One of the most useful features of FrSky radio would be telemetry. This radio protocol is used to transmit the in-flight data from the radio receiver to the radio transmitter (so literally you have a 2-way communication radio system).
D4R-II is a popular Frsky receiver often used in mini quads. It’s compact, light weight as well as having PPM and telemetry support.
Frsky Telemetry – RSSI and LiPo Voltage
FrSky telemetry has an impressive list of features and is capable of sending various types of flight information. As an example, we will show you how to set up telemetry to receive RSSI (Signal strength indication) and LiPo battery voltage readings, and display them on your TX screen.
Some might ask why use telemetry when Lipo alarms or OSD are available. Lipo alarm must be attached to the aircraft battery balance plug. If the quadcopter is a long way from you, you might not hear it when the alarm goes off. OSD is good, but having telemetry as well you have an extra layer of safety, not to mention it can warn you when something reaches critical level by beeping on the TX.
In this example we will use:
- FrSky Taranis X9D radio transmitter
- FrSky D4R-II radio receiver
- Naze32 Flight Controller
Also check out this Taranis Custom Sound Track tutorial.
Connection Setup For Telemetry
In order to receive voltage information from the battery, Naze32 needs to get that information from the battery voltage monitor pins (VBAT) first.
According to the Naze32 manual:
“Optional Battery Voltage Monitor
To enable in-flight battery voltage monitoring and alarm, connect this header to flight battery or power distro board. Up to 25V (6S LiPo) can be measured. No reverse polarity protection – connecting battery in reverse will instantly destroy the hardware.”
What we need is a servo cable and solder one end to the power distribution board or power harness. This connection will get voltage reading from the battery to Naze32. Correct polarity is very important here! You can also setup low battery alarm in Baseflight/Cleanflight, by enabling the VBAT feature, that’s how the Naze32 knows when to start the low voltage buzzer alarm.
D4R-II receiver has an external analog telemetry port. It comes with a cable composed of four wires – black, white, red & green (in this order).
We will use black and green wire only, which is ground and RX. White and red are used for other telemetry information that is no relevant to our setup here. They can be removed completely ,cut short or simply folded for future use if you want. The D4R-II receiver comes with two sets of these cables: one with two male connectors, and other with just one connector and open end wires ready for soldering.
Green and black wires can be soldered to a servo cable with a female end, and that should be connected to the telemetry port on the Naze32. The very important rule:
- Green wire goes to “+” on Naze32
- Black wire goes to “-“ on Naze32
With all cables connected correctly, Taranis should be ready to receive Telemetry information from the RX. Note that Telemetry will only work when the board is armed.
Taranis telemetry screens setup
On the radio itself we need to go through the configuration setup in order to:
- display correct voltage information
- alarm on low voltage
- warning on low RSSI signal.
- display additional information (Altitude etc.)
On the Taranis, when we are in the model menu, it takes us to the telemetry screens by holding down the PAGE button. There are three screens and we can customize how the telemetry information is presented and displayed.
In above examples I setup Screen 1 to show numeric values of Total lipo voltage level (Cells), Per Cell and RSSI Signal Strength. (Apart from telemetry, it’s also possible to get RSSI value via a spare PPM Channel)
Screen 2 was setup to show the same values but with a graph (bar chart). It has the low value set to 3.3V and high to 4.10 (which is 100% of the battery voltage). Tmr1 is a timer which is set to 7 minutes, which is my normal average flight time.
Parameters used in telemetry screens are:
- Cells – displays total voltage of the battery
- Cell – displays Cells divided by a number of battery cells.
Voltage level is detected by the Naze32 of the whole LiPo battery, so it is impossible to get voltage of each single cell. Cell value is used for setting voltage alarms on the radio.
Configuration can be done using radio screen or CompanionTX software (available to be downloaded on the OpenTX website), which allows you to upload settings to the radio from your computer. Setting it up directly on the TX takes longer, but you don’t need a computer. Using Companion app is much easier in my opinion, due to the nice and user-friendly interface.
A1 and A2 are the analog telemetry ports on the receiver. On D4R-II we are using A1. First we need to setup the range. The receiver’s maximum input voltage used for telemetry is 3.3V. Internal voltage divider is 4:1. Maximum voltage reading for A1 is 3.3×4=13.2. In my setup I use 13.3V as this is recommended. Detailed video on telemetry voltage calibration can be found here.
Low battery alarms are a matter of personal preferences. In general we don’t want the battery to go completely flat during flight so setting low alarm to 3.5V – 3.7V is sufficient. Use whatever values that you feel should give you enough time to fly back and land safely. Critical alarm means that at this stage, we need to land immediately, otherwise the quadcopter will be likely falling out of the sky due to lack of power.
Taranis telemetry “talking” voice setup
One great feature of the Taranis is the custom voice capability, you get a lovely lady voice talking to you. Taranis radio can return us the information by “talking” to us while we are flying, so we don’t need to look down on the LCD screen. We can configure the radio switches to report battery voltage, or configure it so it warns us when RSSI reaches low or critical level and so on.
My “talking” custom voice setup has three scenarios:
- Taranis tells me when the battery voltage goes down below 3.44V.
- After the board is armed, Taranis reminds me every 60 seconds what the battery voltage is.
- When SH switch is toggled, Taranis tells me the current battery voltage.
In order to do that I set Logical switch L1 with value Cell set to 3.44V. In special functions I have created SF1 that is constantly looking if L1 switch is true to play the sound batlow (battery low).
Created also SF2 to tell me current voltage when switch SH is pressed down. Switch SF3 and SF4 are applying to my timer settings. Switch SF5 speaks voltage value every 60 seconds when arming switch, in my case SF was pressed down to arm the board.