Electronic – Is it safe to run these motors with raspberry Pi directly

From the software page I can follow what the python is doing but I don't see where it says that pin 18 on the raspberry GPIO should be enabled as PWM? The docs say that Pin 12 supports PWM ? How does this work?

Not knowing the specifics of the RPI GPIO Library that Adafruit uses, but looking at the page you link to, it enables it with set("mode", "pwm"). As it says, The Python program first sets-up the two GPIO pins to be outputs. It then defines the same convenience function (“set”) that we used in Lesson 8, to write to the PWM Kernel Module. This is then used to set the parameters for PWM. The RPI hardware PWM is enabled through that library.

What if I didn't care about speed? Can I just use an regular GPIO pin for output a high on the enable of the L293D? With the Pi I'm assume that would out a 3.3v? And if I'm looking at the datasheet correct the enable pin can take upto 7v as a high? (What speed would it run on?)

Yes, you can do that. It would run at 100%, because you simply toggled the pin on. The voltage at the pin is not what sets the speed, but simply how fast you turn the pin off or on in a given time period that does. Like flipping a light switch on and off fast enough that it seems like the lights are only half on.

Also again, if I didn't care about speed control, I could hook another GPIO to the second enable and control another motor also right. And I can then control each of they directions?

Yes, but you would actual need three gpio to control both directions of one motor. 1 for the motor enable (on/off), and 2 for the motor 2 direction. Exactly as it is now, except without pwm.

There are a couple of issues with the proposed design:

1. The 4n35 is an optocoupler, not a regular transistor. While the output side can conceptually be treated as a BJT, the maximum collector current rating for it is 100 mA, which may be insufficient for driving multiple motors, or even one motor, depending on the motor's current requirement.
   • For the switching element, use a Bipolar Junction Transistor (BJT) such as the 2n2222 (800 mA maximum continuous collector current) or a logic-level MOSFET such as the IRLML2502 (3.4A @ 4.5 Vgs, at least 2 Amperes at 3.3 Vgs) instead. Depending on the motor current requirement, and the number of motors to be run, you may need a higher current rated BJT or MOSFET.
2. The NetDuino IO pins output 3.3 Volts, with an absolute-maximum current rating of 25 mA per pin, 125 mA total. Even if the 4n35 is to be used for optical isolation, which is its purpose, adding an indicator LED in series with the input of the 4n35 is not a good plan, as the voltage drop for the LED (e.g. 2.2 to 3.4 Volts) will leave too little headroom for the 4n35 internal IR LED (~1.2 to 1.5 Volts).
   • Using the LED in parallel with the input too isn't a good idea, since the current for the 4n35 (e.g. 20 mA) and for the LED (e.g. 10 mA) will add up as the load on the IO pin, thus loading the Netduino's GPIO pin beyond its rating.
   • In other words, either drive your indicator LED from a separate GPIO pin, or move it to the output side.

A high-level solution, leaving you to work out the specifics:

^{simulate this circuit – Schematic created using CircuitLab}

• (optional for isolation) Drive the 4n35 input from the GPIO pin with a series resistor to limit GPIO current to under 20 mA
• Use the output to drive a MOSFET (or use a BJT with appropriate biasing if desired)
• MOSFET (or BJT) as switch to power the motor(s).

Note that the motor will be on when the netduino output is low, and vice versa. The circuit can be reshuffled to eliminate this inverting of logic, if required.