Statement of problem:

I am trying to think of a very inexpensive (meaning pennies, not dollars) for a low cost sensor to advertise its position on demand. The sensor could be buried in a structure wall, such as a house, car, boat, etc. Or it could be a few cm underground. The sensor may or may not have a power line going to it, but in the case of being buried inside a wall with no wiring diagram that could't be used to find it anyway.

Imagine that you have several such sensors in a car, for example, and the car has been sold and resold several times. Suddenly, one of the sensors malfunctions. You can see on a display it is sending spurious information. You want to replace it, but nobody has any idea where that sensor is located, or indeed where any of the sensors are located. Assume the person who installed them didn't document anything and is dead so he can't tell you. Further, just to head off the obvious question of "what good is a sensor if you don't know where it is", assume it is just some kind of a generic sensor like "rust detected." Could be anywhere.

Question:

What I want to do is be able to command that sensor to broadcast some kind of transmission that can be located with a probe. I thought of a radio transmission and handheld RF locator, like sweeping a bug detector, but that would seem to require an expensive ($5K+) FCC certification fee to be legal. Transmission could be done license free in the VLF band under 8kHz, but how can you design an antenna for an 8kHz radio wave on a sensor's PCB that is only a few sq. cm? And I have no idea if a detector in that range is even possible. But if it turns out licensing is the only possibility and I eat the $5K, what is the best way to do this? Is there a specific frequency for this purpose, and how to build a very cheap transmitter?

What other options exist that could be done with a GPIO pin, some simple electronics and/or maybe a 10 cent Chinese IC? Ultrasound? That would seem to fail as soon as it hit any kind of air/material interface. Magnetic field and a Hall Effect sensor? Maybe. I don't have any experience with this. Can that be made strong enough to be detectable from a distance (say minimum 10-20 cm) and what materials would block it? Light obviously won't work if it is buried in a wall. Sound wouldn't work if buried in soil or concrete.

Has anyone ever considered a problem like this before, or seen someone implement a clever solution for a problem like this? Any ideas welcome, not matter how unconventional.

 

Hi C,
Have you looked into RFID tags?
C

 

I don't understand what you are trying to do. What would a sensor buried in a wall, underground or in a car detect, and why would it not be documented? If the device is sensing something, it is pointless unless it is transmitting that information by wire or wireless. In that case, its source can be traced.

 

and... you want sensors to be randomly distributed/hidden/buried. their cost is irrelevant (can be high)... but the circuit used to LOCATE those hidden sensors need to be low cost?

 

Without more information, it sounds like tinfoil-hat science on a budget!

 

I don't understand what you are trying to do. What would a sensor buried in a wall, underground or in a car detect, and why would it not be documented? If the device is sensing something, it is pointless unless it is transmitting that information by wire or wireless. In that case, its source can be traced.

Not that I like this activity but it is kind of planting mines in a field and not retaining a graphic reference with the distribution to cross it safely later. Nonsense.

 

If a vehicle or residence has changed ownership multiple times, why would an earlier owner have any interest once the final payment is made?? Wired microphones can be located by standard methods, likewise video cameras. If there is a wired power connection than a suitable command can be delivered that would cause a magnetic signal to be radiated. If the device is not wired, but wireless, the CIA has technology that I can only guess about.