The modems Siemens TC35, TC35i, TC37 are mainly used for deliver/receive sms in embedded systems. Those modems requires a start sequence to power up. Let's take a look to datasheet:
VBatt+ is the main voltage on TC35 (on TC35i this is called only Batt+), located on pins 1-5 of the module. VDDLP is a secondary power source used for RTCC backup, located on pin 30. IGT is the ignition line, used to turn on the module, pin 15.
As you see from the diagrams, the module will no power up with only with VBatt+, but it needs a signal on IGT pin for at least 100mS at low level when you power up the main line. After the 100mS at low level, IGT line must go to High Impedance, but I've tried with a circuit that goes to high level instead High impedance without any damage (please try it at your own risk!).
The schematic is the following (thanks to P. Seminara of robotics-net.com):
WARNING! This schematic is only usable with a TC35 mounted on an adapter board that translates logic levels (5V <-> 3V)
When you power up the circuit (+5V), C1 will let pass some current (until it's fully charged). This current, through R1 will take T1 to saturation, closing collector with emitter and then on IGT line there is 0V. When C1 is fully charged (after about 150mS, RC constant of this circuit is of 147mS), T1 will be interdicted because C1 will stop the current flow and then the base of T1 will tied down by R2.
Actually, T1 will be interdicted when Vbe falls under 0.7V
In this condition the circuit betwewn collector and emitter will open and the IGT goes to high level thanks R3 (pullup resistor).
On the oscilloscope we'll observe this behaviour:
The Cyan line is the main power, yellow is the IGniTion line. After power-up, the IGT line goes to low level for about 130mS (time for C1 charging) and then goes to high level.
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