Question for electrical engineers

[John Bentley]

Actually, I have 3 questions:

1. Can a circuit be devised which is completely passive (draws no electrical power at all), but reacts to a coded radio signal to turn on a power supply?

2. If yes, could the radio signal be as weak as a cell phone signal for instance, and still do the job? In other words, how weak could the radio signal be (how far away from the source), and still do the job?

2. Is there a standard name for such a circuit?

[CurtC]

Probably not, depending on exactly what you're asking. How would this circuit "turn on" a power supply? Doesn't that imply that it will throw a relay or something? That requires a decent amount of power, not the microwatts that you might receive from a cell phone.

[Beanbag]

Around here, such a gadget is called a TollTag (tm). They use them for electronically collecting tolls on toll roads. The tag reacts to an incoming radio signal (from which it steals a few milli or microwatts of power) to return a coded signal of its own that identifies the vehicle passing through the detection zone. The tag has no power source, but works like a crystal radio to convert some of the incoming signal to useful electricity, which it uses to "wake up" and power the rest of the crcuitry.

It works pretty well for what it was designed for. Range is purposely limited to tens of feet at the most. The power source is stationary at the toll lanes. Considering that a 4-function calculator can run off a single solar cell with less than 2 cm square surface, it really isn't that astonishing.

The RFID tags being tested by Wal Mart work on a similar scheme, though much simpler and at less range. They have to pass through what looks like a large airport metal detector to be sensed.

Regards;
Beanbag

[Pragmatist]

I agree with both replies. You can in theory power a circuit from a radio source but I doubt whether a cell phone signal would be sufficient.

And also a lot depends on what you mean by "switching on" the power supply. If you mean sending a signal that will trigger an independently powered circuit, then it's possible, but if the circuit needs to power the switch then probably not.

If the circuit was to be used in a place where there are other strong ambient radio transmissions (i.e. a radio station) then you could possibly tap those for power independently of the cell phone radio trigger.

[TillEulenspiegel]

Originally posted by John Bentley
Actually, I have 3 questions:

[b]1. Can a circuit be devised which is completely passive (draws no electrical power at all), but reacts to a coded radio signal to turn on a power supply?

The answer is Yes , PARC ( Xerox), the people who brought You the GUI( aka windows which S.Jobs stole only to have B Gates steal it from him ), the mouse and other wondrous things, has a system of passive radiation ID badges where all the work is done on the initiator side ( The computer/transmitted ) . There are others such a Olivetti and BIC, but PARC is the top dog.

2. If yes, could the radio signal be as weak as a cell phone signal for instance, and still do the job? In other words, how weak could the radio signal be (how far away from the source), and still do the job?

Yes but the area of the radiated signal is small ( say 15 x 15 ')

3. Is there a standard name for such a circuit?

No, as no standards have been established under the IEEE or similar organizations), but basically it's a passive induction loop fed from a radiator in the ceiling. This is old info , you might wanna poke around some.

To add, no amount of real work can be done by a passively powered device , unless the output of the radiator is massive.

[jj]

I tend to agree with the answers given.

Just to be pedant I will remind us all of Tesla's "power transmission", of course then we have to discuss the efficiency of that all :)

[CurtC]

Originally posted by Beanbag
Around here, such a gadget is called a TollTag (tm). They use them for electronically collecting tolls on toll roads. The tag reacts to an incoming radio signal (from which it steals a few milli or microwatts of power) to return a coded signal of its own that identifies the vehicle passing through the detection zone. The tag has no power source, but works like a crystal radio to convert some of the incoming signal to useful electricity, which it uses to "wake up" and power the rest of the crcuitry.Hey - I have one of those! And before North Texas had them, I had an Oklahoma PikePass (the same thing), because I travelled there a lot. But I think the Tolltags that NTTA uses are battery-powered, not beam-powered.

Amtech does indeed make a beam-powered tag, but its range is limited to less than 10 feet. This just won't do when you drive through the toll lanes on "The Bush" at 70 mph.

The cool technology in these is that the tag doesn't have to actually transmit anything, it instead has a device that changes its reflectance to RF, which can be done with very low power. If it changes the reflectance back-and-forth from higher to lower in a pattern, the receiver can identify which particular tag that is.

The battery-powered tags have a life of about eight years, according to Amtech.

[CBL4]

Since you are talking about turning on a power supply, could you use it to charge a battery?

There are some extremely low current RF receivers (not sure about cell phone but theoretically it would work) that would run for weeks on a battery. If you could use the power supply to charge the battery once a month (or when a signal is received), this might be a solution.

If we are talking theoretically, there are other options. If the device is near something that uses AC current, it would be possible to use the AC magnetic field to power the receiving device.

CBL

[Xeriar]

Recharging a battery would be insanely innefficient.

[cesium]

I bileve that the rf devices for tollways run off a battery, and because
of thier proximity to the antenna, they would not need to draw much
current.

