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looking for some advice...

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hi there everyone,

i am looking for a project or some advice on making a project!
i have searched many sites and still not found what i am looking for...
basically, i need a Transmitter and a Reciever.
The idea is... once the Tx and Rx reach a certain distance, an alarm sounds off.

in other words, once a critical distance between the two components has been reached, then and only then does it trigger an alarm.

Has anyone any knowledge of such a project or can anyone give me any advice?

Peace to all.
i'm no expert but it sounds fairy simple. you simply measure the input to the receiver and once it reaches a certain level it trigers an alarm. I know this isnt much help but i would try researching individual components that will do each task required and then design the circuit.
cheers for reply

cheers for that. i know it sounds simple but there are certain issues that arise when in use. for example, there will always be varying obstacles in the direct path of Tx and Rx. in other words, there will most certainly be objects of varying density between the two components... ie people, furniture etc...

What distance are you looking for ?
Under fifteen feet you might want to look at using ultrasonics, it makes the electronics far cheaper and simpler than that needed for an RF based system.
A PIC micro or dedicated counter timer ic can measure the time of an audio pulse delay between the transmitter and the return pulse sent by the mobile unit to an accuracy of as little as an inch without to much by the way of external components.
thats more like it...

cheers for the reply...

Now we are getting closer. 15ft could do it. but it would be nice to be able to adjust it from anywhere between 10ft and say 30ft.

I would like to keep the cost of production to around £30 - 40.00.

does this still sound possible?

The budget looks realistic, I have had a quick look around the web for an old project kit for electronic croaking frogs, but unable to find link you may have better luck or someone might read this and help out , they worked along similiar lines first listening for a croak then replying , dull when demo'd in pairs but fascinating results when operated in groups of ten or more.
Back to your project:
A fistfull of 555's or perhaps as little as four should suffice, circuits for sound activated switches abound on the web and electeret microphones can be cannibilised from numerous sources (Charity shops here in the Uk yield a seemingly inexhaustable supply of telephones for a £1 each)

Another idea/circuit that is seldom seen these days is that of the Theramin
handy idea for guarding long perimeter fences
cheers Tansis!!!

Thanks for the info...

I am fairly ignorant when it comes to what components do...
I know the 555's are basically timer chips.

I can build complicated projects, as long as i have the diagrams and component list as in Maplin Projects... for example.

i have also made some projects from scratch... but they have been fairly simple.

I am only writing this to give you an idea of my abilities, thats all.

anyway, getting back to your earlier message... i liked the sound of what you were proposing. but would the mechanism still work effectively if objects of varying densities pass between Tx and Rx?

Say i was to use the project for... lets say my forgetfulness and keys.

I am in my office and my keys are on my desk. In this particular example... I would be able to wander around my office, around the cabinets, desks and people. But if i tried leaving the office then an alarm would sound and then remind me about my keys!

Well, that is the kind of idea i want to fufill with the project. As long as i had a diagram i could translate it to the circuit boards with the copper strips and holes (i forget what its called at the mo...)

I am also willing to pay a few Quid to develop this project.
If you have any info on the circuitry and components involved i would be extremely appreciative!

The output of the ultrasonic transducers is small but numerous circuits in the archives claim that normal reflected signal range finders (sonar) operating under ideal conditions can work at distances of twenty feet with some degree of accuracy.
As for "objects of varying density" this does not apply to acoustic / sound waves as they tend to bounce around and off all surfaces.

For simplicity of construction, small size and low power consumption I would suggest using PIC microcontrollers such as the 12C672 or the more common 1684 series. I'm trying to get up to speed on these myself but there a plenty of dedicated PIC sites , you could try and bounce the idea off them.

The sticky problem though is what happens when you forget where you left the lost key warning device.
Perhaps a more low tech solution would suffice :lol:
The sticky problem though is what happens when you forget where you left the lost key warning device.

True, but a bigger problem, is what happens when the Tx and Rx are not facing each other. This would cause the alarm to go off constantly, as you walk around the room.

You will need to use something that doesn't require the Tx and Rx to be aligned, ie RF. Ultrasonics won't be practical in this application.
True , ultrasonics are fairly directional but a pulse or coded signal / chirp at a lower freaquency should compensate , though it it would be a tad irritataing.
After all if you did a walk test with a cheap transistor radio on your desk , no matter where you went in the office you would still be able to hear to hear it playing.
Would be reminescent of the alarm system in a lab I worked at some years ago , its constant beep every two seconds to say that it was working and all was well.
Not necessarily, if you got the exact angle of the Ultrasonic transmitter/reciever, and calculated how many of them it would take to make 360 degree's, it would then tell you what shape the case would have to be to host all the transmitters, and recievers to get a 360 degree broadcast, sure it would be a bit more, but it'd still work, without the irritation of a low frequency transmission.

Also if it comes out to be a strange shape, and you have no access to the machine equipment to mold it yourself, then that's when you go for the low tech way, make it out of pottery clay, then toss it in the oven.

Heh, I know it sounds cheesy, but when you have no other alternatives it works, just remember that the clay expands when you put it in the oven, so make sure you make the inside a bit bigger when you go to cook it.
Fabication of small delicate parts is art, but a usefull trick is the chemical reaction between aluminium and ferric chloride, the technique of "acid etching" is quite common in the aerospace industry.

Please note:
Above reaction is Exothermic (gives off heat) and produces quanteties of hydrogen gas (explosive when mixed with air) perform in well ventilated area, wear gloves,goggles etc
hang on... hang on.

hi everyone....

I have been away for a while so it has been some time since i caught up with this thread.

Anyway, i'm the guy that wanted the info on the project and as i can see, it has generated some 'debate'...

just to clear some things up.... the Tx and Rx WILL NOT always be aligned!

I do not have a Kiln! Nor do i know a friend that does...

Some of the stuff you are chatting about goes way over my head... but it sure sounds interesting!

AND to solve the prob of forgeting the device... i could have it surgically implanted, depending upon the size and my particular tastes at that moment in time. Alt. i could wear it... somehow!

Anyway, debate away, as i say... i am willing to finance this project of there is a positive outcome.

Speak soon,
Perhaps then it would be better to change the type locks , rather than undergo surgery , after all you cant lose your fingerprints.
Granted but loss would be noticed and care would be exercised with remaining digits.............
come on people...

i was only joking guys...

any more relevant ideas pertaining to the projects construction from anyone?

Forgive me, but could not resist, it's a noble project , unfortunatley it's what engineers and philosophers dub as as "Rat Hole Engineering" it could be made to work but is unlikely to suceed on a practical level.

The crux of the problem is that one has to constantly remember where the key finding device is , unless of course it is intergrated into the structure of the building(s) or person.

"COMPUTER..... Locate house keys john Doe"
Click Burr whuzz
chakka , chakka ,neep,wurr
"Search -Query... Request data... Where did you last see them?"
The Project does have a valid use...

I have not mentioned the exact application as i have been told not to release that info. (merely to protect someone else's idea)

I have only mentioned the 'key' example as a practical example of how it could be used.
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