OpenTX - Application Note 18
Voltage Sensor - Voltage Divider
Set up a voltage divider type voltage sensor to send the aircraft battery voltage to the Taranis. Use this data to create alarms when the battery charge gets low.
The FrSky X6R receiver has an analog input socket AD1 which is suitable for a voltage divider type battery voltage measurement but the X8R does not. This means that the X8R cannot use the voltage divider type sensors.
FRSky do make a voltage divider type voltage sensor which is suitable for LiPo battery voltages up to 5S (19.8V) in three ranges. However, a homemade sensor can be made with very few components. This one is designed for voltages up to 13.0 V and is best suited to a 3S LiPo. The AD1 analog input of the X6R has a full range of only 3.3V. To measure higher voltages, a voltage divider is used to scale the voltage down.
The scale ratio is 1.6 / (1.6 + 4.7) = 1 / 3.94.
This provides a voltage range of 3.3 x 3.94 = 13.0V.
The voltage of a fully charged 3S battery is about 12.6V so this divider provides the optimum resolution for voltage measurement.
Making the Sensor
The circuit diagram is as follows.
Resistors R1 and R2 are 0.5W metal film 1%. Capacitor C1 is tantalum 25V. C1 filters noise that may be picked up from the ESC or motor.
The circuit board is veroboard with only three copper strips underneath.
This is the full unit before adding the heat shrink tubing…
… and with the heat shrink tubing added over the board and connector. Note that the 0V pin on the battery connector is marked.
Connecting to the X6R
Remove the link on the AD1 socket which connects the AD1 input to the X6R supply voltage. Connect the voltage sensor so the 0V wire on the plug lines up with the G pin.
Taranis Set Up
On the Taranis Telemetry configuration screen, set up A1 channel as follows.
Set the alarm values to your own preference. For this example …
an audio alarm ‘A1 Low’ is announced when the battery voltage drops below 11.00 v, and
an audio alarm “A1 Critical’ is announced when the battery voltage drops below 10.80v.
These alarms are part of the Taranis operating system. Special functions are not required to implement them.
The A1 voltage is displayed on the Telemetry screen.
The minimum and maximum values are displayed beside the current value.
Logging and Viewing Telemetry Data
Refer to Application Note 8 – GPS Sensor for details on recording telemetry data in a log file and viewing it using OpenTX Companion.