SIM900A power key

I bought a SIM900A module on Ebay but didn't pay attention for the presence of a power key pin.
It isnt there...

Looking at the datasheet there is a powerkey circuit available which also found on the actual module.
(i mean transistors R23, R25 and NPN Q2

Its all there but i am not sure how to make use of it. Google didnt help me either so does anybody have a hint how to solve this?

PWRKEY should be pulled low to power ON or power OFF the system. Just connect the PWRKEY to ground for a second or two to turn the module on/off,No need of Driving Circuit.

Try connecting the NETLIGHT indicator to know weather the system is ON or OFF.

I don't know your module, and if it gives you acces to the PWRKEY pin (what is "vcc_4" ? ) - It is not really needed, since you can power down the sim900 using AT commands and the module takes care of powering it on - But if it does, everything you need to know is here (pages 23 -> 27)

Download the Sim900A hardware datasheet which gives you lots of info, including the power key, which can also be turned off by software AT control.

Also, you need to be aware that the 900A module only works in certain Asian countries, its locked for those areas only.

If you are outside then you need a 900 module not the 900A, or you need to reprogram the 900A with the 900 firmware, lots of detail on the web if you need to do it.

Powering down with AT commands is easy but once powered down it wont be able to receive AT commands any more. So no powering up.

I am no electronics expert but i am learning and trying to become one :wink:
And i did read the datasheet (hence the picture in the first post) but there is just no pin on the board. All the components are there but no pin where i can connect my Arduino to.
I am trying to find out where i can solder a wire to make use of the powerkey.

Just got out my 900A module, which I have only used for testing, my project runs on a 900 shield.

As you can see from the pic below my board is v 3.8 and your is a v 4. and there are a few differences in the layout, plus the schematics do not always match the board, particularly in respect to the component number

What does see common is that Q2 and R23 R25 but on my board there is a spare connection point RST and RESTART .
The RESTART line goes to the base of Q2, on my board via a 4k7 resistor labelled R6 but on some pictures I have seen it as R2 and unpopulated.

So on my board I can feed a pulse in there but R25 had to be removed to allow it to work.

Looking at your Q2,r23,r25 r2 can be seen right next to that yellow connector, so I would check out that bit of circuitry.


Thanks Ricky, very useful info.

Looking at your Q2,r23,r25 r2 can be seen right next to that yellow connector, so I would check out that bit of circuitry.

Did you mean r2 or r24?

R6 is not present on my board. I shoud try to find out if r2 is connected in the same circuit. Maybe if i give the powerdown pulse on one of the sides of r2? (after desoldering r25)

I have a similar board as yours laying here but a SIM800A. A while ago i tried to make the restart pin work but de-soldered the wrong cap (R23) ::slight_smile: So close! When powering off now through AT command the board starts up again after half a minute or so.

I meant R2, but in respect of some other Sim boards I had seen on the web where this was the input resistor for the power on signal.

It may or may not have any relevance to your board.

Inputting a signal without knowing where its going is not a good method.

You should be able to trace out pin1 of the sim chip back to Q2 and R23/R25 and then see if there is any point leading to a siutable connection point for an input signal ?

If not / also, see this diagram from an Arduino Sim900 shield,

You can do something similar if you disconnect R25 from Vcc and feed either a manual switch or signal into the Powerkey pin.

Again without an edge connector pin available you need some delicate solder work.


..So close! When powering off now through AT command the board starts up again after half a minute or so.

Had this problem too, with a SIM808. Problem is that the power key of your board probably is a toggle, not a plain on or off. So if your board was on before your sketch starts it will switch off if you pull the power pin high, and switch it to low again, or if you keep it LOW.

The description of my board says that the RI (Ring Indicator) is high if the module is on, but it is always on, even with the module switched off. So, that signal is useless to sense the power status of the SIM module.

The SIM series have a pin "STATUS" that will tell the actual power status, but my board does not have it connected to a pin of the board, and I don't dare to solder an extra output.

My solution is to test at startup if the module reacts to a simple AT; command. If it does not there are two or even three possibilities: 1. it's OFF, 2. it's asleep if you enabled the slow clock and DTR is high. In that case the module sleeps, saves power, but does not have an active serial connection. To wake up a module in that state you need to pull DTR LOW to wake up the serial. 3. is a deeper sleep: AT+CFUN=0 switches off RF and SIM, so even less power is used.

If you did not enable slow clock mode, or switched the module to functionality=0 (to save even more power), and the module does not respond to an AT; command, you can assume the module is off. Do a power toggle using the PWR pin to switch ON the module. I use this:


I keep PWR HIGH normally, not LOW. If you keep it LOW the module automatically powers on, or off, as you experienced.

If you did enable slow clock (AT+CSCLK=1) pulling DTR to 1 will put the module to sleep, and switch off the serial connection. In that case to wake up you need to put DTR to 0 to wake up the serial connection before issuing the AT; command to test power status, else you may think the module is off, while it is only sleeping.

PS: you cannot power these GSM modules from an Arduino, they need a LiPo battery connected, else they probably don't start.

@ricky101 Please look at my question in some similar context.

Thanks !

@ricky101 Please look at my question in some similar context.

He hasn't had anything to say here for some two and a half years.

Likely isn't listening! :astonished: