Sim 800l suddenly unable to detect the Sim card

Hi everyone
So I purchased this bare sim800l module which only has pins and an LED and some resistors which means it has no voltage regulator or anything. For the past month I was able to use it with 2 different sim cards from different companies. I was building a software for it to work with stm32 MCUs. Yesterday I finally managed to send MQTT messages to hivemq so I let it stay on and send msges for some hours to make sure my software does not freeze. Then I decided to change the program a bit and after I flashed the program, the module stopped detecting my Sim card. Now it only blinks rapidly as if looking for network and my software reports connection errors.

I hooked it up to my PC, it responds to my AT commands but when I enter At + CIEE2 and then AT+CPIN? it responds with Sim not inserted. It does so with both Sim cards(both work OK on phone). The thing is I did not move the Sim card nor did I drop the module or do anything to physically move the Sim so the fact that it suddenly stopped recognizing them is strange and I tried both Sim cards which used to work with the same result.

I was using a 1600mAh LIPO battery the whole time to power the module(it is fully charged now) but I was connecting it to my MCU directly(3.3 volts) everyone says it is 3.3 v tolerant but here Ravi says 2.8 max? Maybe I fried something in there? I do not have a logic analyzer nor do I have a soldering station for SMD parts so I have ordered a new module, I just wanna know what I did wrong so as to not fry the new module.

Any kind of tips or suggestions would be appreciated thank you

Hi. If I was you I would try a more powerful battery, cause the current consumption of SIM800L can be up to 2 A in peaks. Another thing is that the voltages required for this module are 3.4 to 4.4V, so 3.3V can be not enough.

Thank you very much for your answer :slight_smile: but I think you may have misunderstood the problem. The battery than I have can provide 3.7 to 4 volts and when I checked it, It could keep powering my model for about 2 hours with no problem(could probably do it for a longer period I just didn’t check) while it was sending mqtt messages every second. Also usually when there is a problem with power, it will try to connect then restarts. This isn’t the case. The module is right here as I’m writing this, and it keeps continously blinking with no break. It really can’t detect any sim card.

Also when I was talking about 3.3 volts I meant the data from MCU. Arduino TX sends 5 volts, pcs also send 5 volts but stm32 sends 3.3 volts signals. The only thing that could be the problem in my opinion could be that 1:my software keeps restarting the module too fast and somehow damaging it. 2: the module is not even 3.3v tolerant(io pins) so maybe I should use some sort of a voltage regulator between micro controller’s TX and sim800l’s Rx and ground it is a very strange problem indeed :smiley:

You are welcome. Some people use a voltage divider between the RX pin of the module and Arduino TX, for instance R1 = 10K and R2 = 20K, but most people wire directly the module to arduino (RX to TX and TX to RX), and both configuration work fine. I wonder if the module takes care of somehow protecting the SIM800L chip, and gives the correct voltage to RX.
I had a problem as yours a couple of weeks ago, where the LED didn’t blink as expected (at every 3s), and I solved it by change the SIM card. But in my case the LED didn’t blink very fast, but at 1Hz (i.e. one time per second).

OK now that’s a bit embarrassing. I spent an entire day trying to find the problem I even went so far as to send an email to Sim Com support and it turns out that my Sim was inserted wrong. I had inserted my Sim in nano format whereas sim800 requires micro was working with no problem but then for some reason it didn’t. I inserted it in micro format and the issue got resolved.

So remember guys always go for obvious mistakes first, saves you a lot of time :smiley: