I have a question regarding the datasheets on the SIM900. Specifically, I wonder if I can replace a number of the circuits that the SIM900 manual is showing by substituting a TXB0108 voltage translator. For example, on page 45, there is a circuit showing how to attach a LED to the netlight port on the SIM900. The suggested circuit is shown below. .
However, I wonder if I could not send the 2.8V signal from the netlight pin through the TXB0108 and then use that 5V output to drive a 3mA LED (via a 2k2 resistor) in addition to signaling the netlight status to the Atmel MCU via a digital input pin. Similarly, on p. 23, the engineers at SIMCOM suggest a similar transistor setup as shown below to drive the power pin low.
Assuming I keep the 1k resistor in place, shouldn't I be able to drive the powerkey low to my hearts content using the TXB0108?
From what I can tell, the maximum total power draw the TXB0108 will tolerate is 50mA, with all the attached LED's I come up no higher than 20mA. So is this sort of substitution OK? For all I know, I am missing something very important...
The transistor and resistors are there to protect the SIM900 from sourcing too much current. You would still have to use at least 1 resistor to limit the current through the LED.
What it would appear is that the TXB0108 is for changing logic levels for interfacing with components that require different voltages for their logic to work - 1.6V, 3.3V and 5V devices. I would not think that it would supply enough current to power your LED, as it is a logic device, not a current source. You would still need the transistor and other components so you would gain nothing by using it. Transistors are cheap. Resistors are cheap.
I was hoping to use the TXB0108 simply because I can use one of them and take care of all 8 circuits leading to and from the Atmel MCU. The Sim900 runs at 2.8V logic level with a 3.6VDC input supply. I have had very good experience with the TXB series for logic translation like 5V-3.3V for SD card access. I was hoping would be enough power in this chip to allow all these LEDs to come on and off intermittently. Some of these lines are inputs, some are outputs re: the SIM900 chip. Oh well.
If you look you can buy transistors with built in resistors both leaded and SMT parts, NPN and PNP and with various gains too... about 15 - 25 cents each in small quantities.
Thank you, Doc. I'll try to find those integrated units, seems like a logical choice to offer them given the move towards ever-smaller / tighter circuits.
I dropped the LEDs, am using the TXB solely to drive the I/O. A strip header will still allow me to access the signals with my analyzer / oscilloscope, if necessary.
Looking online, I found pre-biased transistors that I believe you spoke of. Those integrated components certainly make life a lot easier. Thanks for the suggestion!
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