Losing it! ( .... and finding it ).
Introduction.
Losing things is something that we all do. Some losses are far more serious than others, for example losing a small child in a crowded place, or your handbag with all your money and credit cards in it. The latter is something that my mate Donald seems to be doing more and more frequently !. So I thought that we would have a look at solving the problem electronically.
When you lose something or simply leave it behind, it may be quite some time later before you discover the loss and by that time it may be gone forever. What we really need is some means to tell us that we are about to lose something !. Maybe I could define that as the moment you start to walk away from the object ... or it starts to walk away from you !. The key is that you start to lose contact with the object and this is what we can detect electronically.
Radio Frequency Short Range Devices (SRD's)
Designing RF circuits has always been a black art and to make it easier to incorporate radio frequency transmitters and receivers into new designs, a new class of license free RF modules was introduced. These work within specific frequency bands set aside for remote control devices, such as car alarms, baby listeners and other short range data links. In UK the most widely used allotted frequency band is centered on 433.93MHz. Other bands are available, for example 868-870MHz (Europe). These bands are intended for low power, generally below 10 mW depending upon the duty cycle (how long the transmitter is on against how long it is off). To limit interference with adjacent devices transmitters are not allowed to use high gain aerials, although any aerial is permitted on the receiver end. Ranges achieved are typically line of sight 50 meters and perhaps 20 meters within buildings ... this is more than adequate for our purpose.
SRD's are available that work on two different principles. The cheaper range use amplitude modulated regenerative receivers which tend to give fairly short ranges. The other type uses a double or triple conversion frequency modulation super heterodyne receiver , which are far more sensitive and can give greater ranges, in the best circumstances, several kilometers. The regulations allow the higher frequency bands to use higher transmitter power levels.
What range do we want for our 'losing it' detector ?. If we allow too great a range a lost child could be completely out of sight in a crowd, before our alarm sounds. On the other hand too short a range could produce false alarms. I think it would be a good idea to include in our design the means to set the alarm range between 1 and 10 meters.
Principle of operation.
The device will consist of two parts. One is a small transmitter module that is secured to the item, you do not want to lose. The other part is a small receiver device that you wear on your person. The way it works is as follows. During normal use, as long as the alarm receiver module is within range of the transmitter module, no alarm is given. As soon as the transmitter module moves out of range of the receiver and alarm is sounded. So if the child decides to stop, or wander off then an alarm is given. If you leave your bag on the counter of a shop and try to walk away, an alarm is given.
For a practical device, Transmitter size will be mainly determined by the battery size. SRD's are very small devices and can be about the size of a lapel badge. The receiver would probably be about the size of a match box.
Keeping it legal.
As I mentioned earlier the permitted transmitter output depends upon the duty cycle of the transmitted signal. For example we do not have to transmit a signal continuously and in practice would send one period of signal followed by ten periods of no signal. This would allow us to use maximum transmitter power because the duty cycle would be 10 : 1. It also has the advantage that our transmitter battery would also last ten times as long.
Multiple devices.
If the device became a commercial success, then we could find the situation where several of these alarms might be operating close together, in a shopping mall for example. The danger would be that if your child wandered off, someone else's transmitter signal might stop your receiver giving an alarm. To prevent this happening we can give each device it's own code so that only the matching transmitter and receiver pair can work together. Special encoder / decoder chip sets are available for this purpose. This would automatically give each transmitter module a unique identity.
Taking it a step further.
The proposed device is only intended to give warning that you are about to lose something. How about the case where a child is actually abducted, or a bag deliberately stolen ?. In the case where the bag is stolen, it is usually discarded shortly after the contents have been removed. In a mall it would probably be discards in the mall or nearby. At this stage the transmitter in the bag is still working and will continue to do so until it's battery becomes flat, so there is a period of up to about 48 hours during which we could locate the bag using radio frequency direction finding techniques, if the equipment were available. This would allow items such as expensive spectacles etc. (worthless to the thief), to be recovered ... along with finger prints etc..
In the case of the abducted child the still working transmitter module could perhaps assist in finding the location of the child. The problem is the short working range of the equipment. To be effective the direction finding equipment must be located at a high location with a good line of sight range. For example in Andover, the best location for the DF aerials would be on top of the flour mills towers. The other option is to deploy many detectors over an area or use mobile detectors, in the hope that the abducted child may be taken within range of one.
Practical direction finding devices.
