Transponder circuit

[disorder]

A friend wants to go as a certain villain from a movie who uses a transponder to track down a briefcase of money. I'm wondering if this is something I could build. The transponder is very "movie like" in that it does the whole beeping in frequency in relation to it's proximity to the transmitter/briefcase. I've found a number of circuits online that transmit out beeping tones on RF, and most of these circuits are very low current and small in size. This is a great start, however, I need to achieve that proximity feature. I think it would look something like a signal strength detector which then feeds a circuit that does the strength--->beeping frequency conversion. I guess my first question is, is there a name for this kind of circuit? And second, is this feasible as a DIY weekend project?

[Kipper4]

You could always use (LDR) light dependant resistors to achieve the proximity thing.
build a signal generator and make the freqauncy pot in your chosen design an LDR so when ever you block out or show more light to the ldr the freqauncy changes.

[disorder]

That would be pretty cool but ideally this will use RF as it seems like building the RF transmitter is fairly straight forward. It's just a matter of building the signal strength detector portion of the circuit.

[thermionix]

I love that movie!  Easily one of the best of the last couple decades.

If you are located in the US, there are buch of legal/govt issues you may run into if you build a device that transmits RF.  If you're only wanting to fake the receiver, I'd recommend somehow simulating the proximity effect, if that's even necessary.

Is this for a halloween costume?  Seems having the chirp and blinking red light would be good enough, even if it doesn't change frequency.

[disorder]

Yes it's for a halloween costume, when my friend grows his hair out it looks just like Anton's in the movie  . My initial plan was to have the transmitter be exceptionally weak, such that during halloween my friend can simply move the transponder closer to his briefcase and have the beeping frequency increase. You would have to be within 12" or so for it to start beeping. Something like those portable fm transmitters people use in their cars, they can't be outputting that much RF power.

If this ends up being too complicated (or illegal) I was planning on having a knob on the transponder simply adjust the frequency beeps so he could turn the knob when demonstrating it to simulate it getting closer to the receiver.

[thermionix]

But what about the suppressed shotgun?  Because that's a whole different set of laws and regulations you'd have to deal with. 

[disorder]

I've never heard a suppressed shotgun in real life but I really hope it sounds like it does in that movie. Sounds so bad ass when he shoots Woody's character at close range.

Found a decent picture of the transponder...

[Kipper4]

I concede not having seen the movie, I think I misunderstood your needs.
This kinda thing might be right up Dead Astronaughts street though.
Houston we have no problem........

[Transmogrifox]

At a distance of 12" and with something the size of a briefcase you might be able to get it to function with frequencies on the order of 100 kHz and simplify the design to more simple layout and construction constraints.  If battery operated then you also avoid some issues with the FCC since you don't have to worry about conducted emissions.  I'm not sure whether there are any exceptions for experimental equipment but certainly prototypes are ok...you just can't go into production and start selling them without demonstrating it meets emissions requirements.

The idea is to make the transponder to look like one plate of a capacitor and the briefcase has the other plate of a capacitor (copper plane on copper-clad board).  The human body serves as the ground reference.

The receiver is just an amplifier taking its input from a "capacitor" (a copper plate).  You can rectify that signal and filter it into DC.  Play with the sensitivity (amplifier gain) to get a voltage output range you can use.

This is a pretty generalized answer, but looking at Theremin circuits might give you some inspiration.

In stompbox terms the receiver is just an envelope detector that works at 100 kHz.

[GibsonGM]

Why not have a pot type thing (spring-loaded lever?) on the handle of the briefcase, and the 'wearer' can slowly change its resistance as the box is brought closer? 

Pot would be set up to change (increase) the frequency of the beeps that are being sent out by the briefcase....so really, all the 'detector' is doing is picking up the beeps that are changing in speed....That would be pretty simple, and no need to reverse engineer a theremin!

[Quackzed]

I'd think part of the fun would be making it beep faster at whatever he wanted it to...
why not just forget the briefcase side, no rf (yay) , and make a pot controlled beeping machine 'transponder' normal lowest pot is slow beeps or none, and anytime he wants he can covertly turn up the pot slowly for faster and faster beeps till 'the thing' is here, fastest beeps.
he could still use it on the briefcase, or anything else as well... bigger fun factor. and much simpler. 

