Small Breakout for SIM900 GSM Module

Posted on Feb 5, 2014

The printed circuit board, has a pin strips (10 pin x 2) used to connect with the circuit board of the remote control device. The pin 1 (ON/OFF) is used by a microcontroller to turn the GSM module on and off. The module is turned on or off according to the logical level applied to their PWR line (pin 1 of GSM1).

Small Breakout for SIM900 GSM Module
Click here to download the full size of the above Circuit.

The PWR line is equipped with an internal pull-up resistance and is active at logical zero; therefore, in order to switch the cellular module on, the microcontroller sets PWR at a high logical level (contact 1 of the pin-strip) and causes transistor T2 to saturate; this transistor will then sets the PWR line at a low level. Remember that the power on or off signal is only a pulse of about 2 seconds. Reset monitoring is handled similarly. The module can be reset by simply causing the microcontroller to send a logical 1 via the RST line, at which time the T3 transistor saturates and sets the NRESET line of GSM1 at a low level. Let`s now proceed with UART`s control lines (i. e. , TXD, RXD, DTR, RTS, CTS, DCD), which are connected to the external area through the pin-strip`s contacts, respectively, 12, 14, 10, 2, 4 and 6. The audio device, with two contacts for the microphone (with differential input) and two more for the loudspeaker, uses contacts 15, 13, 11, 9, which correspond, respectively, to MIC1P and MIC1N (positive and negative of the microphone) and SPK1P and SPK1N (respectively, positive and negative of the loudspeaker). The RI signal (indicating incoming calls) goes out through contact 8 of the pin-strip. Let`s now talk about transistor T1, used here to locally control the cell phone`s reception LED: its base is polarized from the current logical level on pin 52 (NETLIGHT) for GSM1. The transistor`s collector is...

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