Hi electronics experts,
I want to build a circuit that can identify few (4-10) tags wirelessly by proximity. No need for multiple simultaneous detection. I don't need any memory on the tags - just a simple ID - a number from 1..10 would suffice. I have a simple micro controller on board.
The circuit should be as low cost as possible, small, and run on simple AA/AAA batteries. Standard RFID reader/tags solution is not what I need here.
I've found a similar system in a low cost plastic toy for children. Before I dismantled (and beheaded) the product it was a "barby-style" doll which identifies when you place one of its (plastic) pet animals near it and responded with a unique speech sound for each animal. The set had a total of 3 animals.
The doll contains a circuit connected to a custom wire-wound antenna, few hundreds turns I guess and air core. It got damaged unfortunately during the dismantling process. I checked the waveforms going to the antenna (no load) - attaching the osc screen captures basically a ~330us (3KHz) negative pulse every couple of ms (inbetween two 1.8ms positive pulses) The tags insides the pet dolls contain a tiny single layer PCB with just a through-hole inductor (didn't measure inductance) and two capacitors all connected in parallel. It was obvious that the coil values in each tag are different. Don't have a disponsible LC meter to check values right now.. I guess the identification is performed by reading back the effect from the variable L/C values on the tags.
The main IC on the toy was epoxy encapsulated so I can tell what it is. I understand that this circuit works by transmitting pulses periodically and reception & measurement of the response which varies by the tag inductance/capacitance. but
How does exactly does this system work?
more elaborately: Please help me understand how this system works, what and how to measure, how to build a similar one, or point me in the right direction. What is the max number of tags it can identify in this scheme, how to calculate all parameters, etc which ICs/components should I look at.
Basic controller schematic:
The main PCB and the antenna are shown here: Main PCB Main Antenna (was connected to the yellow-green wires) Measure diameter/air-core:
The tags: (top left one coil's was broken off in the process)