[PRR]

> Why not have a pot ... slowly change its resistance as the box is brought closer?

Agreed. It's a frikkin movie! They didn't hide the RF in the mcguffin on the set and let the briefcase find it. OK, they did all that beepery weeks later, you need it "live". Any kinda pot that's not too obvious. Even LED and photo-resistor with a finger more-or-less between; this leads to stupid-simple '555 beepers.

True "signal strength" of RF in a room is very wonky, as the RF bounces to form boosts and dead-spots unrelated to raw distance. Get a very good (dBm) WiFi indicator and walk around your house. I have seen huge peaks/nulls just sliding the netbook 2 feet over, on a table 20 feet from the WiFi hub.

Ah, I hope part of the game is NOT that Ann hides the mcguffin and Bob has to find it. That's too tough unless Carl sees Ann hide and wink-clues Bob. If you must find without hints, you use AM band transmitter (readily available pre-built and nominally legal) and use a *loopstick* AM pocket radio to seek, not by raw strength but by rotating the loop and triangulating two probable locations (one hopefully outside the party room so the other is obviously it).

I just trashed an old Surveying book from when RF (or laser?) distance systems were new. To get sub-feet they have to run many MHz. But then you get the "same" reading every 2 feet. So they start with super low, 25KHz, long-waves to get "1,??? feet", switch to 50KHz to refine to "1,4?? feet", and so on down to small waves (and phase detection) for foot resolution. Far too complex for a party prop.

[PRR]

> prototypes are ok...you just can't go into production and start selling them without demonstrating it meets emissions requirements.

If you interfere, and are noticed, it is your fault and fine.

A one-night stand may not be found before the beer runs out.

If you are going to continue experiments, there are forms for experimental transmissions. This puts you on record for a certain area and frequency, so if someone is bothered you can quickly be found and told to stop.

Most party-size transmitters SHOULD be hardly any bigger (in range) than the size of a house. If everybody in the house is cool, OK. If your landlord lives downstairs and you wipe-out his over-the-air reception of the big ball game, you could be out on the street.

[nickbungus]

Seems like a very interesting project. 

I've been working on a sonic transducer.  Its a kind of audio-vibratory-physio-molecular transport device.  A device which is capable of breaking down solid matter and projecting it through space...and who knows, perhaps even time itself!

Sorry, just while we're talking movies.

[GibsonGM]

I'd think part of the fun would be making it beep faster at whatever he wanted it to...
why not just forget the briefcase side, no rf (yay) , and make a pot controlled beeping machine 'transponder' normal lowest pot is slow beeps or none, and anytime he wants he can covertly turn up the pot slowly for faster and faster beeps till 'the thing' is here, fastest beeps.
he could still use it on the briefcase, or anything else as well... bigger fun factor. and much simpler. 

That is what I said...this is how MythBusters might do it, in a sleight of hand way. You THINK the 'locater box' is WORKING, but it's only a radio receiver picking up a (faster and faster) tone whose speed is controlled by the TRANSMITTER.

[amptramp]

You always have the example of sonar tracking a torpedo where the transmitted pulse comes after the reflected signal so the pulse repetition rate gets higher as the torpedo gets closer.  If you go with RF, your local conspiracy theorist may have just the design you need:

[duck_arse]

doesn't anyone play with ultrasonic transducers any more? they used to be available at 25k and 40kHz, if I recall right. with an envelope detector/signal strength thingy, might werk.

maybe find one of those microwave doppler units they used to use as movement detectors in alarum installs. split the horn - rx here, tx there, bosh.

[anotherjim]

I was thinking ultrasonic too -  gets you out of worrying about RF. You can get the important parts cheap as one of those little interior alarm add-ons to put inside cars. 12v of course. Only plug in the transmitter for the "target".
Receiver -  amplify and envelope detect. 4046VCO for beep pitch from envelope (VCO will cut off when envelope = 0v). 555 or other logic for beep duration.

[thermionix]

I concede not having seen the movie

Rich, you have an assignment...

[thermionix]

your local conspiracy theorist may have just the design you need:

That's funny he foiled everything BUT the roof.  At least the tree is protected from the NSA satellites and black helicopters.

Also...Florida...LOL