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UNTEngineer
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« on: November 15, 2012, 09:35:35 pm »
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Could I use the RSSI signal to determine signal strength? Can someone suggest a simple circuit that I can visually display the signal using a LED bar graph.
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oric_dan
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« Reply #1 on: November 15, 2012, 09:50:09 pm »
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According to my d/s, RSSI is a PWM output. You can filter this using a simple RC low-pass filter, and feed
to an LM394 or LM3915. They apparently still make these things.

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_24230_-1
http://www.jameco.com/1/1/2533-lm3915n-1-lm3915-dot-bar-display-driver.html

For ckts, do a google image search on LM3914 or LM3915,

http://www.google.com/imghp
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UNTEngineer
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« Reply #2 on: November 15, 2012, 10:41:35 pm »
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Well I have a PCF8574N. Will that work?
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oric_dan
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« Reply #3 on: November 16, 2012, 01:03:12 am »
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Ok, take at look at the chips I mentioned and see how they compare to your [what I presume is an]
I2C expander. You want a display, no?
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UNTEngineer
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« Reply #4 on: November 16, 2012, 01:42:04 am »
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Quote from: oric_dan(333) on November 16, 2012, 01:03:12 am
Ok, take at look at the chips I mentioned and see how they compare to your [what I presume is an]
I2C expander. You want a display, no?

I looked at the LM3916, and I realized the difference after I posted that. One is a multiple LED/LCD driver and the other is an I2C expander for an LCD.

What values would you suggest for the Low Pass Filter?
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UNTEngineer
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« Reply #5 on: November 17, 2012, 08:51:03 pm »
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bump!
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oric_dan
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« Reply #6 on: November 17, 2012, 09:12:35 pm »
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You need to know the PWM frequency out of the RSSI pin, and then select a cutoff frequency
roughly 10X lower, according to

F3db = 1/(2*pi*R*C)

Use R=4.7K or 10K as a starting point and calculate C accordingly. That's where I'd start.
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« Reply #7 on: November 17, 2012, 10:19:52 pm »
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take a look to dev interface schematic, were a lm339 is used to drive 3 leds (basic comparator) feed with rssi output through RC filter


http://ftp1.digi.com/support/images/XBIB-R-dev%20schematic.pdf
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Jack Christensen
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« Reply #8 on: November 17, 2012, 11:16:32 pm »
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I'm working on a project right now that does this. Connect the XBee RSSI pin to a digital input pin and use pulseIn() to read the PWM pulse width.

S1 or S2 XBees?
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