SIEMENS TC35 GSM Development Board, GSM-TC35



GSM modem is a specialized type of modem which the wireless network is likely similar to our hand phone device. Now it can directly connect to our computer via COM port (serial port) and using the HyperTerminal (UART), we can communicate with it and make call, accept call, send SMS and also receive SMS.  Cytron now carry a SIEMENS TC35 GSM Development board, and I would like to talk about using it  with SK40C 🙂

Just a brief introduction. This board comes with following features:

  • Build-in with SIEMENS TC35 GSM module
  • 4.2V Voltage Regulator, rated 3A.
  • Proper level shifter for RS232 and DB9 for serial connection to computer COM port.
  • Jack for Mic and Earphone (Audio in and out).
  • High quality antenna for better signal reception and transmission.
  • LED and Buzzer as ring indicator (incoming call).
  • Ready with standard 2.1mm DC socket for DC adapter.
  • Power from 7 to 12VDC.
  • Push button for manual calling and accept call.
  • Extend out all TC35 GSM module IO for development usage.
  • 2.65V TTL UART pin is available for microcontroller interface.

Device picture:

Top view:


Bottom view:


Device short introduction



I am going to show you how to interface this GSM development board with SK40C and further we will send and receive some SMSs 🙂 Let’s look at the hardware list first.

Hardware required:

1. SIEMENS TC35 GSM Development Board


2. SK40C + P16F877A


3. LC04A


4. UIC00B


Software required:

• MPLAB IDE & HITECH C PRO Installation


Related References:

• Getting Started with MPLAB IDE

• Getting Started with SK40C

• User manual for GSM-TC35

• Project on GSM-TC35

• Tutorial on GSM MODEM


Getting Started

I have divided this tutorial into 4 sections and 1 partially developed application to demonstrate the possible usage.

  • Section 1: Basic calling
  • Section 2: Basic send sms
  • Section 3: Basic receiving call and sms notice
  • Section 4: Basic read sms
  • Real world application



Following photo show the interface (wiring) I did for this tutorial, from SK40C to LC04A and last to the TC35 GSM Development Board. I added five push button for mode activation purpose.



Please look carefully at the connections. This example shows the connection to SK40C or any microcontroller that have 5V TTL UART example Arduino series of controller. SK28A, SKds40A, etc. We need to make sure both the controller’s UART and GSM development board’s UART are at correct level, a level shifter is needed. I choose LC04A because it is quite easy to use. You will still need to power up the GSM development board using a proper power, I am using AC to DC adapter, 12V, 2A. Here are the connections from TC35 GSM to LC04A Low Voltage site, and from LC04A High Voltage site to SK40C:

VDD at P1 + + UART’s VDD
GND at P3 – – UART’s GND
T0 at P3 LR1 HR1 UART’s TX
R0 at P3 LT1 HT1 UART’s RX

Here is the simpler look schematic using Fritzing. The push button is push low logic.


If you wanted to connect the SIEMENS TC35 GSM Development board to microcontroller using the RS232 cable and the DB9 socket, remember to ensure both jumpers are placed correctly as shown. You will need to use a proper MAX232 board for the interface.


So that’s the interface, hopefully you get it right, we continue with the code on SK40C, or the PIC16F877A.

Section 1: Making Call



To make call

1. Send: AT<enter>     Reply: AT

2. Send: ATD<phone number> <enter>   Reply: (nothing) (actually calling to that phone number)


Section 2: Sending SMS




To send sms

1. send: AT<enter> Reply: AT

2. Send: AT+CMGF=1<enter>       Reply;AT+CMGF=1   (SWITCH TO TEXT MODE)

3. Send: AT+CSCA=<messengers services center number> <Enter>

Reply: AT+CSCA=<messengers services center number>(set number)

4. Send: AT+CMGS=<target number><enter>

Reply: AT+CMGS=<target number> (target phone number)

Reply: >

5. key in the messenger u want and send by sending as ending 0x1A

Reply: +CMGS: XXX


Section 3: Waiting Call and SMS



To receive notification

1. send: AT<enter> Reply: AT

2. Send: AT+CNMI=1,1,2;<enter>

Reply: AT+CNMI=1,1,2;

3. if call receive : RING        accept call ATA;      reject call ATH:

if messenger receive: +CMTI: XXXX     refer section 4;


Section 4: Read SMS




To  read message

1. Send: AT<enter> Reply: AT

2. Send: AT+CMGF=1<enter> Reply;AT+CMGF=1 (SWITCH TO TEXT MODE)

3. Send: AT+CMGR=N(N=which messenger) Reply: AT+CMGR=N



Simple Application:



Most protocol and instruction has been shown in above examples.

Please refer to the datasheet of TC35 GSM module for full protocol or User’s Manual of TC35 GSM Development Board.


Caution and Not Working?

  • There are lots of pins which pull out of the TC35 pin, wrong voltage will most likely kill the GSM module.
  • UART using 2.65V to receive data, provide 5V will damage the device in long term usage.
  • Please make sure the connection is correct. Especially the power and UART.
  • If there is still problem, please discuss in our FORUM.




17 thoughts on “SIEMENS TC35 GSM Development Board, GSM-TC35”

  1. My gsm module is sending SMS but mobile cant read it can u help me with it please ? sorry for my bad english

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  3. hellow…i want to know the connection between this gsm and ic max 232? if i using ic max 232,maybe i must using lc04a??

  4. hye there… i see at “+” sk40 to vdd sk40 and “-” to GND sk40.. but i not used sk40… i used arduino uno… at arduino not have vdd.. were i need to put the “+” sk40 to my arduino?

  5. Not directly, you can still refer to the code, but you cannot copy and paste in Arduino IDE and hope it can be compiled. They are not compatible. Anyway, is good to continue the discussion in our technical forum.

  6. hye… im new here…
    it is code for sending sms for tc35 gsm can used in arduino uno?
    because i want to connect tc35 with arduino uno..thanks..

  7. Hi Ameen,

    Yes, the connection is same but the programming is slightly different.
    Please continue discuss in our technical forum.


  8. Hi. i new to cytron..i already bought SK40C and PIC16F877A..all the component are completed.
    However, if i want to connect the PIR sensor using this kit, are the connection still the same with the PIR sensor project using SK28A?

  9. Please check the connection and check whether your SIM card have credit, also is the SIM card locked? Please continue in our technical forum where you can share photo of your hardware setup.

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