RFID tags either respond to an rf signal by resonating at the same or a different frequency which is then responded to by the sensing device. other rf tags are powered by either battery or RF and use a microchip with encoded information.

If you need to switch a power source, why not just build a simple low-current reciever and a simple switch like a relay. Even if the power source is limited, this device should draw far less then the device you need to power.

[TillEulenspiegel]

Main problem being you cannot design anything until you know the purpose of the device.

Why passive , when it has a power supply ( presumably in keeping with the passive idea) that will probably be batteries that need charging ? Why not siphon off a few mV and uA to make the device active, ala the solar calculator? They ( for the most part) don't have batteries but large value capacitors* which can store enough energy to operate the device for days before re-exposure to sunlight?

I'm guessing here but I think the approach is less then efficient.
There are active MOS components that consume <2 V and < a few uA.
Until we know what the poster had in mind we can only answer questions directly.


*Actually called ESD AKA Energy storage devices, a mythical creature ( since spotted and exploited ) My prof swore would be as large as bathtubs.

[Ove]

They use a similar device to register dogs. The device is inserted just beneath the skin with a hypodermic needle and from then on you can allways identify the dog.;)

[John Bentley]

Sorry to have abandoned my own thread, but I've been away. I didn't mean to be intentionally cryptic. I was simply wondering if it was possible to design a circuit that wouldn't use any power until "queried" by a coded signal, thereby lessening the demand for power when not in use (and hence, size of battery), and lengthening the "standby" time indefinitely.

I was wondering if the device inserted beneath the skin in animals for ID purposes could be expanded into a tracking device for lost or stolen pets, and could respond to something as weak as a cell phone signal, without having a huge battery requirement, or having to be frequently recharged.

Since that post, I have discovered commercial devices on the market for just such purposes, along with explanations in laymen"s terms for why the power requirements and battery size have an irreducible minimum with current technology.

Thanks for all the replies.

[epepke]

I know this is possible, because I did it when I was a kid. I had this tiny relay, about 1 cm square. It was quite sensitive.I hooked it up to an AM band RC circuit and a diode. The source for the signal was a little potted AM transmitter module. It only worked from a couple of feet away, but it worked.

A better design would probably be an SCR instead of a relay, in which case you could probably omit the diode.

[RussDill]

Originally posted by epepke
I know this is possible, because I did it when I was a kid. I had this tiny relay, about 1 cm square. It was quite sensitive.I hooked it up to an AM band RC circuit and a diode. The source for the signal was a little potted AM transmitter module. It only worked from a couple of feet away, but it worked.

A better design would probably be an SCR instead of a relay, in which case you could probably omit the diode.

The problem is the exponential dropoff of signal strength combined with the difficulty in decoding a complex signal pattern like GSM or CDMA. To get anything usefull out of GSM or CDMA you'd need an amp, some a2d's, and some dsp's. We have passive cards at work with a wire loop with an area of about 5" square, you need to get within inches of the tranceiver for it to work.

[epepke]

Originally posted by RussDill
The problem is the exponential dropoff of signal strength combined with the difficulty in decoding a complex signal pattern like GSM or CDMA. To get anything usefull out of GSM or CDMA you'd need an amp, some a2d's, and some dsp's. We have passive cards at work with a wire loop with an area of about 5" square, you need to get within inches of the tranceiver for it to work.

What you describe (the cards) are what we used to call grid dip. This was back when we had grids, which were in vacuum tubes. (Vacuum tubes are actually pretty decent devices because of the high impedence. Nowadays, we use some sort of insulated-gate FETs.) Basically, the transmitter has to pump out enough signal, and the passive circuit has to use enough power that the transmitter can detect it. It's a bit of a different problem.

Passively receiving a signal from a transmitter is much easier. My father used to do radio tower maintenance, and moderately far away, it was possible to light a neon bulb in the hand to a ground connection. That's 80 volts, though not much current.

From a cell phone, from the requirements in the OP, I suspect that you'd just have to be able to detect the short high-power bursts that a cell phone sends out from time to time. Detecting only a particular cell phone, of course, would be much more complicated. But presumably, the passive circuit could switch on another circuit for a short time to do that.

[jj]

Originally posted by RussDill
The problem is the exponential dropoff


Exponential? Are you referring to some interference phenominon, or did you mean square-law?


of signal strength combined with the difficulty in decoding a complex signal pattern like GSM or CDMA. To get anything usefull out of GSM or CDMA you'd need an amp, some a2d's, and some dsp's. We have passive cards at work with a wire loop with an area of about 5" square, you need to get within inches of the tranceiver for it to work.

I think you meant square-law.

[RussDill]

Originally posted by jj
I think you meant square-law.

my bad. In any case, if you find that you would need an amp to get a usable signal, you are out of the self powered realm.