Personal Tracking Device
After posting that a personal tracking device wasn't feasible because of power consumptions issues it got me thinking, is it?
I haven't worked this out yet so as I write this I genuinely don't know if it is possible to construct one but I plan to work it out, considering both size, power and practicality.
I have seen devices on the market which do this type of thing but none do exactly what I want, my specifications are as follows
- Small - It needs to be small enough so it can be carried without much inconvenience
- Power - The battery needs to power the unit for at least 24 hours ideally longer
- Accuracy - The GPS needs to be accurate enough to pickup a signal wherever the device is placed on someone
- Frequency - The system needs to transmit its location in real time, this means a constant communications link and no stupid SMS systems.
The other requirement for this system is that I can build it, that means no (BGA) ball grid array devices but I can deal with pin pitches down to 0.1".
The system will be composed of 4 main parts, GPS, communications module, processor and battery. I will look at the GPS first.
GPS devices require a lot of power, they average around 30-40 mA at 3.3V which is a lot of power for something that needs to run all day, there are also different types of antenna, the common ceramic one needs to be pointed up which isn't very practical, they can work at other angles but they aren't as sensitive, thankfully you can get helical antennas which don't have orientation problems.
The GPS to the right is a high performance GPS which should be ideal, the body is only 2.5cm by 2.5cm and it has a small connector which can be soldered to, unfortunately it draws up to 80mA which is way to high.
After another look I have found a GPS module, a Lassen IQ which draws 26mA at 3.3V which is much better, it doesn't have a built in antenna but a simple ceramic one can be connected, this will need to be pointed up but I will probably have to compromise on that option. The module is about 3cm square.
The antenna also draws some power, I don't have an exact figure but it may be up to 10mA
The other important part of the system is the communications module, I have recently used a GSM module from Telit with another project so I will see what they have that would be suitable for this.
Telit make some small devices but unfortunately they are all BGA devices, this means I will need to use the same size device I used for the other project, this was 4.5cm square. They also make a GSM module (the same size) with a built in GPS, I initially had difficulty dealing with this signal so I would prefer to avoid using this device.
The module I previously used, the GM862, draws 24mA at 3.8v normally and averages about 150mA during transmission.
The processor would be a small pic microcontroller from microchip, its power consumption would be negligible.
This is the important part, I would use a 3.7V lithium-ion battery, this would power the GSM module directly and a 3.3V regulator would be used for the GPS module, the GSM module has a built in battery charger so it will deal with battery management.
The GPS module will draw about 36mA at 3.3v, this works out out be 120mW of power.
The GSM module would draw 570mW assuming continual transmission.
These two together come to 690mW which can be rounded up to 700mW.
To run for 24 hours the battery would need to be at least a 4.5 Ah battery, this is large but not big enough to be impractical.
The biggest part of this system is the GSM module at 4.5cm square, the GPS module could be placed to one side of this and the voltage regulator and capacitor in the 1.5cm by 3cm gap next to it, the microcontroller can go on the other side of the board and the battery would sit flat next to the board.
The battery would probably be about 1cm thick when you connect enough up to provide the power required, the GPS and GSM modules with the circuit board are about 1cm as well.
The GPS antenna can sit at right angles at one end of the board, this would add 5-10mm to the length and should be within the 2cm depth of the system, the GSM antenna is a piece of pcb and can attach to the side adding negligible thickness.
Overall the system would be about 8.5cm x 5cm x 2cm.
It seems a personal tracking system would be feasible, I could make something that could be clipped to a belt or kept in a pocket, it would need to be charged every day but it would transmit its location in real time.
With technology as it is it's unlikely I would attempt this, maybe in a few years when the power consumption of GPS systems has dropped further or battery technologies have improved.