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I'm trying to do high value asset monitoring within a panel van/truck with RFID tags. So this would be a case of a 3m+ range with metal everywhere - mowers, digging equipment, jackhammers etc. A local antenna would interrogate all the tags within range and then relay the information via 3G or WiFi.

Olin Lathrop answered another thread saying passive RFID is impractical for purposes like this.

Active RFID requires batteries however. I feel a practical battery replacement schedule would be over 2 years. For a requirement like this - relatively low range, low scan speed (ie the equipment in the van is there for several hours at a stretch, it's not just passing by), and maximum battery life:

  • Is there a preferred frequency (VHF vs UHF etc). Does higher frequency mean lower battery life?
  • Is there good transmit interval time or is it the case that the longer you leave it between transmits, the longer the battery life? Is this a problem with many tags in the one location? ie: something has to sort between all the RF collisions.
  • Would receiving as well as transmitting (so, waiting for a valid RF signature) badly affect the battery life? Wouldn't this be more secure?

(Edited to ask more specific questions)

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    \$\begingroup\$ There are similar things using bluetooth available, e.g: techcrunch.com/2013/06/20/tile Otherwise I'd look at the Nordic Semiconductor devices. I can see a couple of 434mhz commercial solutions already: findtheneedle.co.uk/companies/jpa-electronics/products/… \$\endgroup\$
    – pjc50
    Jan 2 '14 at 16:30
  • \$\begingroup\$ That's quite interesting, thank you. I think the mistake I was making was searching on UHF (868MHz) - not sure I remember why now! My main issue is battery life - I'd really prefer something in the 3yr+ range if I can. Cheers \$\endgroup\$
    – carveone
    Jan 2 '14 at 16:41
  • \$\begingroup\$ Note that if you get something that works on coin cells for a few months, you might be able to swap in a couple of long life D cells and get extra battery life at the cost of size. \$\endgroup\$
    – pjc50
    Jan 2 '14 at 17:21
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I once designed a small transmitter that woke up every minute or so to measure temperature and transmit the value along with an ID. These were used in a room which had a stack of freezers and if one of the received transmissions indicated the temperature was too high an alarm sounded. Collisions of data was expected but each transmission only laster 40 milli seconds every minute or so if you had 100 of these all randomly transmitting, the total transmission time of all one-hundred was less than 10% of one minute. There was also a random factor built in based on ID.

Battery life was paramount on this but 1 year was got (from memory) from a PP3 style 9V battery. The engineering to get more battery life is keep the electronics powered off as long as possible, ensure the quiescent current (when off) is as small as possible and keep the transmission as short as possible. I used 433MHz off-ther-shelf transmitter modules and one off-the-shelf receiver acting as the central collection point for data.

Incorporating a receiver is not a good idea i.e. transmit on request would need a receiver that takes up valuable real-estate and consumes power almost constantly.

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  • \$\begingroup\$ I think this is a valid way to proceed. I have a few nRF24l01 modules handy so I'll have a go and see how low I can get the battery drain. \$\endgroup\$
    – carveone
    Jan 6 '14 at 17:29

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