Trying to understand electric basics ...


I've reviewed my electric lessons, but i have some difficulties understanding the schematic at

It's said: "Finally, we have to connect the pin 6 in arduino with pin 9 in XBee, since Arduino is working with 5v and XBee with 3.3 V, we shouldn't connect them directly, we are going to use a resistor bridge to adapt these levels (see image)."

I really don't understand how sensor-networks has established that a resistor bridge with 10k and 15k resistors will do the trick

Voltage from arduino pin 6 (or 7) is a fix value (isn't it ?) 5V, and voltage for xbee pin 9 is a fix value (isn't it ?) 3.3V

But about current, what it is ? I thought it was 40mA from pin 7, but it's complete non sens: 5 V = 10k * 40mA + 3.3, it makes 5 V = 403.3 V, something wrong...

I've done a pastebin for naming current and voltage (I goes from up to down)

Spec for Xbee is at and information on xbee sleep mode is at page 22 (section 2.5.3)

Any idea to how have been calculated those resistors is welcome...


When he says "Resistor bridge" he really means voltage divider (soemtimes called potential divider). Try to google voltage divider and it will all be clear to you.