Very short range RF communication?

C

CURTISBEEF

New Member

• Aug 18, 2006

• #1

Hi im about to begin working on a project where i need very short range RF communication. It involes Keyless entry for a car when you are in less that 4 foot range. Ill be using Pic microcontrollers.

I need to have a Tranmitter that is small and portable(in pocket) and i need the receiver to only just see if the tansmitter is in range or not. I would like it to be somekind of checking of a to make sure it is an athentic device(for security).

anyone have any ideas? must have no longer than a 10 foot range. is this possible?

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Aug 18, 2006

• #2

CURTISBEEF said:

anyone have any ideas? must have no longer than a 10 foot range. is this possible?

Click to expand...

No, setting a maximum range in that way is impossible - particularly such a short one, even IR would work much further away. You 'could' reduce the power until it wouldn't work at 10 feet, but it's not likely to be reliable at 4 feet then either!.

Why does the maximum range matter anyway?.

 

JimB

Super Moderator

Most Helpful Member

• Aug 18, 2006

• #3

As Nigel said, defining an exact range is not a practical proposition, in theory yes, but practically there are just two many variables.

Also consider, you are not alone! There is a million and one other low power RF devices out there. Most of them well engineered, and a reasonable number made by people who dont have and never will have a clue as to what they are doing!
All this junk is assaulting your nice little device. (Or should that be piece of junk?).

JimB

 

Hero999

Banned

• Aug 18, 2006

• #4

Range limiting is possible but it isn't simple.

The controler and unit in your car need both a transmitter and reciever. The unit in the car transmits very low-power low duty-cycle pulses when the reciever in your controller detects a pulse it sends a pulse back. The reciever times how it takes from sending and recieving a pulse and figures out how far you are away from it and it you're near enough it unlocks the car.

You'll need a very expensive custom-built circuit made with special parts for it to be small enough to cary around with you.

 

C

CURTISBEEF

New Member

• Aug 18, 2006

• #5

ya see here is the thing im tring to make a keyless entry and start for my car like the high end Mercedes have where you can walk up to the car with a device (in your wallet or pocket) and it unlocks the car and also start the car without a key.

The range matters because if i walk away from my car i dont want it to still be unlocked/startable.

any help?

 

Hero999

Banned

• Aug 18, 2006

• #6

We've already tried to help you, read my previous post it should give you a clue.

What you are trying to do isn't easy, didn't your car come with a key-fob to unlock the doors?

 

K

kchriste

New Member

Forum Supporter

• Aug 18, 2006

• #7

I think you are looking for something like this:
https://www.electro-tech-online.com/custompdfs/2006/08/01024A.pdf
https://www.microchip.com has TONS of good app notes like this.

 

Marks256

New Member

• Aug 18, 2006

• #8

Hmm, sounds like an interesting project. Couldn't you have a function that required you to program times that this function is available, and when it isn't available, then have it make you push a button on the transmitter to unlock/start your car?

huh, just make sure you don't mess up the PIC program, have your car unlock and start in the middle of the day. Ooh, that wouldn't be good. Even if nobody stole the car, with the current gas prices, you would be on the ground crying!

 

eblc1388

Active Member

• Aug 18, 2006

• #9

A magnetic loop input, if placed inside the car, would certainly has a very limited sensing range. One would be lucky to get range of even a few meters.

 

C

CURTISBEEF

New Member

• Aug 18, 2006

• #10

eblc1388 said:

A magnetic loop input, if placed inside the car, would certainly has a very limited sensing range. One would be lucky to get range of even a few meters.

Click to expand...

Can you give me any more info on that?

 

eblc1388

Active Member

• Aug 18, 2006

• #11

It is just a loop of coil, either with a capacitor(tuned) across the ends or without(untuned). One just connects the two leads to the input of an opamp and ground and amplify the signal, follows by some filtering.

The transmitter has a small coil and creates some magnetic flux around the transmitter. The signal get pick up by the coil if the transmitter is brought near to the loop.

This is sort of how it works. It is the reverse application of the magnetic loop that are commonly used for the hard of earing where the transmitter is outputed via the loop and the senser actually picks up the flux and strip the audio information from it.