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Is this possible in keyring size?

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New Member
Since I keep losing my mobile phones, I'm thinking to make a thing to remind me whenever I drop my mobile phone and just walk off. I'm thinking of using a range detection sort of thing and whenver I'm too far away from the mobile, it either sounds and alarm or shakes so I can walk back and pick it back up. Is this a possible design?
Any help is gladly appreciated


New Member

Your problem poses some design problmes:-
As mobile phone could be left anywhere, you cant juz add a proximity or object sensor, a phone left near a book might still think there is someone nearby, so the only way is the intelligently sense if there is a human preset nearby.

One way I can think of is to use the newly introduced Motorola "E-Field" sensor, this sensor uses a very low electric field to sense the presense/absense of a human amongst others.
You might want to google for Motorola "E-Field" or try

The chip no is: MC33794



New Member
how about a PIR sensor? it could work....you could make it less sensible, so it will only respond to heat from your body....adn sound as soon as it doesnt sense it.
or you could use some sort of short range rf transmitter located into the phone, and another receiver located at the keys. when no signal is present at the receiver it beeps. you could use the trnasmitter to send short signals every 2 seconds, and if the receiver dosn't receive the signal for like 3 seconds, it will beep.


Active Member
I may have this wrong but I think cell phones transmit periodically to look for cells within range - and it lets the cell (therefore the cell system) know where to route a call should one come in (otherwise every cell would have to look for your phone when a call came in). I think mine does the query thing once a minute or every few minutes anyway. I understand that when it thinks it's well out of range it saves power by doing the query thing at wider intervals.

If this is true then it would seem that a small receiver that would sense these "queries" might do what you want it to do - within limits of course. A receiver and PIC might be combined to look for queries within certain time limits and advise when a query has not been detected. This removes the need for you to transmit anything.

Almost all electronic devices generate some undesired stuff - RFI if you will. At very close range the RFI might be enough to be easily sensed - and if not sensed might suggest that the phone was out of range - or the battery quit. This might be better in that as soon as you move far enough away the alarm could trigger.

Good idea. Might actually be easier to implement than you thought.


Well-Known Member
A piece of string seems like a good low-tech solution. A piece of string on a spring-loaded reel would be my high-tech solution.
Sorry - I can't relate to misplacing my cell phone, but I suppose that if it were my only phone, I might lose it when I'm wandering around the house naked. The options of what to hang it on are extremely limited.
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