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Distance Sensor

Thread starter #1
Hello. I'm would like to build a gadget, made up of 2 separate components, whose main objective is to beep / alarm when they are distanced from each other - say about 2 meters. Is it possible, and can it be built cheap and compact - about the size of a 9-volt battery or smaller?
Thread starter #4
How exactly...

I'm not really an electronics techie, so how exactly do I go about making this gadget? I've been searching the net but couldn't locate schamatics for the shoplifting alarm. Know where I could find one? Thanks!
Thread starter #5
Re: How exactly...

I'm not really an electronics techie, so how exactly do I go about making this gadget? I've been searching the net but couldn't locate schematics for the shoplifting alarm. Know where I could find one? Thanks!
Thread starter #8
Infrared Sensor

Okey, will try a search on that. But does infrared work without line of sight? I mean, the gadget should work from all directions, and preferably even if there's a person blocking the line of sight between the two components...


New Member
First of all, why don't you share what you are building so we might assist you more precise? That goes out to a lot of ppl posting, not just you... I mean... you're not inventing something new, if so, you shouldn't even need to ask would you :)

Anyhow, I don't have a clue, but when you first mentioned it, I though of a high frequent sound and a reciever, that would work around bodies for instance, and I also thought about magnetism, although that would be quite sensitive to other objects.

But no, I do not know how to build this thing...

Could radio be the answer?

First, I agree with Thec about the lack of info when requesting help ... some people leave a lot to be guessed (the more info offered the better will be the guesses aimed at helping :wink: ) sorry to be such a neg.

If line of sight is NOT available the IR and ultrasonic would be impossible -- could radio be used?
If the passive (if feasable) device is some sort of 'tag' as used in clothes shops and the powered detector contains the buzzer then maybe some form of absorbance of radio power could be detected by the transmitter; the 'tag' could be a tuned loop to soak as much RF power as possible. The transmitter could then sound the alarm when it detects that no power is being absorbed by the 'tag' (which has wandered too far away)???
It would use a fair amount of power ... batteries might be a problem :!:

If such a set-up could work - how big could the transmitter be? how tolerant of false alarms would you be?
Thread starter #11

Sorry for the lack of info, non-techies like me try to keep it simple :) i'll try to explain more in detail :)

I was thinking of a gadget to use for - let's say handphones. It will be made up of two components: 1) Alarm / Beeper that will house the power supply, and 2) the part to stick on the handphone. If I accidentally leave my handphone somewhere, or if someone tries to steal it, the alarm / beeper will sound if the component on the handphone is distanced from the alarm / beeper component by at least 2 meters. Kinda like a shoplifting alarm like Star882 mentioned, but this one will be cheap and compact.

I like the idea of the shoplifting alarm, but the way the gadget should trigger should be the opposite - alarrms as they are distanced from each other. Infrared looks possible, but the problem is the line of sight. The connection between the alarm / beeper and the other component should be intact even when there is something blocking the line of sight (example: handphone is at my back pocket, the beeper is on the front).

I also think that radio waves is the answer. But can each gadget using radio waves be unique? What if two of the same gadgets happen to pass each other? Can micro chips be used (same ones used in modifying PlayStations) in tandem with simple radio transmitters / receivers to embed a unique serial number?

Thread starter #12
I found it!

Try checking the Electronics Project on this site, the one labeled as Anti Car Theft Alarm, is actually what and Mechie were thinking of. It was posted by the webmaster I think (your one hell of a techie guy!).

Anyways, the gadget looks big and has separate power supplies. Can someone please decipher the diagram, I don't know what it means :)

Following this diagram, how big will it be? And more importantly, how much would it cost to make one (estimate). Anyone know how to "fine-tune" the existing diagram, and modify it to be cheap, compact, and with one power supply?

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