Hi, lets talk about some IoT. I have found a very useful module which is working as a combo module for GPRS and GPS and its a complete positioning module just need few interfacing designs, that i am going to discuss in the following blog of mine. The blog contains some very rare design considerations that you’ll find tough if using it for the first time. Firstly let me introduce the features I found in this module.
SIM808 is integrated with a good performance GSM/GPRS engine, a GPS engine and even a BlueTooth engine. The GSM/GPRS engine is a quad band GSM/GPRS modulethe works on the frequencies GSM850MHZ, EGSM900MHZ, DCS1800MHZ and PCS1900MHZ. SIM808 is capable of GPRS multi slot class 12 and supports GPRS coding scheme CS-1, CS-2, CS-3 and CS-4. The module integrates a GPS engine that has an aquisition of -147dbm which is a pretty attractive sensitivity at cold start. People have a misconcept that GPS doesn’t work good with GPRS in combo module but i have tested it and it is working nicely for me. In short its reliable for GPRS and outdoor GPS. Have a look,,.. 🙂
SIM808 Hardware Design
- Used for ultra low consumption requirement. Like Gas meter. Petrol meter, water meter. Which might change battery only once per year.
- Purple line for RTS and CTS is optional if not used, like for small data transmission.
- the diode and 10k Resistor forms current leak protection circuit. One side, it can decrease the influence from MCU to SIM808 module.
SIM808 Module-Microcontroller Interface
The communication between SIM808 module and microcontroller is such a pain when you have to use a level converting circuit. Our SIM808 Module works on a voltage level of 4 Volts and internally it gives a 2.8 V signal while the microcontroller you may use can be of 3.3 V Vdd or 5 V Vdd as in Arduino. The schematic for both the microcontroller cases is shared below.
3.3V microcontroller <-> SIM808
5V microcontroller <-> SIM808
Power Supply For SIM808.
The SIM808 feeds on 4 Volts and may require upto 2 Amperes Current burst while initiating GPRS engine. So it must be taken care that the Power Supply for the SIM808 should be capable of producing 4 Volts and 2 Amperes of current. If you want to make a mobile IoT device, you should go for a Li-Ion Battery of typical 3.7 Volts as in any normal Mobile Phone. And if you want to connect it to Adapter power source you should go for an Adjustable Buck Converter to emit 4 Volts. both of the circuits are below.
- For better power performance, it is recommended to place a 100uF tantalum capacitor with low ESR (ESR=0.7Ω) or 470uf aluminum capacitor and ceramic capacitors 100nF, 33pF(0603) and 10pF(0603) close to the VBAT pin.
- Place a zener diode close to the VBAT pin for ensuring stable input voltagee.g. 5.1V/ 500mW (MMSZ5231BT1G)
- For PCB layout:
- To ensure less voltage drop during a transmit burst. The width of trace should be no less than 2mm. The longer route, the wider trace.
- Keep away from the RF trace. The VBAT line should be isolated from the RF line with ground if these two lines are closed.
- Power supply range of SIM808 module: 3.3V~4.6V
- Typical supply voltage of SIM808 module: 4.0V
- Desired high current capability of power supply: 2A (at least)
Sometimes you may get trouble with the resistance divider circuit, try using smaller resistance on the basis of following formula R1 = R2 x (( Vout / 1.240 ) - 1 )
GSM Antenna Considerations.
The next big consideration you need to use is a GSM Antenna for Cellular Communication. GSM antenna you can use a Surface mount Antenna or an external antenna whatever you like. Make sure that the traces to the Antenna must be straight and the impedance should be calculated to 50 Ohms. I’ll explain you how to calculate impedance of traces on my next blog, but for now you can keep the width of the trace 21.54 mils and ground planes cleard with 13 mils, only if you are using a standard FR-4 PCB with copper thickness 0.35 microns/1 Oz. Its a calculated trace width for 50 Ohms impedance you can apply it in your PCB Design and enjoy a good cellular signals.
GPS Antenna Considerations.
GPS Antenna also needs same 50 Ohmic impedance so you can use the same design as in GSM antenna. If you are planning to use an Active Antenna with your design, you need to power the LNA of the Antenna, while i was using it for the first time i didn’t knew that the LNA of an Active antenna will need an external power supply to work, i studied on it and implemented a bias Tee to power it up following is the Bias Tee for powering up the Active Antenna. Cap here I used is 100pF and inductor 270nF works fine as 50 Ohms impedance.
Rest of the things rely on how beautifully you play with the AT&T commands of SIM808, that i’ll explain in my upcoming blogs, till then Happy Designing 🙂