Hello all,
I have several Arduinos that I would like to get talking to each other and to a Raspberry Pi. They would be 100ft apart at a maximum (outdoors). What would be the best cost-effective solution for this? Sorry this post is a bit vague, I can provide more details if necessary.
An inexpensive serial link can be made with some 315 or 434MHz transmitters and receivers.
RF Link Transmitter - 434MHz
If you want something a bit more robust and that would work in both directions, you might have a look at the RFM69.
SparkFun RFM69 Breakout (434MHz)
There’s hookup guides for both to get you started. 
Another question in the same spirit of “best radio for xxx”.
Given similar requirements of 100’ range, a few dozen bytes per transmission - one way data only at a slow data rate: What would be the best receiver for a battery operated display where access to batteries is limited to about once per year?
In the quest for long battery life I have used a naked ATmega328P with most of Nick Gammon’s power saving tricks, an ePaper display (no power needed except for updates), an nRF24L01+ receiver and a SparkFun Power Timer TPL5110 which activates the receiver and 328P and display at roughly 7 minute intervals. The heart of my question relates to the 14 (or so) mA that the nRF24 receiver needs. I see on some Chinese websites a number of 433 MHz receivers that claim to require less than 5 mA (even as low as 2 mA). Is there anything that comes close to this power consumption in SparkFun’s bag of tricks? Or should I sacrifice my Monday morning’s coffee budget and spring for something from China, possibly reinventing the wheel in the process?
The big picture: Trying to build a temperature display for outdoor temps with no power connections - battery only, and no more than one recharge per year. The transmitter/temp sensor is the easy part. Powers up at 30 second intervals, sends data and shuts off after about 1 second. The receiver power up at 7 minute intervals to save power with an average wait-for-data of about 15 seconds. I have the gut feeling that I should be able to cut that 7 minutes down to 1 minute and still be able to achieve a once-per-year recharge of the 18650 batteries. With a little help from my friends here, of course. Thanks!
Don’t know that a 18650 will last a whole year on it’s own without a recharge, lithium ion batteries do self discharge just sitting around.
That being said, as long as your micro is drawing very little power when asleep, you could just switch off your transmitter when it’s not in use. A mosfet could do that and then your radio doesn’t use any power at all while it’s not in use.
You might have to experiment to find the best ratio of on too off. Like program some settings and then monitor how many mAh the circuit uses in one week then extrapolate that out too a whole year and see what your power budget is. Tinker for the lowest number of mAh per week and then see if you have enough in your battery to last a whole year. You should probably count on only using 1/4 to 1/3 of your batteries capacity to account for self discharge and the effects of temperature on the battery.
A couple of points: The transmitter is indeed off between transmissions, using a SparkFun TPL5110 (which draws nano-watts while off). The transmitter power budget is fine for a year, no problems. It is the receiver that must be adjusted for a reasonable ratio of OFF-ON because it is not synced to the transmitter. I settled on 30 seconds ON (worst case) to 450 seconds OFF. The battery supply for the receiver is a pair of 18650 - 3000 Ah each. They run at room temperature and have trivial self-discharge over 1 year.
It is the receiver that I would like to optimize. Currently drawing 14 mA for 30 seconds, mostly doing nothing until a signal is received. A received signal - Wait(*msg) - triggers an update to the display, saves data in long term memory and triggers its own TPL5110 to OFF (for 7+ minutes). The batteries are big and ugly and are on the border of not lasting a year per power calculations and I would like to use smaller batteries if possible.
Receivers that use less power appear to be available but I have not seen them at SparkFun. Am I just looking in the wrong places? Does anyone have strong recommendations for Ultra Low Power receivers? (Wake-On-Receiver seems to exist only on paper.) Thanks!