If you want to buy bare PCBs of this project then have a look here: Surplus
You can download the schematic here
[…] the way that can be used with any microcontroller. From here, I want thanks to Ioannis Kedros from Embedded Day his support in the design of the battery charger, and to Sonia Muñoz, FAE from EBV, a great […]
You’ve made amazing work with your breakout board! Congratulations!!!
Excellent blog and amazing projects ! Thanks a lot for sharing !
I am really glad you like it! I can see that your blog is wonderful as well!
Πολύ καλή δουλειά!!!
Thanks a lot ! I am interested to find out more about your GSM project (especially the power-hungry module), however the dropbox link for the schematic is broken. Could you please check it ?
The link s fixed!!!
Thanks a million for sharing your excellent design ! I hope that you don’t mind a few questions and comments on your schematic. I am using the same charging scheme on my ongoing project (www.seismobug.com) and I would like to make it ready for future additions like GSM capability.
1) Why did you prefer 2.8V for powering the MCU instead of the usual 3.3V ? I suspect that this is because the GSM module TX/RX bus is rated at 2.8V, is that right ? Otherwise, is level conversion required ?
2) What is the max GPRS speed (kbps) of the module ?
3) The BAT_VOLTAGE sensing design is great. I am not using the filter capacitor C8 but I think I will add it. I am sending you my calculation sheet for your reference if you like (http://www.sendspace.com/file/1am1hz).
4) Since you are using load sharing between an external 5V source (e.g. USB) and a 4.2V battery, isn’t there the possibility for +Vgsm to exceed the maximum rating of the GSM module (4.2V) when USB is connected ? Or maybe D2 and D3 do the trick ?
5) Are C1 and C2 sufficient ? I read in another blog that it is better to use a minimum of 470uF.
6) Why do you drive the module RESET pin via a transistor (Q2) instead of connecting it directly to a I/O pin by enabling its internal pullup ?
7) I am currently using a 5V/1A source for my device. Is it sufficient to run the GSM module or should I raise it to 2A ?
8) Are there significant differences between your GSM module and the popular SIM900 ?
Thanks very much in advance and I would very much appreciate your help. Keep up the excellent work!
Questions and comments are always welcome! No worries about that! So, about your questions:
1) Yes, the primary reason that I am using 2.8V for the sensors and the PIC is because of the direct communication with the TX/RX pins of the Telit module. The secondary reason (and the most critical) is the handling of power. A system powered with lower voltage will stay alive longer (using the same design/battery) compared to another one with a higher voltage rating. On top of that during data/call transmissions of the Telit module you are going to have voltage drops on the power rails. A lower voltage with good decoupling circuitry will handle better those drops compared to a voltage more close to the power rail of the module. If you want to use a higher voltage then a voltage translator is more than fine. Depending on your voltage range maybe a voltage divider will be also fine but not a proper solution.
2) The short answer is something around: 16 – 24 kbps upload / 32 – 48 kbps download. In other words the theoretical speed of Class 10 GPRS that is supported but the GL865 module. The problem is that this rate is close related to the signal strength and the network the module is registered! For just data uploading and downloading this rate (even half of this rate or even lower) is more than OK for most embedded systems. To be honest I never benchmark the module (at least speed oriented).
3) Is always a good idea to use filter capacitors. In parallel with the C8 I will add and a 1nF cap. But this depends of your design. In general (or as a rule of thumb), if you have RF transmitting elements on a design then use good ground planes and filter caps everywhere.
4) No, the module will not exceed its maximum voltage rating in any case senario. The voltage drop on the diodes together with the P-Mosfet will do the job without any problem.
5) C1 and C2 are sufficient if you have the LiIon battery always connected. In any huge voltage drop the battery will supply the needed power. That means that your power line between the battery and the Vin pins of the module should be as short as possible and as wide as possible. The internal resistance of the battery also should be low (and this is translated to not a cheap $5 battery from eBay). Of course, 470uF caps will be even better but in SMD format it means a few more dollars per cap!
6) I am doing that because it is specified like that on the data sheet: “Using pull up resistor may bring to latch up problems on the GL865 power regulator and improper functioning of the module. The line RESET* must be connected only in open collector configuration”. Again it depends on your design. Sometimes, even if the datasheed was not specifying anything like that, its not a good idea to use the internal weak pull-ups of any micro.
7) Go with the 2A 1000% and use wide lines for the power rails!!! If you can and your design can support it then go even further to something like 2.5 – 3A. I am saying this because when we are saying 2A PSU we refer to the maximum power during the average load. A system with a GSM/GPRS module on board will have burst voltages. A 2A PSU (on average system load) will fail to supply your design during those burst phases.
8) Yes and No! It depends of so many factors. If you just want to upload a few bytes or even KB of raw data any module is OK. If you want 100% reliability then move away from Simcom and the popular SIM900 and stick with the Telit ones. If your project is a hobby one and is cost sensitive then go with the SIM900. I’ve test both of those modules and in any professional project I will use for sure the Telit without any second thought.
Have a look on this picture (http://farm4.staticflickr.com/3119/12975344665_0ba0ddb03e_c.jpg) for the same system with the SIM900 together with Temperature, Pressure, Humidity, Acceleration and Gyroscope sensors. This is still under testing and prototyping…
For an extremely compact project you can eliminate the micro and use Python and the internal micro of the Telit module…
If you have any other question then please let me know!
PS: Your SeismoBug is amazing!!!
Many many thanks for your detailed answers Ioannis !!! I really appreciate it, your answers are mostly helpful for my understanding of GSM requirements. Να είσαι καλά ! :)
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