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? possible to design such a unit .....

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Oct 15, 2001
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may be silly, but it is of general interrest for me.

I think on a unit, as big as a wrist watch to locate my cat or my child (or my wife :lol: ) - only max 30 meter distance.
Receiver unit has a display and show me in an analog bar the distance to the transmitter unit.
Transmitter unit send every second to the receiver unit.
If the contact is lost - I get an alarm on the units.

can be done this in such a small system like a watch?
can this be done on 400Mhz?
are there electronic parts (receiver/transmitter units) or system on a chip .... to do this.
Please give me some links (in the right direction), so I can study and read about it such kind of electronic parts.
do you think ist is possible to handle with one receiver unit a lot of transmitter units? (If I have a lot of children or girl friends and cats)
- and my neighbor has also such a equipment.

are there other technical risk?

if someone have interest to send a comment will be nice


They use this technology to keep prisoners in "home prison" instead to stay in a "real prison", you intend to do this with your wife ? :agrue:



Ha Ha

think invers,
some people are happy if the wife is not at home and they will know if she comes back. :spoko:

i am in industrial automation and not a electronic designer,
i need some hints to understand and read and think ..... on how to.

hugo :bye:

I think you would need something bigger than a watch size.
Problem is power supply, even a low power receiver needs a few milliamps to run, you cant get a small battery with high capacity in a watch case!!!

it is useful sometimes,when you have a baby and take him or her to a shop,then you will not lost your baby.
it is easy in tech,ans i have seen some kind of products ever before.and a watch size is also easy,even just like a coil size is possible.
i have seen a products add to a key,just like a coil size.when you lost your key,you can use a handheld looking for it,the device will show the distance of the key from you,ans direction also be can find 4 keys.i forget whith magazine i have seen the message.maybe you can buy one in web.if somebody interesting about the device,PM me ,i will try to fiind out more info.

ok,the small things is called I-SPOT,produced by a US company,but i do not know the web site.i seen it in a magzine.anyone interesting can search it himself.
if anybody want design such a thing,i think at least 3 methods:
1:just a very sample FM senter in baby,the can sent out a low level FM signal,such as 300m-400MHZ FM signal,the you receive it ,if the signal is lost ,you will know the baby is far away from order to save power,you can sent 10-100ms every 1 second.when it is work,it only takes about 0.5ma 3v,and can be receivesd about 50 meter.the higher sensitivity are ,the longer distance can be the device will consume only about 0.15mW in one second.
2:use a data RX/TX chip to design,and maybe a 8 pin cpu is needed.use the cpu to control the RX/TX and you can do more things than method one.
3:use a bluetooth modle,it is a easy way,also need a cpu.maybe it will have many functions but will be more expensive.

8 pin cpu can be found in:
**broken link removed**

use a li-ion ,which is 3v 200mah,size like a coin,can continuous drive such a device about 2000-4000 hours,about 3-6 months.
you can get some info from
**broken link removed**

and some of RX/TX chips can get from:
**broken link removed**

I got it!

I have designed the product.

I saw somewhere that a device like this has been designed to detect
peoples whereabouts in a building using their Bluetooth enabled mobile phones, The acces point that they are nearest to reports there approximate locatio to within a few meters to the security system.
It then knows if they have left the building.

Phil 8O

it can be built in very small size.

If you use the bluetooth, you need not use the access point.
and get one point to more point(>7).


Why don't you point your effort to proximity card's, maybe it will be the way. It uses no batery for the power of target cards, it has a resonant circuit, when the card or the transmiter is inside the resonant magnetic field gets the needed supply, so you can put an smallest chip in transmiter like a watch. The biggest power is used by the receptor than must create a big alternated magnetic field. Be carefully with this, is the same concept of a microwave cooquing machine, so you can maybe tost your wife.... :lol:

I don't have the links, but I heard that in the UK are using something like this, they put a microchip inside a teenage girl, to locate inside London, it's terrible, but it's true. :(

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