[Linux-ham] soundmodem

[R Radford]

Couple comments first:

    * RS232 voltage levels can be as low as +/- 3v and as high as +/- 15v
and still be in spec, so they are not always at 12v.
    * The BC237 transistor has an "on" voltage of 0.55v
    * the resistor sets the current at the base of the transistor and must
be at least the load current divided by the transistor gain (120 min, 600
typical)

So the 3.3v GPIO voltage of the RPi is at least as high as the minimum
RS232 voltage spec.  In addition, the voltage drop across the resistor is
3.3v - 0.55 = 2.75v, which gives a current through it of 2.75v/10k = .275
mA.  This means the transistor can sink up to 120 x .275mA = 33mA, or 165
mA with typical gain.

So check your radio to see the current sink required for the PTT, but I
suspect that 33mA will be enough, and that is using the minimum gain.  If
more is needed, simply drop the value of the resistor.

An op-amp is not needed, nor recommended in this design.
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