# Use of RFID or suitable alternative to remotely communicate change in system state

I have a physical system which is dispersed over about 20-30 metres cubed and at random points within this space there are electronic nodes which have single binary outputs.

I want to monitor every time any of these outputs goes high from a central location accepting occational losses. The essential criteria in the solution is that each node communicates wirelessly and utilises the minimum amount of power possible.

My question:

• 1) Is there a way to leverage (hack) RFID technology for this purpose, perhaps powering an active RFID tag for a transmission and reading this transmission from the central location?
• 2) If this is not possible, what would be the most common solution in such a scenario where power is highly constrained and fault tolerance (acknowledgement / confirmation) is important but not paramount? Would Bluetooth LE be my best bet or a different solution?

## 1 Answer

You can get small FM or AM transmitters that can transmit a message when the output changes state. These can transmit to a central receiver and if the number of transmitters is small and the occurrences of the signals changing state is low you can probably "collect" this information but with the occasional data collision. A more sophisticated regime can be engineered where each transmitter transmits on a certain time frame and therefore collisions (if they happen) will not happen on the next time frame.

You would need a small MCU with each transmitter to encode an address and set up a transmit preamble and possibly append a CRC byte.

This sort of system works an I developed one for a bunch of food freezers in a warehouse. If the freezer got a little too warm it would transmit that fact to a central receiver and this would alert staff to take some corrective action.

Uing RFID is not going to work because the distance the tag has to be from the "reader" is quite small i.e. a few centimetres.

• Thanks for that. I'll look into setting up an MCU. As far as the RFID, don't active RFID tags have a much larger range? Passive tags are a few centimeters but active ones can reach much further? – Ian Belcher May 7 '15 at 9:27
• How will the active tag know when to wake up? At the end of the day you can buy a transmit module for about $4 and a micro for about$1. You are then in control of the design rather than trying to hack something you are not in control of. – Andy aka May 7 '15 at 9:35