Grumpy_Mike:
While you can run it at 3V 3 it is:-
- running outside the specification for a 16MHz clock.
- you need to disconnect any 3v3 peripherals when you are uploading code because then the USB runs it off 5V
Could it be that you have fried the TX during upload?
Running the ATMEGA328 at 16MHz and 3.3V hasn't been a problem for the most part. For the Arduino Buono board I'm using this on, I think 3.3V/5V switch works for both the USB power and the AC adapter power...but thanks for pointing that out. I should verify.
fungus:
Yes, it could be sending 5V to the module.Reason: Arduino I/O pins have protection diodes on them. If you apply a voltage higher then Vcc to a pin it will go down the diode into Vcc, possibly raising the voltage coming out of other pins.
You need to put a resistor divider on the outputs of the ultrasonic sensor to drop it to 3.3V.
That's a good idea. Thanks. It's a quick modification. I'll give it a try and report back.