A simple but effective direction finder can be built using two SRD super heterodyne receiver modules with separate yagi aerials. At 433.92 mHz small highly directional high gain Yagi aerials are practical. When the two aerials are pointed directly at the transmitter the signals received by the two aerials will be in phase. If the distant transmitter now moves, one aerial will be fractionally closer to it than the other, giving the signal in that aerial a phase lead over the signal in the other aerial. So by measuring the phase difference between the two receiver outputs we can accurately determine it's new bearing. Two such bearings give us triangulation and the exact location of the transmitter. What I have basically described is known as a dual diversity direction finder and could be built for about £25.
Since our transmitter is working on a ten to one duty ratio a better direction fining equipment would be based on the commutated aerial direction finding principle. This is really a development of the simple dual diversity equipment using a minimum of four receiver aerials instead of just two. The equipment has the advantage of a graphical presentation, fast response and back bearing resolution.
Future developments.
When I last looked at this subject, I decided that the best chance of success would be to operate the DF equipment from an aircraft, such as a helicopter. Many helicopters are actually fitted with UHF homing receivers that are capable of working on 433.92 mHz. Helicopters are expensive to operate and it takes time to get flights authorised etc. So eventually I came to the conclusion that a model radio controlled aerial platform was required. With modern minaturised electronics it would be fairly easy to install the direction finder, control link and a video camera into a small fixed wing design with a wing span of about four feet. I think that a satellite based system would be too bulky for this application.
Links ....
http://www.ofcom.org.uk/radiocomms/ifi/tech/interface_req/uk2030.pdf
http://www.ofcom.org.uk/radiocomms/ifi/licensing/classes/rlans/short/
http://www.rfsolutions.co.uk/acatalog/Radio_Modules.html
http://www.lprs.co.uk/about/index.php?language_id=1
http://www.futurlec.com/Radio.shtml
http://www.radiotelemetry.co.uk/module.htm
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John Kent,. June 2007.
Note. Since I wrote this I have seen such a device on sale on Ebay, being sold from China. Prices are relatively high at the moment at £29, but will eventually drop below £5 as they are very simple devices
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September 2007
Finding it !
Personal GPS tracker.
Well, not only have the prices of the lost child alarms already dropped below the £7 mark, but a 'locator' has actually come on the market to locate the alarm tag once it has gone out of range of the alarm receiver. This locator is being sold by Maplins Electronics and is based on the GPS system, so there are really no range limitations. The tracker is priced at a very reasonable £129.99 and full details can be found at ......
http://www.maplin.co.uk/Module.aspx?ModuleNo=99114&C=Maplin&U=SearchTop&T=ALARM&doy=14m9
Another related device also sold by Maplins is the Loc8tor Homing device. A small hand held device for homing in on the lost transmitter tags. It will guides you to within 2.3 cm of the lost item, up to 183m away. Priced at £59.99 plus. Up to 24 tags can be registered per system. Full details can be found at .....
http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=99295&doy=14m9
Conclusions.
In the case of the abducted child,, at a medium walking pace of 5 Km/h (3 MPH) the abductor and child would be out of range of the Loc8tor device in 2.19 minutes, probably the length of time to reach an exit from a mall. I feel in a mall type situation the range may be much less than 183 meters stated, because of steel structural screening. After the tag has moved out of range, a single hand held tracking device inside the mall, it is unlikely recover the signal.
The GPS system appears to offer the ideal solution except that perverts and kidnappers may also be aware of the latest developments, such as miniature satellite tracked transmitter tags and search the victim at a very early stage of the abduction and remove such devices. One solution is to make the device un-removable ..... and also give an active alarm to attract attention. Another solution is to implant the tracking transmitter inside the child. The technology exists to make very tiny transmitters that could be implanted but the problem is in powering them from conventional power sources. For example an implanted transmitter could also contain a battery to power itself and that battery could be easily recharged inductively .... but the charge in that battery still has a finite life. So design becomes a compromise between the duty cycle of the device, the time required to set up a search and rescue, and the self discharge characteristics of the devices implanted batteries.
I am coming to the conclusion that mall owners should be responsible for providing a child abduction detection system at every exit of the mall. The way it would work is that as soon as a tag moves out of range from it's parent receiver, it generates an active RF alarm signal. A receiver at each mall exit would detect such signals and stop the child being abducted further. Considering the Madeleine McCann case, it could also be applied to amusement parks, play area's, hotels, schools etc.. If we take this idea a little further, then the short range nature of the system requires multiple detection sites, which further increase location accuracy over much larger area's. We have the "Smart dust" theory .... all we need is someone with the IQ of a lamb chop to apply it. Good heavens .... did I said that ! .
In the case of Don's lost or stolen bag, either system would work admirably, in the recovery of the bag and much of it's contents, providing it has not been dumped in a metal skip !. As usual it pays to cast ones nets wide.
John Kent 2007