[41south]

My initial thought was to replace one resistor with a 10/20 turn pot, one of the small blue ones. Then use that to set the correct divider with a multimeter.

I may go that way to begin with, then I have solved the problem as it was designed pretty much

[werk_ag]

..... find this line of code on the Pro2 PLUS RX or Pro2 Compact [Receiver] software: .....

This is a matter that I have considered raising for some time.  If the "error" is in the Transmitter hardware, then it would be more logical to "calibrate" the formula in the Transmitter software.  This is particularly relevant if you are using two (or more) transmitters (or receivers).

Hello Alan,

I understand your thought, however let me try to explain why this, as some other things too, aren't done at the TX level. As much as possible, I tried to minimize the number of  calculations done on the TX units. In almost cases where data is obtained using the Arduino ADC, what is sent to the RX is the ADC RAW reading, and the required calculations are done on the RX. This approach may have some disadvantages, but also have advantages, as it may be easier to do small changes on the RX software, compile it and reprogram the receiver, than do it on the TX unit which most of the times is installed on the roof.

Regarding the case we are talking about, in the TX parts list its clearly mentioned that for R4 (3K) and R5 (1K) should be used Metal Film resistors with 1% tolerance. Using this type of resistors the error on the measurement of Vbat voltage should be no great than 0.1V or 0.2V, which seems perfectly acceptable considering that the purpose of this reading is give the user a quick idea of the state of the battery.

[werk_ag]

My initial thought was to replace one resistor with a 10/20 turn pot, one of the small blue ones. Then use that to set the correct divider with a multimeter.

I may go that way to begin with, then I have solved the problem as it was designed pretty much

Even doing it, you may still need to do some adjust on the formula referred before.  

[AllyCat]

....  the precision of the reading depends mainly of the precision of R4 and R5 resistors, as also the precision of the internal reference of the ADC, which you can't control.

Yes, even with exactly the "correct" resistor values, the value of the "Reference" voltage may produce a significant error.  But I believe the reference is actually the "external" (5 volt Vcc) rail, not the Atmel's internal regulator.

If the Transmitter is "on the roof", it is likely that so will be the battery and the calibration multimeter?   IMHO it is more logical to "tweak" a value in the Transmitter software than to change resistors (or to fit in a potentiometer).  This also covers the use of more than one Transmitter (with different calibration necessities) and avoids the need to edit every version of the Receiver software (which is generally updated more frequently than the Transmitter software).

Obviously the value could be "calibrated" within the transmitter software and the Receiver software left unchanged., but currently there isn't a transmitter "calculation" in which to "tweak" the value.  Also, I was trying to reduce the software size in the (original Pro) Receiver, because I want to modify the Fan (aspiration) control, which may make the program too large (but that's for another thread).  Similarly, I've been considering modifications to the Transmitter power supply arrangements (e.g. using Lithium Cells) where a higher measurement accuracy may be required and with perhaps the Nano's "5 volt" supply rail no longer accurately regulated at 5 volts.

Cheers,  Alan.

[werk_ag]

Hello Alan,

I agree that it will be more logical to "tweak" a VBat value in the Transmitter software, specially because, as you said, it also covers the use of more than one Transmitter.
Doing it doesn't seem difficult, but the changes should be done in both the TX and RX software, what would imply that users will need to upgrade RX and TX units. Well, I liked the idea, so I will gonna work on it for upcoming software release.

UPDATE:
It's already done, and under test! For now is working like a charm.
To avoid that all users had to upgrade the TX software (its winter in some places of the globe) to use any new RX software version, a new setting was added on the Config_Options.h file.

[hornychz]

....
UPDATE:
It's already done, and under test! For now is working like a charm.
To avoid that all users had to upgrade the TX software (its winter in some places of the globe) to use any new RX software version, a new setting was added on the Config_Options.h file.

[werk_ag]

Released today a new TX software version which allow the fine tuning the VBat voltage reading.
To use this new TX software (v7.x), RX software must be upgraded too.
Note that there are some new settings on both Config_Option.h files, so don't use older versions.

Thanks Alan (Allycat) for having insisted on the advantages of having this setting on the TX software instead of on the RX.