Need a low latency wireless transceiver. - Page 1
Forum section: electronics / beginners

author: vikisonline
author_rank: Contributor
timestamp: May 18, 2013, 07:34:30 pm
content: I am working on a project that needs a very low latency signal to be transmitted wirelessly.I asked on other forums, I know I can buy these, but I want to make one.One example usage would be:A wireless flash controller that takes a signal from a cameras hotshoe adapter, sends a wireless signal toa flash device, or something that drops, shoots an object.Timing of course is critical. So the requirements:-A signal must be fully sent and received withing 1/500 of a second at least, faster is better of course-Have at least a 50-100m range-One master must be able to individually talk to on of at least 15 units and broadcast a message to all (at least 15) units at once one data packet-should be small, relatively low power (batteries) and relatively cheap.So far I have looked at:-wifi: possible collisions and congestion issues, tcp packet processing can be intensive for slow microcontrollers, etc etc.-bluetooth: constantly frequency hopping so not sure if the latency would be reliable-zigbee: looks promising but want to know if there is anything better/best, and not sure if its capable of broadcast messages.-nRF24L01P: also nice, cheap ($2.5 from china) but can at most have 6 slaves for one master.Between price size and latency, latency and size is the most important.And of course I should be able to control this with an avr/pi/etc... I can write the libraries myself, that in itself is not an issue.Thanks

author: Simon
author_rank: Unknown
timestamp: May 18, 2013, 07:38:47 pm
content: you might want to give more details like what is the use so people can better help

author: vikisonline
author_rank: Contributor
timestamp: May 18, 2013, 07:40:39 pm
content: One use case:-arm processor receives an interrupt for a camera hotshoe. Sends wireless signal-slave receives wireless signal. Triggers flash unit x number of times. has to happen all within an exposure.Generally under 1/500 of a second.

author: Simon
author_rank: Unknown
timestamp: May 18, 2013, 07:44:47 pm
content: sounds like you need to have a 4 bit address system that gives you 16 addresses so you can use the 16th as a universal address for all 15 units, or do that same with more bits if you want over 15 make 0000(0) your universal address, then you need a few bits of data to tell the receivers what to do. you will need a frequency not used by anything else or with a way of distinguishing itself.

author: vikisonline
author_rank: Contributor
timestamp: May 18, 2013, 07:51:41 pm
content: Any wireless modules you could recommend?

author: Simon
author_rank: Unknown
timestamp: May 18, 2013, 07:54:57 pm
content: No idea, I don't do wireless

author: Rasz
author_rank: Super Contributor
timestamp: May 18, 2013, 08:27:04 pm
content: Quote from: vikisonline on May 18, 2013, 07:34:30 pmI suspect flash units wont have anything to say back to the master, so you dont really need slaves, just listening radiosNext Hope Badges were based on this chip and Travis Goodspeed was able to sniff arbitrary traffic with them = you dont need any pairing. You can just send with master and sniff with slaves for "magic packet"

author: vikisonline
author_rank: Contributor
timestamp: May 18, 2013, 08:50:54 pm
content: Hmm that might just workThanks. I'll look into that further.Out of everything this chipset has the lowest latency I have seen so far. With the right config you can get down to something around 250 micro-seconds for an 8bit message. If they don't actually need to pair and I can just sniff that is great. I wanted them to be able to report back, maybe for collecting battery stats, but like I said thats not important.And an other interesting project to look at

author: G7PSK
author_rank: Super Contributor
timestamp: May 18, 2013, 09:23:35 pm
content: All the slave flash guns I have seen have been light operated, the master flash is fired and all the slave units have a photo cell that triggers the unit, works in all the instances I have seen.

author: codeboy2k
author_rank: Super Contributor
timestamp: May 19, 2013, 12:40:27 am
content: light works well as a slave flash.. but I think there are problems with it when there are more than 1 slave, and you have multiple slaves, I don't know if they all flash at the same time.here's a table from a PhD students dissertation thesis, he studies clock synchronization in networks of wireless embedded devices.here's the full paper. Actually a good read.http://e-collection.library.ethz.ch/eserv/eth:4579/eth-4579-02.pdfIn that paper he tests 4 different wireless chipsets. It turns out the TelosB platform has the least latency, and that platform uses the msp430F1611, the TI CC2420 radio, and TinyOS.

author: amyk
author_rank: Super Contributor
timestamp: May 19, 2013, 10:50:32 am
content: I don't know where you are but if all you want is on/off type signaling then 433MHz transceivers with an RF remote control encoder/decoder will probably give the lowest latency:http://www.laipac.com/easy_434a_eng.htmIf this band is legal to use in your area it might be less crowded/susceptible to interference than 2.4GHz

author: vikisonline
author_rank: Contributor
timestamp: May 26, 2013, 06:04:19 am
content: Thanks.Didn't know you could simply sniff like that with the nrf. The latencies are not great but should workThat research paper looks like an awesome read. I have a thing for well written papers. :-D