and make the pads long enough to have space for some solder (when doing it by hand).

    On my stamp size node I also used the smd version and that layout was not difficult to solder when using a fine tip.


  • Hero Member

    @GertSanders said:

    @NeverDie

    Check the datasheet for the atmega328p-au. It has a section on PCB layout.

    Thanks! I'll give that a look. On my first attempt I just used the default pad layout in the diptrace library for an atmega168 (as it didn't explicitly have one for an atmega328), so I should probably verify that the dimensions are correct.

    I would go for same width as the actual pins, and make the pads long enough to have space for some solder (when doing it by hand).

    There might even be a case for making them a little extra long, if the board real estate allows. I believe there's a solder technique where you add solder to the pads in advance, and if they're extra long you can kinda push it onto the pins after melting it, provided you've also layered a lot of fresh flux everthing, including the pins.

    On my stamp size node I also used the smd version and that layout was not difficult to solder when using a fine tip.

    What kind and size of solder tip do you use?


  • Hardware Contributor

    @NeverDie
    0.8 mm solder by STANNOL (HS10), packs of 100gr
    The tip is around 0.8mm wide (narrowest tip I could get for my Weller station, I have an old EC2002)


  • Hero Member

    Thanks, I just ordered Hakko 0.2mm (curved) and 0.5mm (straight conical) tips from amazon. Both got good reviews. Hopefully one of them is a good match for the task, but if not, it's good to know that 0.8mm will do the business.


  • Hero Member

    I received the board from post #1 in the mail today. I'm planning to mount the atmega328p DIP chip using machine pin female headers. It certainly can do that, but I'm thinking that on the next iteratioin it might be preferable to have the entire machine pin--not just the thinner leader part--go through the PCB, because then the plastic part of the header can be flush with the PCB (as would be the case if more typical female headers were used). Anyone tried doing that? Any reason not to do that?


  • Hero Member

    So, to clarify, what I mean by a female machine pin header (also known as a Swiss machine pin header) is this:

    alt text

    Unfortunately, the only dimensioning I've been able to find is this:
    alt text
    which lacks the dimension I want to know, which is diameter of the larger pin body that's just above the 0.51mm diameter part of the leader pin that's meant to be pushed through the through-hole.

    Purely eyeballing it, it looks like it's maybe, what, twice as wide? Anyone happen to know what that dimension is?


  • Hero Member

    I've one of these at home. I could take a measurement, but maybe someone has a better idea. ☺


  • Hero Member

    1.35 Millimeters!


  • Hero Member

    @TimO
    Thanks! That exactly matches what my micrometer says also, so I'll go with that.

    So now the question is: what diameter hole should I have drilled for that? Is there a rule of thumb? e.g. Would 1.45 be too snug? Too loose?


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