Can I use the circuit provided here ?

[milan.rajik]

Can I use the circuit provided here ? Will this work. I have seen some poor quality SIM900 boards which give garbage data sometimes on UART lines.

I want to make sure if this board works. I will be making some modifications. The board will have only connections for

PWRKEY
nRESET
RF Ant
Net Light
RxD
TxD
GND
Vcc
SIM holder

The same components mentioned at the above page will be used but I need it to work from a 3.7V Li-Po battery and the Vcc has to be 3.3V. So, can I use a LM1117 LDO regulator to provide 3.3V from 3.7V ? Can LM1117 SMD version can provide 2A current needed while sending SMS ? If it is better to use any other kind of power conditioning circuit then please provide a sample circuit.

I will also be using 3.3V interfacing method.

https://www.open-electronics.org/small-breakout-for-sim900-gsm-module/

If anybody can provide a better all SMD circuit with component list then it will be helpful. I want to integrate a PIC32MX270F256D TQFP chip with SIM900 and make a small board for SMS alerting system. There will be a Li-po charger circuit on-board and also I need a circuit for providing 3.3V from 3.7V 6600 mAh Li-Po.

I am planning to make the board not more than 6.5 cm X 4.5 cm x 1.6mm.

- - - Updated - - -

Can I use the DC-DC power supply provided here http://electronics.stackexchange.com/questions/123240/powering-sim-900-gsm

to provide 3.3V to SIM900 from 3.7V Li-Po ?

 

[ud23]

can I use a LM1117 LDO regulator to provide 3.3V

If you read its datasheet you can get your answer.It is no because LM1117 LDO comes with 800ma output. And other thing you are just designing the supply according only sim900 also consider the other device power consumption too.So I think you need at least 3.3v and more then 2amp ldo.

For choosing the LDO in battery operated device it is always need to choose LDO which has low quiescent current for lesser battery charge loss.

You can use booster IC to convert the battery voltage to higher level to provide it to the LDO input like lm2576 with 3amp output.

Or just look at below link that might help you.

http://www.atmel.com/Images/Atmel-42212-SAM-D20-GPS-Tracker-Reference-Design_AP-Note_AT03821.pdf

 

[milan.rajik]

I somewhere read that DC-DC converter is only good when there is a large difference between IN and OUT voltages like if IN V is 12V and OUT V is 3.3V and I also read that if there is not much difference between IN and OUT voltages then LDO is a good choice for voltage conversion.

So, In my circuit there will be a 3.7V Li-Po battery and it will be always connected to a charging circuit. The battery terminal voltage while not charging can be 3.7V and while charging it can be 4.2V or 4.5V. So, what LDO or better thing can I use which convert these (3.7V, 4.2V, 4.5V) to 3.3V. The LDO or any other device should be able to deliver 2A burst currents required by SIM900/900A while sending SMS. The PWRKEY and nRESET pins of SIM900/900A will be controlled my PIC32MX270F256D. The SIM900x will be OFF and only turned ON when accelerometer triggers the PIC. An ADXC335C is used to detect vehicle movement. If detected it will power ON SIM900x and sends alert SMS. So, SIM900 will be OFF most of the time. When vehicle is parked a button has to be pressed and this will start the ADXL335 monitering. Once the system is activated by pressing the button then only way to deactivate it is by sending an SMS.

Can I implement any better method to deactivate ? Like if activated it waits for 3 vehicle movement triggers and if there are more triggers then it will send SMS. This way if there is no vehicle movement and then owner wants to retreive the vehicle then he can press the button and deactivate it. Is this a good idea ? The unit will be fixed below the seat of vehicle and only if the seat lock is broken then the system can be deactivated by anyone. Is this better ?

 

[Surendhar M]

Better dont use LDO for this purpose. Use any switching regulator which will give you 3A or use a capacitor of 2000uF as a charge pump in your design which will be able to provide the burst current which is required for you and also dont try with battery I dont think battery will able to rovide the 2A current required.

 

[milan.rajik]

For another product I planned to use mikroElektronika GSM2 Click which also need 2A burst current while sending SMS and while registering to network. So, I asked whether I can use 3.7V 2000 mAh Li-Po for the project and they said that the Li-Po can provide the required current but now I am using a 3.7V 6600 mAh battery. Whay do you say that it will not provide 2A burst current. The current is drawn only for a small time. The SIM900/900A will be off most of the time as I mentioned. Only if vehicle movement is detected it turns ON SIM900 and then sends alert SMS and then waits for say 10 seconds and if again there is no vehicle movement then it turns OFF SIM900 power. So, only PIC32MX 3.3V device will be running. The battery can be charged once a week. I can't eleiminate batteries. I can't use vehicle power to power the unit. The system has to be failsafe and hence battery is used.

