Hello,
I bought BT module BTM-112 working at 3.3V. Although Arduino has both 5V and 3.3V power output I need to switch between 3.3V and 5V, so I made a voltage divider from TTL 5V to CMOS 3.3V on serial link with R1=1.8k an R2=2.2k and the same divider on power line. Voltage divider on serial link works fine, but on the power line is low voltage. When I replace 2.2k by 3.9k (R1=1.8k, R2=3.9k leads to ground) is 1.7V? How is it possible, when Uout=R2/(R1+R2)*Uin? What values of resistor should I use?
The current draw will be constant, but I dont know its value. The maximal peak current of BTM-112 is 80mA...
Can I use max232 circuit instead voltage regulator (7803)?
Use a regulator or buy an Arduino like the pro series from Sparkfun that runs at 3.3v by default. The only issue there is using a proper programmer, i.e. one that won't blow out the Arduino in the process (3.3v FTDI cable, for example). One downside is that the Atmel CPU has to run at a lower speed to be stable per the manufacturer (Sparkfun uses 8MHz in general, though their openlogger clocks at 16MHz and still seems to work)
So if you're going to be passing signals back and forth between a 5V arduino and a 3.3v daughterboard, use a voltage translator chip rather than relying on resistor networks for the signal conditioning. Bidirectional voltage translators like those sold at Adafruit are your friend if you want to run 5V and 3.3V side by side. For $8 it's cheap insurance and guarantees performance. There are also other versions of this chip available to translate for I2C networks.