Cheap wireless trigger

Wireless & IoT
1

I’m looking for a way to activate a large number of small devices at the same time at a distance of ~1 mile away. Does anyone have an idea of how to do this?

The system requirements are

  • Range: at least a mile would be best (free air)

  • Bandwidth: 1 bit / infinity (ok, maybe more like 1 bit/hour to allow testing)

  • Latency: <3 seconds

  • Error rate: <1 false positive / ~50 hours run time, <1 false negative / ~400 activations

The receiver requirements (at quantity 100) are

  • Output: has to be something a microcontroller can distinguish (digital ok, medium LF analog signal ok, small HF signal not good enough, etc)

  • Power consumption: has to be reasonable (<~100mA @ 3V)

  • Cost: the receivers need to be less than ~$4, not including power supplies

The transmitter requirements are

  • Cost: can’t cost more than ~$300

It seems like this should be pretty doable but I haven’t come up with anything viable yet. The 50mW & 60mW XBee modules are fantastic (I’ve tested them to ~1.5 miles with good results), but $35 comes in a wee over budget. Off the shelf hobby RC receivers seem like a pretty good option but the best price I’ve found is $11 without crystal. The Sparkfun RF link receivers seem like a great solution at $3.95, but they say they’ll only go 500 ft (far short of the ~2500 ft desired). With a pretty heroic effort and a 3" lens, I got IR to go ~800 ft, but that’s far too short as well. The physics on an ultrasonic setup requires a pretty darn powerful transmitter so I don’t think they’re practical. It seems like cheap FM radio receivers should work great with the transmitter transmitting a low-frequency saw wave or similar and the AVR looking for consistent peak-peak spacing, but I can’t find decent, cheap FM receivers.

Any ideas on how to do this? The Sparkfun RF link seems like the most promising option if a higher-power transmitter could be bought/constructed. Anyone have any links or suggestions on that?

Thanks for your help!

–Ben

2

You aren’t exploding lots of bombs. I hope. :sunglasses:

You will need to budget lots of $$$ for certification, unless you buy hardware that is already certified. Putting that sort of system together is very different from a hobbyist playing with a couple of modules.

Leon

3

No, definitely nothing harmful :slight_smile: I don’t want to say exactly what it’s for, but the only payload for each receiver is going to be a set of LEDs.

I’m only looking to produce one of these systems and it will be for my own use so I was hoping to avoid certification. I have a small army of friends to help me assemble the 100+ receivers, but yes, anything with Q100 is quite daunting.

It’s unfortunate (but expected) that Sparkfun’s RF link receivers aren’t as good as the (more expensive) long-range Linx ones:

viewtopic.php?t=13596

…so that seems to count out using a Linx long range transmitter with cheaper receivers (makes sense; the long-range Linx transmitters are something like 10 dBm less transmit power than the Sparkfun RF links). I’m still somewhat optimistic that I might be able to buy/build a higher-power 315 Mhz transmitter to make the Sparkfun RF link receivers work at ~5000 ft; any suggestions or comments?

Also, it still seems like those cheap little FM radios should work well as receivers and it’s not too hard to buy an FM transmitter that would go a mile in free air. Any comments on the viability of that?

Thanks,

Ben