I’m looking to develop a system that will transmit the angulation of an accelerometer every 10 seconds. It must also receive this pulse of angulation data from other accelerometers in the area every 10 seconds. So each individual system would have to be always “listening”.
It’s range must be around a .5 mile in dense trees. It would be powered by consumer batteries, AAA’s etc. I’m looking for a starting point on what wireless system would be best used. I have little experience in the rf field so easy set-up would be preferable.
If you want the sensors to last any amount of time on a battery it’ll depend a lot on how you program your microcontroller. You need something designed for low power usage like a TI MSP430, or an ATMEL AVR with picopower and sleepwalking. You can save power if you use an integrated radio/microcontroller chip like this.
You get your battery pack and radio/microcontroller all in one so all you need to do is add your sensor, program it, and stick it in a weatherproof enclosure.
Aggressive power saving will be a must, and maybe some power harvesting techniques so you can drive the radios for some range through trees.
cmor:
I’m looking to develop a system that will transmit the angulation of an accelerometer every 10 seconds. It must also receive this pulse of angulation data from other accelerometers in the area every 10 seconds. So each individual system would have to be always “listening”.
It’s range must be around a .5 mile in dense trees. It would be powered by consumer batteries, AAA’s etc. I’m looking for a starting point on what wireless system would be best used. I have little experience in the rf field so easy set-up would be preferable.
Thanks for your thoughts.
Half a mile in trees - I'd use a 900MHz data radio with at least 100mW (0.1W). Digi has some. Not the 2.4GHz XBees.
If they don’t move, you can trade transmitter power for antenna gain using directional high gain antennas.
With that data rate I’d be looking at 433MHz or even VHF frequencies, as it will have much better range through trees etc. Radiometrix makes some nice modules:
stevech:
433MHz is limited to a few bytes per transmission and a very small number of transmissions per hour, by FCC regulations.
VHF MURS band (in the US) is your best bet for long range.
Otherwise, it’s 902-928MHz in the US.
Agree that VHF is best in this case, but there are exemptions in the FCC rules that allow 433MHz transmission every 10 seconds, with duty cycle < 1:30.
I haven’t seen anyone/anything doing more than wireless thermometer and garage door opener type data comms in 433MHz.
What are those 433MHz exemptions? The limitations are because few/no 433MHz devices use CSMA (listen before transmitting, to avoid interference in unlicensed bands). Or alternatively, as in high speed for the 902-928MHz band, fast frequency hopping.
stevech:
I haven’t seen anyone/anything doing more than wireless thermometer and garage door opener type data comms in 433MHz.
What are those 433MHz exemptions? The limitations are because few/no 433MHz devices use CSMA (listen before transmitting, to avoid interference in unlicensed bands). Or alternatively, as in high speed for the 902-928MHz band, fast frequency hopping.
There are plenty of 433MHz modules that will do listen-before-transmit (eg the RFM22 sold by Sparkfun).
I’m not very familiar with the rules on using the 433MHz band, and the rules vary from country to country. My understanding is that in the US, you can transmit once every 10 seconds, with duty cycle <1:30, if you keep the power below 10mW. Happy to be corrected on this though…
You are mostly right, for those Hope RF modules. I’d looked at them in the past. They do have a clear channel assessment (CCA, similar to CSMA) mechanism based on a threshold on RSSI. It looks like the attached user microprocessor is responsible for CCA immediately before attempting to transmit.
With the 1:30 duty cycle limit (transmit time) over 10 seconds, at some bit rate, we could do the math to see what capacity such a device can have and comply with the FCC.
My weather station (La Crosse) uses 433MHz to send wind speed/direction and temperature from an outdoor unit, wirelessly. It only sends every minute or so IIRC. Probably more for battery conservation than regulatory compliance. It runs on C cell batteries. Their newer 900MHz version has a faster reporting interval, in answer to users’ complaints.