Frequencies, range, raw RF, and cheap WiFi

[Richard]

I'm curious why you picked the 900MHz range over the other alternatives like 315 or 433. In theory, lower frequencies will suffer less signal loss from obstructions, but they also carry less data. I'm just starting out on this journey, so I don't know if those bands are overused.

In playing with range, did you try a much lower baud rate? If you don't need the speed, you should be able to get greater distance at lower baud rate. Again, in theory. Sometimes that just means using a lot more robust encoding scheme, which lowers the effective throughout. At least for my sensor application, 150-300bps is probably plenty.

Have you given any thought to using raw RF modules and doing the encoding and networking in your own code? Primarily to drive the cost even lower.

Conversely, have you looked at the ESP8266 WiFi modules? They are also serial-only, but use WiFi for transport. Their price tag is also in the $4 range, but I'm sure they're much more power hungry for battery-based applications. Check out esp8266.com for a starter.

Cheers,
Richard

[camblonie]

I've got some ESP's on order. I've seen a lot about hacking them. The RMF69HW's have been working great for me though.

[Admin]

Jan 5, 2015 6:54:02 GMT Richard said:

I'm curious why you picked the 900MHz range over the other alternatives like 315 or 433. ...

Cheers,
Richard

Hi Richard,
All of those are good points. It would be nice to be able to define the exact parameters for the ideal home automation transceiver. But realistically, you have to use the chips that are out there, in the form factor that you can handle, and not spend the rest of your life developing a library for the chip before being able to build anything useful . At $4 per transceiver, and with a couple of Arduino-clones built with this transceiver available, I thought this was the best option at the time.

I've read about the ESP8266 after starting my project. At the time, there wasn't much info on deep sleep and how wifi handles waking from sleep. Good sleep performance was one of my major requirements for battery powered sensors, so I went ahead with the RFM.

[Richard]

Thanks! I agree, you've got to put a stake in the ground and stop the analysis at some point. It's a constantly moving target; I'd just started down the path with 315MHz when I learned of the ESP8266 hitting the scene.

I don't consider that chip an option for my stuff because WiFi is too power hungry and heavy-duty for long-life battery applications, and this chip is too limited for more advanced uses. But a $4 WiFi module has a lot of appeal for many applications. One of the more interesting things is that it has scripting and general-purpose I/Os, so no micro controller is needed for very simple sensor applications.

To your point, while I'd like to nerd out and code in assembler on raw chips and do my own radio encoding, I've opted to use Arduino for its speed of development. I suppose I should do similar with the radio module to get started.

Shifting back to finer points of your project, did you specifically select the frequency and baud rate based on your applications needs, or was it more a case of "this works well enough, let's move on"? Just wondering if there are factors I should consider that I may have overlooked.

Cheers,
Richard

[lawrencium265]

I'm going to be trying out a different module, the nrf905. They are about the same price on ebay as the rfm69HW. They do use slightly less power and have greater bandwidth than the rfm69HW, they are on the same frequency, but with less range (~150m instead of ~400m.I'm also going to try to use it directly with the pi instead of through a gateway, there is documentation of folks using this chip with both the arduino and the pi. Another benefit is that the modules I ordered already have pins attached so no soldering to the chip if you don't want to, also The main reason I'm doing this? I accidentally ordered 3 of them instead of the rfm69HW, so I might as well try to make them work.
really if you are not concerned with power consumption (where you have access to a wall outlet)or range (everything is within the confines of a normal sized home) it seems like going with an nrf24l01 is the most cost effective approach at about $.91 each, coupled with a nano you could make each sensor node for ~$3.40 + the cost of sensors, wiring, connectors etc.

[Richard]

Lawrencium,

Thanks for the pointer to the nRF905/nRF24L01. At a glance, they look interesting; it looks like they might support frequency-hopping / spread-spectrum even in the 905, which would be different. I don't see mention of crypto though, which is important for securing links.

However, I didn't see price points around $0.91 - rather, closer to $4-5. Where are you seeing price points that low? I've so far only seen pricing that low for very simple OOK transmitters, not transceivers (especially with any kind of added functionality). (I do see that pricing for the 24L01, but not for the 905 - in fact, I just bought a 10-pack of nRF24L01 on eBay for $5 to dabble with.)

Cheers,
Richard

[Admin]

Jan 6, 2015 15:20:47 GMT Richard said:

Thanks! I agree, you've got to put a stake in the ground and stop the analysis at some point. It's a constantly moving target; I'd just started down the path with 315MHz when I learned of the ESP8266 hitting the scene.

I don't consider that chip an option for my stuff because WiFi is too power hungry and heavy-duty for long-life battery applications, and this chip is too limited for more advanced uses. But a $4 WiFi module has a lot of appeal for many applications. One of the more interesting things is that it has scripting and general-purpose I/Os, so no micro controller is needed for very simple sensor applications.

To your point, while I'd like to nerd out and code in assembler on raw chips and do my own radio encoding, I've opted to use Arduino for its speed of development. I suppose I should do similar with the radio module to get started.

Shifting back to finer points of your project, did you specifically select the frequency and baud rate based on your applications needs, or was it more a case of "this works well enough, let's move on"? Just wondering if there are factors I should consider that I may have overlooked.

Cheers,
Richard

ESP8266 is pretty interesting. I read about a working MQTT library for it. It's tricky though - once you use wifi, you start having to deal with the overhead of wifi and making it work stably. It'd be an interesting transceiver to use for powered nodes, but it might be a few more months until the wifi library gets easier to use and more stable.

I haven't seen any ~300MHz receivers with the same set of benefits as the RFM69HW. I also did not fine tune the baud rate on the RFM69 I'm using. It's just default - no doubt it can go farther if the baud rate is lowered, but the default was more than adequate for home automation.

[Richard]

ESP8266 does look interesting, but at the same time limited. It's a serial-only interface, though it does have some ability to run Lua scripts (but why Lua??). Its big benefits are price and WiFi connectivity method - I think it might be a fit for machine-to-machine communications in line-powered applications; WiFi is too power-hungry for low-power use and the 8266 is too feature-limited for more robust applications. But it probably has a niche.

I've heard that at least one group is trying to port an open OS onto it to gain more control and utility. That will make things more interesting.

I agree on the 315MHz - I'd forgotten there's not an RFM69 module for it. I'd started down a path of bare transmitters and receivers, where this frequency is readily available. But to the choice of 900MHz vs. 433MHz - was there any decisioning there, or did 900MHz just work well enough for your needs? I'm wondering if there's a benefit to it that I need to explore. (Generally higher frequency = shorter range but higher data rate. For things like temp sensors, an extremely low bandwidth is fine.)

FYI, I've since been spending some time at www.LowPowerLab.com, where the topics are very much along these lines of interest. Felix is running a small shop selling related goods and a small-scale Arduino platform that he's also coded an over-the-air reprogramming capability for. Pretty slick.

Long-term, I have a crazy interest in using bare radios and coding the transceiver functions in software, along with the crypto. No small task, so I'll be using RFM69 for the immediate.


Cheers,
Richard