I'd like to send a simple trigger pulse by RF at irregular intervals (seconds to minutes). No other data need be sent. No special requirement on the length or type of trigger signal. All that's needed is something like the momentary pressing of a light switch. The triggered device will be locally reset in between each triggered event.
The transmission distance is roughly half a kilometer in unobstructed line-of-sight. I'm thinking of using some readymade transceiver modules, perhaps in the 2.4GHz band, so the RF portions are not really part of my question. My main concerns are -
Delay between transmission and trigger output from the receiver should be less than 1 msec
Avoiding false triggering by RF interference sources like vehicles, cellphones and other spurious sources.
What approach do you suggest to satisfy the above requirements? For example, will a tone burst transmission and a tone decoder at the Rx be sufficient? Thanks in advance.
If you have a direct line of sight, why not try for infrared? The radioband is extremely polluted, and unless you protect your connection with encoded fingerprinting, like they do with a garage door opener, you will get very frequent unwanted triggering. I would try a bunch of IR diodes, modulated with say 1MHZ, and a receiving diode connected to the antenna input of a cheap AM radiochip. There are cheap ones on the market, to cannibalize.
Infrared is far harder due to the solar background. You can do it at night, but in the daytime, 500 metres is hard. Also out in Pimpom's neighbourhood the airwaves are probably a lot clearer than where you are!
With a nice narrow bandwidth signal like that, Pimpom could use the AM band with a multi-turn loop antenna on each end.
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
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845-480-2058
email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net
Thanks for the reply. But IR at 500m? Is it feasible?
That's my biggest concern. I'll go for encoding as a last resort, but would like to avoid it if a reasonably simple alternative is available.
Can you point me to an example of practical applications of this technique at comparable distances? If this were for a fixed installation, I'd go for a cable system - copper or fiber optic. But the application is to be used only a few times a year, transported outside town, set up and dismantled after each use.
I'd think so too. I've used IR with standard remote control receivers at much shorter ranges (up to ~15 metres in direct sunlight), but what Sjouke proposes is an entirely different matter.
Probably! But I wouldn't want to depend too much on that.
To use the AM band with that range, I'll probably have to break a few laws. Not that there's much danger of anyone in authority noticing it. But I was thinking of using an approved commercial module, beamed with yagi antennas at both ends. I've made and used yagis for other purposes at VHF, but not at the higher UHF freqs.
100 mW in the AM band is legal most places, I think, and that would work with a narrow enough bandwidth--e.g. a 1 MHz crystal osc at one end and a matching crystal at the other. (You'd have to make sure the load capacitance was the same at both ends too, of course.)
Depending on how fancy you want to get, there are more commercial approaches...a wifi bridge and a couple of Yagis would get you data communications end to end.
The nice thing about low frequencies is that you get good reception with simpler antennas--the higher you go, the more antenna gain you have to use to keep the same intercepted area (i.e. the same RX power).
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net
I certainly haven't tried it, but I can suggest some thoughts to consider about the idea. The sun travels across the sky and you probably cannot select your direction from xmtr to rcvr. So there is probably some "worst case" situation with the sun in the sky that you can't do a lot about. But you can do some things, I think.
One of them is to figure out ways to limit the bandwidth in optical wavelength and modulation frequency and limit the field of view of the receiver, since you have line of sight. Your source might be broadband (xenon strobe?) but you can limit the received optical wavelength to some very narrow band using optical filters. This will exclude a lot of light from the sun. What you will then need to do is somehow make the transmitter emit enough against that background to at least make a PLL function, which can certainly "find lock" at night time and perhaps hold it during the day. Which is the next thing -- using a precision modulation frequency on the transmitter ... as precise as possible ... and use as narrow as possible acceptance filtering on the receiver. The sun's energy will be spread out (perhaps there are some low energy frequencies at whatever wavelength you limit yourself to) and the narrower you can make this the less energy from the sun you need to deal with during the day. And of course there are optical means (long tube, telescope, baffling, etc) that you can do with the receiver, as well. These can be more restrictive in one plane than another (vertical vs horizontal, for example.)
Perhaps a combination of these could get you there?
Then there may be some problem with human dangers from your emitters. I don't know.
