daemondust wrote:I don't have much of a formal electrical background, but why wouldn't it work? There's almost no current flowing into the RX pin.
As far as I know, the reason most people avoid resister dividers is because they're horribly inefficient if you're drawing any appreciable current.
Someone else here can speak up if I'm wrong.
The RX input is probabaly a high impedance input to an IC. Most likely it's just the gate of a fet, so when the device is started there could be a very small amount of current to charge the gate capacitor on that fet, and then current should drop to ~0. (I think actual current would normally be in the pico amp range or less, but I'd have to check some notes I have which are halfway across the globe from me). Thus a resistor divider should work just fine. I expect you can even use ones in the M range, if that extra ma is that essential to save.
The reason resistor dividers are not always the ideal solution is because they are most accurate when current draw is 0. But again, current draw here should be so minor that it should be, essentially, 0.
One thing though - you can probabaly get away with connecting the rx pin to a 3.3v line. I believe the whole EM-406 module runs off of 3.3V, so that should not be a problem at all, and it is unclear to me why they would reccomend 3.5V.