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I'm trying to get a SIM800L module working with a USB-to-serial adapter using this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

(All of those resistors are 1000 ohm.)

I have a 5V 3A supply (connected to a plug socket), which I'm converting to ~3.8V with an LM317. This is then being used to power the SIM800L module.

I also have a USB-to-serial adapter (connected to a USB hub on my PC), where I've connected its serial lines to the SIM800L's serial lines, and also connected its ground to the power supply's ground.

When this is turned on, the SIM800L blinks its light every second (which I believe means it's on but hasn't connected to a network.)

If I open screen or an Arduino serial monitor (at 9600 baud) and send the AT command, it usually doesn't respond with anything. At best, it'll respond to AT a couple of times with OK but then stop responding entirely.

I'm aware that the SIM800L can require quite a lot of current, hence why I've chosen to use an external USB supply. I measure roughly the expected 3.8V across the SIM800L module, and a draw of about 55mA when first powered on, setting to 14mA after a few seconds.

Why is my SIM800L not responding to commands properly?

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    \$\begingroup\$ The LM317 is probably not up to this especially without capacitors. You are likely also operating it in dropout. Your measurements scheme us not capturing the pulse loads of actual operation. \$\endgroup\$ Jun 10, 2020 at 18:47
  • \$\begingroup\$ Many folks here will see a Fritzing diagram, and just move on. Consider replacing the Fritzing diagram with an actual schematic (which can be generated with tools on this site). \$\endgroup\$ Jun 10, 2020 at 18:48
  • \$\begingroup\$ 1. The LM317 requires a minimum difference of 3 volts between the input and output. 5-3=2. Your regulator isn't regulating. 2. Run the calculations for the output voltage as given in the datasheet. I think you will find that your resistor values are wrong. \$\endgroup\$
    – JRE
    Jun 10, 2020 at 18:58
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    \$\begingroup\$ Perhaps there is a voltage level issue for the serial lines? Maybe your USB/serial adapter is not compatible with the 3.3V levels from the SIM800L? What is the USB to serial adapter? \$\endgroup\$ Jun 10, 2020 at 18:58
  • \$\begingroup\$ What USB to Serial adapter are you using? \$\endgroup\$ Jun 11, 2020 at 12:09

2 Answers 2

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Making an answer from my comments:

I think you have a couple of small problems:

  1. The LM317 requires a minimum difference of 3 volts between the input and output. 5-3=2. Your regulator isn't regulating.
  2. Run the calculations for the output voltage as given in the datasheet. I think you will find that your resistor values are wrong.

You need a powersupply with a higher output than you have. To get 3.8V out, you need at least 6.8V in.

The correct values for the resistors can be calculated using the suggested circuit and formula given in the datasheet.

Circuit:

enter image description here

Formula:

enter image description here

Rewritten to solve for R1 when given R2 and the desired output voltage:

$$ R_1 = \frac{R_2}{\frac{V_o - I_{adj}R_2}{V_{ref}}-1}$$

Where:

  • \$V_{ref} = 1.25V\$
  • \$I_{adj} = 50 \mu A \$

Given \$R_2 = 2000 ohms\$ and \$V_o = 3.8V\$, I get R1 as 1020 ohms. My "eyeball estimate" of the resistors was wrong, and your original values were pretty much spot on.

Really all you needed was enough input voltage for the regulator to work.

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Seems there's a problem with powering the Sim module. LM317 doesn't work for this circuits. You need a low-dropout/high-current/adjustable voltage regulator for that. Try using RT9183 or MIC29302. By the way, you can also use a Lithium battery (3.7v nominal) directly connected to the module with no external component.

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