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+BAT input dumb question
#1
Hi All,
I'm just starting on this journey of building the TX unit and noticed a connector labeled "+BAT". In the schematic it seems to be connected, through a couple of resistors to ground and also to A2.
Am I correct in assuming this is to monitor the battery voltage?
The power supply is on the 12V/GND inputs?

I'm sure this is only the beginning of dumb questions from me!! Blush

Thanks,
Gord
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#2
(16-12-2018, 19:54)WetCoast Wrote: ...
Am I correct in assuming this is to monitor the battery voltage?
The power supply is on the 12V/GND inputs?
...

Yes, exactly.
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#3
(16-12-2018, 20:18)jarekh3 Wrote:
(16-12-2018, 19:54)WetCoast Wrote: ...
Am I correct in assuming this is to monitor the battery voltage?
The power supply is on the 12V/GND inputs?
...

Yes, exactly.

Thanks.
I just read some other posts discussing this same subject...
I'll have to read a bit more before I post again!! Sorry.
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#4
And the other question is how to properly calibrate it to read the correct voltage. Mine reads under by quite a bit so I suspect I need to play around with the resistor values?
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#5
Depends on R4 and R5, the should have a ratio of 3:1. The better is it, the better results you will get.
The next uncertainty is the internal reference of the ADC, but this you can't influence directly.

engolling
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#6
(18-12-2018, 20:58)41south Wrote: And the other question is how to properly calibrate it to read the correct voltage. Mine reads under by quite a bit so I suspect I need to play around with the resistor values?

I think there is a simpler way...

Indeed 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. Even using 1% resistors they always have some tolerance. However there is a way to calibrate the Bat voltage reading to a very acceptable level, without touching in anything.

-Supposing you have a good precision voltmeter, measure voltage at the +Bat pin.
- Compare the reading with the value given by the Pro2 Plus receiver. They differ by more than 0.1V?
- Then find this line of code on the Pro2 PLUS RX or Pro2 Compact software:

Code:
TX_Unit[TX_UnitID].BatVolt        = (int)(((RX_Data[1] * 20.23) / 1024) * 100.0);

Adjust the value of 20.23, raising or lowering it until you get a Battery Voltage reading as close as possible to the measured with the voltimeter.
Theoretically the value should be 20.00, the value of 20.23 is the one that works better for me, but due to the reasons explained above it may be different for each one.
MeteoCercal - Air Quality Data
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#7
Yes, that looks much easier than playing around with resistor values. From memory I didn't have the correct resistor value at the time I built it - I had 3k3 not 3k (if memory serves me correctly) so I am not surprised at my readings being out.
I will just need to remember that change when upgrading  Wink
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#8
(19-12-2018, 01:36)41south Wrote: Yes, that looks much easier than playing around with resistor values. From memory I didn't have the correct resistor value at the time I built it - I had 3k3 not 3k (if memory serves me correctly) so I am not surprised at my readings being out.
I will just need to remember that change when upgrading  Wink

Smile That explains why it is reading under by quite a bit.
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#9
(18-12-2018, 22:33)Werk_AG Wrote: ..... 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).

I have already changed the software in my own hardware, but I may get fed up with having to convert every re-release of the receivers' software.  Wink   Also, I cannot be sure that I have not introduced any "range" (overflow / underflow) issues in the numerical values transmitted, because I don't fully understand the Arduino C++ / Radiohead data structures.  However, I do note that the value re-transmitted by the Receiver is in the calibrated form?

Of course a problem is that both the transmitter and Receiver software must be changed at the same time (or an additional "flag" introduced into the transmission / software).  Therefore, I accept that this would not be an "easy" modification, but I propose that it should be considered for the next time that there is a significant change to the Transmitter software / wireless protocol.

Cheers,  Alan.
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#10
In my opinion it still would be easier to buy precision resistors - 0,1% type are also available.
So you can tune the divider very precise and you do not need to worry about after each update.

Regards,
engolling
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