Hello Everyone!
I’ve gotten the designs of my three next boards to a point I’m comfortable talking about them!
The boards:
- Hand-held Transmitter
- Battery Powered Receiver Light
- Battery Powered Dimmable Light
I’ve built some prototypes based on the Feather 32u4 RFM69 (915MHz version) and they are working well for the most part. I need to fix up some programming bugs and track down a potential hardware failure that I caused. But I’m comfortable with moving forward to custom PCBs. As a side note, I’m only building a few of these each (5 in the first run) but I’m trying to keep some ideas of manufacturability in mind.
My questions:
- With a target range of 100-200 meters through moderate forest, does this unit have enough transmit/receive power/sensitivity with this spring antenna link
- Is the RF layout good at all? Or am I missing something
- Has anyone used a MIC1557 timer
- Besides a uCurrent or Joulescope, do I have any way/hope of measuring sleep/idle currents?
Range Calculations:
I have done some thinking/used a few link loss calculators and from my guesses about certain things, I seem to have more than enough power to reach 200m. The RFM69 has a max transmit power to +20dBm, a receive sensitivity of at least -100dBm, and the antenna has a gain of +2.15dBi with a Vswr of ~1.5.
RF PCB Design:
I tried implementing what I know about RF design, which isn’t a ton, but this is where I got to. I tried to keep the track from the RFM69 to the antenna connection point as short as possible. I also used the KiCAD PCB calculator to try to match the track’s impedance as close to 50 Ohms as possible (ended around 51-52 Ohms, which I am comfortable with).
I removed the copper ground planes from underneath most of the antenna, although I’m not sure if that is what I should do. I’m also not sure if I need to add via stitching around both the trace going to the antenna and the ground exclusion zone.
As a side note, the antenna I would like to use is the spring antenna, but I’m keeping a u.FL connector available just in case I do need to use a different antenna. In that case, I’m not entirely sure which one I would use or where I would end up mounting it, but one problem at a time.
In both photos, U401 is the RFM69 module.
MIC1557 Timer:
The MIC1557 seems like a nice timer chip, I want to use it to create an adjustable PWM signal via a potentiometer. So far, I’ve set it up like below, which is based on the datasheet and the similar circuit when using a 555. I could just use an ATtiny with a small amount of code, but I like the idea of doing it in simple hardware only. And I can’t use an NE555 due to the temp range that I would like to hit (ideally -40 to +85 C).
I’m open to doing it with another PWM chip if anyone knows of anything that is similar, and is in stock.
Also curious on thoughts around if I’m over-complicating this one at all. I’m starting to think I might have.
Thanks in advanced, this was a long one.