The RSSI device is built around the famous LED bargraph driver IC LM3914. The radio IC CXA1019S when run from 3V supply generates a variable voltage of 0.23V to 3V in its METER pin (pin 20). In this LM3914 circuit R2 and R1 was so chosen that the all the LED are OFF when METER pin at 0.23v and all LED are ON when METER pin at 3V. Now this design is functional, but it has a drawback, this I found out later when the circuit was actually constructed. The radio IC generates 3V in its METER pin when there is no received signal and 0.23V when there is full receive signal and anything in between 0.23V-3V when signal is fluctuating. That means the relation between received signal strength and METER pin voltage is inverse. Due to this the RSSI indicator shows all LED ON when there is no signal and all LED OFF when there is strong signal. This is kind of weird, but I got used to this. Another problem with this circuit is that each LED consumes around 5mA of current, when all LED glowing the total current of RSSI circuit exceeds 50mA. For an RSSI indicator the power consumption of 50mA is too much. The workaround is to use a series resistor to each LED to reduce the LED current to 1mA per LED. This will make max power consumption to approx 10mA peak. Final value of R1=12K and R2=20K linear POT.
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