Hi
Can anyone can tell me if a L293DNE would need a bipolar transistor as isolation for the arduino.
I have a small 4V DC lego motor and also a larger 9v DC lego motor.
My previous circuits using motors have used this TIP120 to offer isolation for using seperate power supply with more current.
Thanks
Firstly you are talking about amplification, not isolation.
Isolation means two circuits have infinite resistance between them, and is accomplished
with a transformer or opto-isolator.
Here we are boosting logic signals to many amps to power motors - this is simply
amplification (aka buffering).
The L293D is a darlington output H-bridge chip capable of upto about 1A
without efficient heatsinking. It takes logic signals in and a 5V supply on
one side, and controls a motor fed from a motor supply on the other. grounds
are common (no isolation here).
The darlington transistors in the L293 are taking the role of the TIP120 that you have used previously: they take a small signal and amplify it. No additional isolation is required, as such.
However if your power-output stage is particularly risky, e.g. has moving parts near mains voltage or lots of stored energy that might blow up the L293, then you might want an opto-isolator between the arduino and the L293. Its only role is to protect the arduino (and your safety, presuming that user interface things like buttons are connected to it) if the L293 gets blown up or the load touches a dangerous voltage.
For a simple battery operated or otherwise low-voltage thing (lego motors), there's no point in using additional isolation. The L29x series are designed to be directly connected to 5V logic.