Learn about the SIM800/SIM900 auto power on feature

Why do we need auto turn on :

Let me explain why this auto turn on feature is important and where to we need this feature,

Sometimes in applications where there is no microcontroller present, that time we need the GSM module to boot up by itself and start functioning. One such application is a Auto answering device. This device’s job is to just turn on once the power is given and wait for incoming calls, and then auto answer the calls so that the person who is calling can listen to whats happening near the module through the mic connected to the SIM900 or SIM800 GSM module.
This is just one of the example where we need auto turn on feature. But the auto turn on feature is also useful wherever we don’t have a microcontroller to intervene and turn the module ON by pulling down and holding the PWRKEY pin to GND for some time.

How do we auto turn on the SIM900/SIM800 module :

The only way to turn on these modules is to press the PWRKEY pin to GND for atleast 1 second to boot up.
The way the PWRKEY pin is designed on the SIMCOM modules is if you pull it to ground for 1 second, it will boot the module if its already turned OFF.
and if you pull the PWRKEY pin to GND for 1 second when the module is already ON, it will shut down the module.

But to auto power on the module we need to connect the PWRKEY pin to GND permanently. Meaning, On power ON, if the PWRKEY pin is connected to GND then the module boots up. Since we are not making it high we don’t have the risk of shutting down the module again. So power key will stay low for the lifetime of the module operation. I tried it and it works. Go ahead and try it and comment below.

Hope it helps someone

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