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RSSI / Lost Packet Indicator

When looking for a way to access an RSSI (Receiver Signal Strength Indicator) input, I came across the excellent Lost packet indicator article on the RCExplorer website, and was able to successfully tap into the LED output on the Futaba R617FS receiver.  Running the output through a 68K/10uF RC filter (as described in the article) results in a signal of about 0.18V when the red LED is off (100% RSSI) and 1.67V when the red LED is on (0% RSSI).

The platform I'm using (ArduPilot Mega with MinimOSD) expects an RSSI signal ranging from 0 to 5 volts, with 5 volts corresponding to 100% (good) RSSI.  So, I built an inverter/buffer circuit to convert the signal levels.  A "rail-to-rail" op-amp (like the TLV2371) is needed because it can generate output voltages very close so its 0 to 5 volt supply power.  Here is the circuit I came up with:

The 51K/12K resistor-divider provides a bias voltage to pin 3, which is compared to the input voltage going into pin 2.  The R1/R2 values of 1.5M/270K define an inverting amplifier with a 5.56 multiplier.  I did some trial-and-error testing of the values to come up with a circuit that was sensistive enough to respond to small dips in the signal while still have a full-RSSI-strength output near 5 volts.

I did a quick-and-dirty construction of the circuit by soldering the components directly together.  Here was my diagram for the physical connections:

I used a servo connector for the +5V/GND power supply, and M/F pins from red-JST connectors on the I/O wires.  Here's a mid-construction pic:

I insulated the bare spots with electrical tape and covered the works with shrink wrap:

Other receivers may provide an LED-output signal similar to the R617FS, and I've read that some of the FrSky receivers produce RSSI output as a 0-3.3V pulse train, so a circuilt like this with different component values could also work in those cases.  See here for an rcgroups thread about finding RSSI outputs on various receivers.

This circuit has worked quite well for me; it's really nice to have an RSSI display while flying FPV.  Here's a video showing it in action:

Recording of downlink video while flying my X550AIOP quadcopter via FPV in Belleville, Ontario. I was inside a brick house while flying, so lucky to get any range at all, but makes for a good demo of the RSSI (Receiver Signal Strength Indicator) display, the percentage at lower-center of the screen. The keycam video from same flight is here .

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