Need help in designing a GPS tracker - Battery and Antenna selection

Hi Ravi, I’m the one who email-ed you. I just want to ask something:

  1. As far as I know, the GSM module draw out maximum 1A (plus MCU’s current draw, the total maximum current needed would be really large) How do you sustain such a large current draw?
  2. The GSM module works at 3.7-4.2V and the MCU at 5V ? (as I’m not really sure) How do you provide enough voltage with just your 3.7 LiPo battery, I’m concerned about how long the device will last with just one piece of battery like that.
  3. How many UART are you using? Or you are using just 1 with a MUX?
  4. Correct me if I’m wrong but the GPS antenna should be in open space? If so, how can your device work if it it put inside a motorbike?

Hi Thai,

The GSM modules draw 100mA to 300mA in normal operation, but when an network intensive operation takes place, they draw around 2A in short pulses. So 1A is not the maximum draw actually. MCU draw hardly around 10mA in normal operation at full frequency like the STM32L series.
But you can use sleep modes of GSM modules where they draw only 1mA waiting for a call or a SMS or a wake up signal.
Similarly MCU can also be put into low power run modes where their consumption can go down to 100uA.
The large current draw is sustained by providing strong power sources like batteries or high current switching regulators like LM2596 or LM39302 etc.

The 3.7V batteries normally provide 3.7V to 4.2V. They reach 4.2V when fully charged and drop to 3.7V slowly as current is discharged to the load.
You will get MCU that can run at 1.8V to 3.3V also. Its wiser to use a 3.3V MCU as you wont need a boost circuit to provide 5V from a 3.7V source. Just use a 3.3V LDO to drop 3.7V to 4V battery voltage to 3.3V or whatever voltage is needed by MCU.
The device up time depends on battery capacity and current draw. My VALTRACK-V2 GPS tracker can run for 72 hours on single 1000mAH battery in SMS tracking mode with sleep enabled.

The MCU which i use has 2 separate UART’s. No mux needed.

GPS modules can catch signal through plastic casings also. The signal from satellites can pass through them to some extent. But, Just don’t use metal boxes.

Thanks for your promt reply

In my case, I intend to use the ATmega328 which can be run at 1.8-5.5V and at 3.3V of operating condition, it will run at a slow clock speed (up to 10Mhz) do you think it’ll be okay ? and I’m using 9600 baudrate with the GSm module
Do you recommend any other low power MCUs?
Thank you inadvance

You can use this one, no problem. Better to runt he MCU at 2.8V to 3V instead of 3.3V. Because SIM800 GSM module can take max 2.8V IO level. Some modules can take max 1.8V like SIM5360.
So choose a LDO that can give you 2.8V or 3V and run the MCU on it and the IO level of MCU will also remain within range, otherwise again you have to do Voltage level translation for UART and IO lines.
The lower the frequency lesser is the power consumption. You can run at 8MHz also, no problem.

Thank you.
One more small question.
You are saying that SIM800 GSM module can take up to 2.8V IO level, so how can it be interfered with by the arduino (for example) if I’m powering the arduino with my usb cable (which is 5V)?

If your GSM module board doesn’t have voltage level translator built in, then you should use level translators like TXS0104 or TXS0102 to convert GSM levels of 2.8V to 5V of Arduino and vice versa. Connecting Arduino pins directly to GSM without level translation will damage device.

A post was split to a new topic: Which Antenna to be used with SIM808? Internal or External?