[quote author=Nick Gammon date=1433569419 link=msg=2264595]
Can you test your circuit by just hooking up to the +5V on the Arduino, rather than the Tx pin?
That is what did it. Now I am getting ~3.4v. Thank you, now I know I am not insane.
[quote author=Nick Gammon date=1433569934 link=msg=2264598]
Mathematically it is correct, so either the input voltage isn't what you think it is, or the resistors aren't what you think they are. Or the load is different.
I just loaded up "blink" onto my Uno. This doesn't output to the Tx pin at all. Despite that I measured 5V on the Tx pin. However loading it up with 33k load, it then reads 2.3V. So I am guessing you are not outputting anything on that pin.
The whole point of me doing this was me assuming that the Tx pin on the arduino was outputting 5v, which is too high for the bluetooth module's Rx pin (only can take 3.3v) that I am using. Do I still need this voltage divider I made even though it only yields ~1.4v when it is hooked up to the arduino Tx pin?
Looking at your image you have posted.
Gnd is connected to Row11, the resistor is then connected Row11 to Row15, the next resistor is Row15 to what looks like Row20.
Your yellow loop to the next resistor and the reduced output is connected to Row19????
Is that right? Or is the image to blurry?
Yea the image is blurry. The original picture I took was too large for an attachment : /.
[quote author=Nick Gammon date=1433571977 link=msg=2264623]
To explain the reading of 2.3V, even when the pin is input, I think you will find that the Atmega16U2 chip, which does the USB interface, has that pin set to input-pullup. That way it doesn't get garbage readings (and send them via USB) if the Tx pin is not configured as an output.
Now the spec for the Atmega16U2 shows that the input pull-up resistor value for I/O pins is in the range 20k to 50k, so let's assume half-way which is 35k. Plugging that into a voltage divider, along with the 33k load I used, plus the 1k resistor between the Atmega16U2 and the Atmega328P, you get an expected voltage of 2.5V. Pretty close to what I measured.
So do I have to configure the Tx pin as an output? I thought it was output by default.