I need to know what other methods I can implement to decrease the power consumption.

 

[ud23]

https://electronics.stackexchange.com/questions/31083/how-to-power-a-gsm-modem-from-a-li-po-battery
**broken link removed**

 

[milan.rajik]

Ok. I will read. I have one month to decide the bare minimum SIM900A circuit, Li-Po charger circuit and Power supply circuit. Battery is finalized. It will be Li-Po 3.7V 6600 mAh. I changed the PIC32. Now it will be PIC32MX534F064H the same used on mikroE MINI32 board. It has USB feature and costs the same as PIC18F4550. I will use one USB port which connects to Li-Po charger circuit and also PIC32MX USB communication lines. mikroE provides free bootloader for this device and also the free mikroE HID bootloader software. So, I will be providing option to update the application firmware from USB port. A 5V 500 mA charger or USB power can be used for charging the battery.

The board size and approx unit size dimensions will be 6.5 cm x 4.5 cm x 3 cm. An aluminium enclosure will be used and an optional rubber jacket will be provided which can cover the USB port and switch. This is to make unit water proof. One push-pull switch will be used (similar to one used in seeedstudio DSO Quad). Pull it to right activates 2 minute timer, pull again to right within 2 minutes stops the timer. After 2 min timer expires system will be active.

Detect more than 3 movements then send SMS.
Once the system sends SMS then it can only be deactivated by sending SMS from owner mobile.

ADXL335C is used. Is there a way to detect vehicle fall from G's of accelerometer. For example if vehicle falls then I don't want to consider it as a movement and want to discard it. Is there a way to detect vehicle movement like tilt etc... from G's obtained ? The accelerometer will be placed horizontally when the system is active.

 

[Surendhar M]

Run with battery itself.Battery spec 6600mAh means it wont able to give you 6.6A actually it meant for charging rate, battery supply current will be different. As it is burst current it draws heavily thats the reason I have suggested you to use high rating capacitor as a charge pump. Try to maintain the battery voltage to the level which you have specified. Keep on charge the battery. For 3.3V use a switching regulator which will be able to provide the high current which is required for your application. Check in TI you will find lot of parts.

 

[asdf44]

I somewhere read that DC-DC converter is only good when there is a large difference between IN and OUT voltages like if IN V is 12V and OUT V is 3.3V and I also read that if there is not much difference between IN and OUT voltages then LDO is a good choice for voltage conversion.

So, In my circuit there will be a 3.7V Li-Po battery and it will be always connected to a charging circuit. The battery terminal voltage while not charging can be 3.7V and while charging it can be 4.2V or 4.5V. So, what LDO or better thing can I use which convert these (3.7V, 4.2V, 4.5V) to 3.3V. The LDO or any other device should be able to deliver 2A burst currents required by SIM900/900A while sending SMS. The PWRKEY and nRESET pins of SIM900/900A will be controlled my PIC32MX270F256D. The SIM900x will be OFF and only turned ON when accelerometer triggers the PIC. An ADXC335C is used to detect vehicle movement. If detected it will power ON SIM900x and sends alert SMS. So, SIM900 will be OFF most of the time. When vehicle is parked a button has to be pressed and this will start the ADXL335 monitering. Once the system is activated by pressing the button then only way to deactivate it is by sending an SMS.

Can I implement any better method to deactivate ? Like if activated it waits for 3 vehicle movement triggers and if there are more triggers then it will send SMS. This way if there is no vehicle movement and then owner wants to retreive the vehicle then he can press the button and deactivate it. Is this a good idea ? The unit will be fixed below the seat of vehicle and only if the seat lock is broken then the system can be deactivated by anyone. Is this better ?

Linear regulators have an efficiency that's dominated by the input to output drop. Basically efficiency is Vout/Vin. In the examples you give thats in the 70% territory. A DC-DC would be better (if you operate it within it's high efficiency areas), probably ~90% for an off-she-shelf module.

In your application 2A would be a problem for a linear regulator because it would be well over a watt. Even a D2PAK/TO220 doesn't sustain 1.5W without rising an unreasonable amount. You could convince yourself that the bursts are small and that you can get away with it, but it's pushing things.

The DC-DC is more efficient and shouldn't have the thermal issues. It may also be smaller because it doesn't need to be sized to dissipate the heat. I'd go on digikey and find a small integrated DC-DC solution. Tons of them exist for those voltage ranges.

Though warning: doesn't a discharged lithium cell drop below 3.3? Double check that and the minimum input of whatever regulator/DC-DC you chose. I think you may have an issue there. Though the good news is that you're not the first person to try and get 3.3V from a lithium cell. Google pre-made solutions.