Wireless trigger

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We are designing a wireless trigger system for our acquisition devices.
The purpose of the trigger system is to remotely start two independent
systems at the same time. We are envisioning the operatior starting the
two system with a "keyfob" or similar device.

The latency from when the button is pressed to the systems are
triggered should be less than 1s.
The time difference between the triggering of each system should be
known and short (~ 1ms).
The wireless triggering should work reliable at a distance of 15m
minimum and preferably up to 100m.

Does anybody know of any RF components/solutions that fulfill these
requirements?

Thank you in advance,
Thomas


Re: Wireless trigger
The first answer is yes, there are probably hundreds of these devices
out there.  [A lot depends upon the range and the SIZE needed.]  You
can buy they in sets if you want, keyfob and a little RF
receiver/decoder PCB.  The receiver has the ability to set a "code" and
if it is set manually (i.e. you set it yourself) then there is no
problem with setting two receivers to the same FOB code.

Latency of < 1 second... Piece of cake.

Simultaneous triggering < 1 ms.  That's going to be the problem.  These
things work in the unlicensed band, usually 315 MHz.  Lots of noise,
lots of multi-path reception.  When you press the button on the FOB it
just starts sending out a train of data packets, over and over.  The
idea is that eventually ONE of them will get through intact. The data
rate is slow, and there could be 50 to 100 ms or more between the start
of one packet to the start of the next.  Your problem - one receiver
will almost certainly see a valid packet that the other one missed,
causing the second one to trigger on a latter packet.  At close range
this probably won't happen (often), but at longer ranges (like 100m) it
almost certainly will, very often.

I'm not sure of a good solution to that (one that isn't too involved).
If I were to pick a starting point, I would pick a receiver that has an
RF Signal Strength output (lots do, but not all).  I would then trigger
not on the reception of a valid START packet, but rather on the loss of
signal FOLLOWING reception of a valid start packet.  That would
probably work, but you would have to run some tests to see how well it
works in your specific situation.  [In your living room, probably
great.  In a factory filled with heavy electrical machinery... probably
not so great.]

If you are looking to buy COTS, you might also look at the little rigs
that photographers use to trigger remote flash equipment.  That would
seem to be very close to your requirement (except maybe for range).  If
you are looking to design and build custom - look at the Micrel "quick
radio" chip sets.


Re: Wireless trigger

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If you send your own data over the link, you can embed an updated
"time-to-trigger" value within each packet.  Regardless which packet
is received, the designated trigger time is the same.  Since the OP
wants the trigger to occur within a second, the initial delay can be,
say, 800 ms.  This value is decremented to account for elapsed time
between each packet transmission.

Thad

Re: Wireless trigger
Damn... that's clever.  Why didn't I think of that!  [Note: rhetorical
question. <G>]


Re: Wireless trigger
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A standard thingy is the 434MHz transmitter here:

http://www.futurlec.com/Radio.shtml

You can get them elsewhere. Here is the datasheet:

http://www.laipac.com/datasheets/Easy%20Link%20Wireless/TLPRLP434A.pdf

They can be easily used with holtek encoder/decoders, like the HT12D/E
pair, and driven with microcontrollers on either side.

Better antennas give better range, but they claim 200m with no
obstructions. I suspect that is pushing it. 100m might be possible with
matched antennas.

You could build your own using the microchip rf stuff, but it'll cost
more than these, particularly when you go to get it certified by the
FCC. Microchip also sells little cards that have been pre-certified,
that use this band.

--
Regards,
  Bob Monsen

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