I'm interested in building a basic lc circuit that could ring up another circuit to identify it. Basically I want to have something like a wacom board. A sensor that is stationary and is powered by a cable that is able to tell what is brought near it by the responding frequency? Is that possible to do? And what components would I need to build such a circuit?

Just to clarify a little, lets say I have a stationary, powered coil A, and I have few other unpowered, free floating coils X, Y, Z. Can coil A, using a single frequency identify the separate free floating circuits? This is how I imagine it, coil A induces oscillations in coil X, coil X circuit uses that power to create a seperate oscillation at different wavelength unique to coil X. Station A has a separate reciver that senses the oscillation of X and identifies it as X. Is this possible? Also that's just off the top of my head, it would be much better to have this all done by single coil.

 

It's hard to tell what your ultimate goal is. It sounds like you might be interested in learning about RFID technology.

 

Well the goal is to be able to id items without having to power them or connect them by cable. The distance I'm looking for is very small too, so something like 5mm to 2cm would be just fine. I'm aware of RFID but if you're talking about commercial rfid, it's too expensive. I think what I'm asking about is the same thing except home made.

 

I think maybe people are getting thrown off by extra information in my post? The basic question is this, how do I make two nearby circuits resonate without touching? Do I just have a powered LC circuit, and then bring near it an unpowered LC circuit with same resonance and it should automatically catch the wave?

 

This is similar to an RF detection system that I was involved with some time ago. (not as the designer). They wanted to catch people stealing things, so the frequency wasn't checked. Bit basically they had a relatively large antenna that put out a signal across a 36" corridor.. Any tags in that corridor would hopefully respond when they were hit with the transmitting signal.

The tags are simple devices. A flat coil and a capacitor that resonate at the frequncty of interest. When they are hit with energy near their resonant frequncy they will ring and that signal can be detected. ( or tags were something like 2 inch squares)

But, it gets tricky. We went through a range of frequencies because the tags varied somewhat. It would have cost a bunch of money to make precision tuned tags in high volumes.

So, essentially we were looking for tags within a frequency range. To do that, the system would transmit a short burst and listen for a response.Then it would up the frequency and trpeat the process. Since we knew the transmitting frequency, it would have been possible to identify tags that were at specofoc frequencies.

But it's a fairly complex system. Pulsing the transmitter was done with a phase locked loop and some kind of switching IIRC. However, while the system was transmitting, the receiver had to be protected. This system used some special FETs to short across the receiving antenna to keep out the transmitted signal. They used multiple FETS in parallel, and if one failed, the system wouldn't work. It was hard to figure out why without some major disassembly. Possibly in a low power application, you could get by with one FET or other device. In any event, the RF design part was fairly tricky. The designer was an experienced RF engineer.

RFID is actually more straightforward from the transmitter side, and the receivers are probably pretty common these days. The tags cost more because they actually have a little IC inside that generates the signature. The transmitter is slightly off of the tag frequency and acttually serves to power the tag.

These systems are actually a type of RF transformer as opposed to normal radios.

hj

 

A number of related technologies are used to chip runners in foot races. Some use a copper wire antenna wound into a large loop around the runner's number tag bib. Others use something laced into the shoelaces. Each chip produces a unique code to identify the wearer.

 

@chv_sck
that's interesting, but you did have tracking on rather large scale, and I assume the tag in the items was kept very simple to keep the size down. I'm thinking that in my case, where I only need millimeter range and the circuitry in the item doesn't have to be flat or small, I could maybe get away with a much simpler design, like if interference is a problem I can do something like delay the responce of the reciever. Lets say I ping the coil first with a power signal, store the energy, stop the ping and then use that energy to transmit back. But I'm just guessing about this since I really don't even have a hands on on how these circuits behave, that's why I wanted to build a small mock circuit to test it first. In other words I just want to get a resonant response in an unpowered circuit and I'll go from there.

So really don't worry too much about the implementation, I just threw it in thinking people here were experienced, but if you can just help me with parts/schematics for the circuit I need it'd be great.

 

Maybe this will give you some ideas.

 

Again that article describes commerical RFID and has me buying premade components, it doesn't explain how to build a similar system. Of course I can buy pre-built tags and scanners, but I came here for help in building a resonant circuit. I guess it's some kind of esoteric subject since I can't get a straight answer for 5 days. All right, just never mind I'll go elsewhere.

 

You didn't get a straight answer because you haven't posed a straight question. We've only just now established that, while it appears RFID is probably on the right rack, you want to build your own version. OK, that's a start. From your first post it seems you don't need to distinguish lost of different IDs, maybe just a handful? How selective do you need it to be? I mean, how different are your 3 x,y,z coils? Are there size restrictions? Cost/complexity restrictions?

Do you have the skills and experience to build your RF receiver, "A".

 

Great lets start over. Yes I want to build a transmitter/reciever similar to what RFID uses but I without buying proprietary chips. I need about 40 ids. I don't need spacial position at all. Perhaps I mentioned Wacom, that was just to give you guys an idea of the circuit I'd like to build, since Wacom boards use same principles. In my case all I need is tag ID. So I just have one coil that pings the object's coil to get a response with ID if that object is in range. Also there will never be two objects in the same place at the same time. That's the gist of the project, however it's not that important right now to go into exact details. If you have some ideas I'd appreciate it but, I think right now it would be more realistic to build the basic circuit. Just one receiver and one transmitter, so I can at least get a sense of it.

 

I think the requirement for 40 IDs would rule out using 40 frequencies. It would make the receiver design difficult. I think you will find it simpler using a digital encoded responce. Ruling out the use of chips like keyfob encoder chips will also increase the amount of design work.
Les.

 

I thought maybe there was a way to to receive multiple frequencies on one coil, but if it's not possible then yes I would have to encode them. That shouldn't be a problem though, I could just use simple PWM to transmit it back. But what about the circuit that I mentioned, do you know a basic layout for that? Or a schematic?

edit: The circuit I'm referring to, if you didn't read the thread, is a simple 1:1 resonator, one LC circuit powering another. Is it as simple as just putting two LC circuits in vicinity or is there some other way.

 

I don't know what schematic you are referring to. I goggled "wacom board" and it seems to be some kind of tablet device. It would not be impossible to use 40 frequencies but it would be difficult. Using a system similar to DTMF (Dual-tone multi-frequency signaling) may make it possible. (Standard DTMF only gives 16 combinations.)

Les.