View Full Version : Flipping a circuit?

01-27-2012, 07:59 PM
Not sure how to even word the question, but the xbee turns on it's PWM0 pin when receiving. I would rather it light an led when not receiving as a warning you have moved out of range.

What kind of circuit would I need to flip that? So that when that pin goes low it causes led to come on and when it is PWM the pin, to keep the led off.

Phil Pilgrim (PhiPi)
01-27-2012, 08:11 PM
That's the RSSI output. If you want to do something really cool with it, consider using a dual-color red/green LED (common anode). The green segment gets driven directly from the RSSI output, through a resistor. The RSSI output also drives a transistor or logic inverter, the output of which couples to the red segment through a resistor. What you end up with is an LED that glows red when there's no signal, gradually changing to yellow, then green as the received signal strength increases.


01-27-2012, 08:30 PM
Or simply use a hex-inverter (http://search.digikey.com/us/en/products/74VHC04MTCX/74VHC04MTCXCT-ND/965533).

01-28-2012, 12:58 AM
Turn the LED around and tie it to the +(Vdd) instead of -(Vss)
When the I/O pin "PWM0" goes low,It will light the LED.
A logic value of 0 does not necessarily mean the pin(PWM0) is at or near - voltage(Vss), Nor does a logic 1 mean at or near +(Vdd)---Measure the voltage at PWM0 when you know its at logic 0 to make sure if 0 is - or + with respect to GND (Vss) if your not sure.
I like Phil's idea of a bi-color LED.