Just thinking out loud about this:
(1) Laser emitters, because of their narrower bandwidth (and especially gas lasers instead of diode versions), would be preferred because they can benefit better from thin film narrow band optical filters as well as provide much better point-to-point directivity (lower divergence) and in the end reject more of the background illumination from the sun. (You don't need the higher modulation bandwidth, necessarily, but lasers will probably allow you more options there, too.) In short, I'd probably first look into the selection of appropriate laser diode emitters (cheaper than gas), but wavelength may be a factor in making choices.
(2) Weather, rain, fog, smog, some wild animal standing in the beam for all I know, may affect things aversely. I've no idea here.. but you need to think about it.
(3) Collimated emitter is wise together with the use of a very narrow field of view for the receiver. (Keep in mind reflections that bang around and wise use of baffling and absorptive coatings on the baffling parts -- at whatever wavelength you choose to rely upon.)
(4) The sun path, given your two locations, may inevitably overwhelm whatever you do with narrow receiver FOV, etc. So you may not be able to avoid disruption at certain times.
(5) Avalanche detectors are more sensitive than PIN, I gather, so that may help with a transimpedance amp over distance. But solar radiation may saturate the system, so this brings back in making an amplifier front end that attenuates DC signal (and anything 'out of band') as much as possible. I've read that InGaAs APDs operating at 1.55 micron might help, versus those operating in the usual 780 to
850 nm, which are probably more influenced by solar illumination when taking into account variations in the background levels. I don't know, though.
(6) Modulation and limiting acceptance at the receiver is likely very important to achieve final success and lock out still more of the solar illumination.
I've also probably failed to think of something important.
All this seems to suggest RF. Too many different facets to worry about and get right with IR. But you did say "line of sight" so maybe it is doable??
Oh, well. Interesting to at least consider for a moment.
When you can use a fixed path, you can shield the receiving diode with say a 1 yard 2inch pipe, the inside painted with black non-glossy paint. That takes care of most of the daylight. Modulating the transmitted beam takes care of the rest of the static light, because of ac amplification. We did something like that years ago, with transmittor consisting of a 100 diode square. Today I would use a laser diode, they are quite powerfull these days. For a range measurement I used a 80 amp IR diode with a pulse length of 4 Nanosec, 10 times/sec. That was enough for a distance of ~40 meters, with passive reflection, so reduced by the fourth power of the distance. Such a diode, at a lower current and 50% dutycycle, 1Mhz, should have no problem with the distance, but you need a colimator? lense. The receiving diode would need to be a wide area optical diode. Both diodes are EXPENSIVE. This is from memory, I dont work there anymore, for more info try google.
There will definitely be LoS, with flat open ground along the Tx-Rx path and trees and power lines some tens of metres running parallel on each side. Transceiver modules intended for serial data transmission with stated ranges of 1-3 km in optimal conditions are not all that expensive even here in India.
My main concern is with avoiding false triggering due to interference, and this is where I'm seeking opinions and suggestions from those with experience in this area.
you need a coded signal because just the carrier alone may give you false triggers.
One could use a date carrier Transceiver and encode a serial stream to be read and decoded. If CRC passes along with Key code, then it can send it's pulse to the device.. The system should send a ACK back to the transmitter to validate that it did receive it.. Otherwise, you may want to try again, if it applies.. This could be done at high speed but there will be a delay and this has to be factored in..
Sending a continous carrier with a low freq PLL signal on the carrier most likely will work better. This way the receiver will be desensed due to your close TX unit and constantly in lock with the PLL freq on the carrier, a slight shift on this tone can generate a Phase shift at the RX end very quickly and the phase detect can be used to generate the pulse.
Jamie
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You can do one-way reliable signaling, just make sure there's lots of redundancy sent, and that the receiver ignores event duplication. I spent a decade doing that sort of stuff for security systems where false alarms are as bad as not receiving an alarm. It's quite easy to implement the concept with today's toys.
Transmit time-'til-event. Receiver loads time-'til-event into a counter on each receipt. Make timer and transmission as accurate as needed. So what if a packet gets dropped?
Part of a system involving a timer. Max resolution 1ms. START signal local to the RTC, STOP signal by the remote trigger. (Just have time to dash that one off. Lots of social obligations. Will reply to the other posts later. Sorry).
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