Designing a Low cost GSM/GPS tracker – V2.1 Design Update – Part 4


Battery charger :

The TI battery charger i had used in Valtrack-V2.0 was supposed to deliver exactly 4.2V at the output for the GSM module to operate along with the entire circuit. But the device was way off from it and was putting out 4.38V whenever the charger was connected. The GSM module has a maximum operating voltage of 4.3V which made device enter into overvoltage shutdown. Over voltage shutdown and under voltage shutdown are a safety mechanism inside the GSM module which prevents it from operating at voltages outside its specified maximum and minimum voltage range. Since in my vehicle tracking application the device was supposed to be always connected to the charger source i.e vehicle battery , the device was always putting out >4.3V and i was unable to keep the GSM module running. As a work around i put a Schottky diode in series to the GSM module power supply which reduced the voltage to 4.1V from 4.3V. Schottky diodes have a very low forward voltage drop (around 0.2V for the one i used), which is what is needed here in my application. The normal Silicon diodes would give me a voltage drop of around 0.7V which would be very high for the application.

With this change the device was operating well and i had been doing the other development work on it. However i am getting rid of the TI battery charger in this design. 


Inductor change : The Inductor chosen in V2.0 was a 7.3 X 6.8 mm size part, which could be replaced with smaller 4 X 4 mm size inductor and still work and thats what i did in the new design. This will save me some real estate on the PCB top side.


Shrunken Board width : The old board had dimensions of 51.40 X 17.70 mm which was not fitting correctly into the box i had chosen, so i cut down the board into 49.4 X 17.7 mm  size which might help a little bit in fixing the device easily into the enclosure. 


SIM card holder change : The SIM card holder connector has been changed to a Nano SIM from a Micro SIM connector. This has helped a great deal in saving some space. The cost of both the connectors are identical, so no problem with the cost. Also i like the Nano SIM card it will make the product look more CUTE 😛


Layer count reduction : I desperately needed to do this, because the V2.0 had gone to be a 6 layer board, It did work well but the PCB cost made the device expensive when manufactured in low volumes. So we are reducing the layer count to 4 layers from 6 layers. We should be able to achieve this by doing track routing for power lines instead of  power planes. We also reduced the size of some resistors and capacitors and we also merged some connectors into one by making common ground and power connections. The SIM connector change helped in this to get some more extra space for routing. 


Additional MOSFET driver : There was only one MOSFET driver on board earlier, but now it will be two drivers which will help in controlling a Ignition control relay and also a Siren or a alarm device. 


Addition of Analog and Digital Input : I did add two inputs to the IO connector which will help in sensing a Digital input through a driver and the other input is connected to the ADC channel which will help in measuring analog voltages, as it may be needed in case of a fuel sensor. 


Watch this video for more details on the changes and the demo :

Stay tuned for more updates 🙂

Also see : 

 

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