new feature idea: battery voltage

[UFO]

Originally Posted by bobski

Sounds safe. :/
Maybe if you use high value resistors for your divider. Low values might pass enough current to out-feed the power regulator. If the regulator can basically run backwards, that might not be a problem. If it can't, you'll end up with greater than 5V as VCC.
Also, the forward bias voltage of the diode would mean the input hits 5.7 volts (or so, depending on the diode) before it starts clamping down.

Low resistor divider values would be mistake - they would load your source and you might not get an accurate reading. And circuits that run on 5.0V generally have absolute maximums well above 6V, so a diode drop above 5.0V is not a big concern. If you could handle the leakage current, Shottky diodes have very low drops, they might be a good option.

[mcmancuso]

can someone post a simple wiring diagram of what exactly was done here? And maybe an example of the code added? I mean really, what's the idea of posting hey I got mine to do this! and then not posting anything substantial on how you did it?

[glug]

I don't think we've found a safe way to do it yet.

Another thought - maybe the 12v could be monitored via the analog input for the buttons. That way, it wouldn't tie up another input.

[bobski]

Originally Posted by glug

I don't think we've found a safe way to do it yet.

A 5 to 1 (or thereabouts) voltage divider consisting of 2 resistors should work fine.

Originally Posted by glug

Another thought - maybe the 12v could be monitored via the analog input for the buttons. That way, it wouldn't tie up another input.

I guess you could use the battery voltage signal as the pull-up for one of the button lines. You wouldn't be able to monitor the voltage while the button is actually being pressed, but that shouldn't be a big deal. Also, very low supply voltages (unlikely to occur) could be mistaken as a button press.

[glug]

Originally Posted by bobski

A 5 to 1 (or thereabouts) voltage divider consisting of 2 resistors
I guess you could use the battery voltage signal as the pull-up for one of the button lines. You wouldn't be able to monitor the voltage while the button is actually being pressed, but that shouldn't be a big deal. Also, very low supply voltages (unlikely to occur) could be mistaken as a button press.

I wasn't sure if there would be spikes and noise that might be a complication if the 12v vehicle path is tied to an analog input, etc.

I'd guess the drop in voltage due to the starter would also be a gotcha. The code would probably need to detect that, to avoid false button triggers. Configuring the voltage dividers in a fairly narrow range would probably help reduce false triggers.

The more inputs that are free for pressure, temp, etc, the better.

[mcmancuso]

That's a good idea, most cars won't start if battery V is less than 10V or so, I know a Metro's computer needs at least 9V to run. You could essentially ignore any voltage below 10V or so since if your voltage is below that you will know it via less technical means.

[bobski]

Yep. You could set the minimum even lower if you wanted to be able to diagnose bad battery cells or something; 5V maybe? That would be 1V after the divider, or 200 or less when read in code.

if(analogRead(pin#) < 200) {doButtonAction;}
else {batteryVoltage = analogRead(pin#) / 40.92;}

[meelis11]

Here is my working function:
Voltage divider ratio could be bigger but I had these resistors and it is working.
* correct R1 and R2 value to resistor values you use. For calibration, you can fine-tune these numbers
* you also must define voltagePin with correct pin number

#define voltagePin   2

unsigned long batteryVoltage(void){
   float vout = 0.0;
   float vin = 0.0;
   float R1 = 21500.0;    // !! resistance of R1 !!
   float R2 = 9500.0;     // !! resistance of R2 !!
   int value = 0;

   value = analogRead(voltagePin);
   vout = (value * 5.0) / 1024.0;
   vin = vout / (R2/(R1+R2));

   return vin * 1000;
}

Schematic is like this.
12V...R1...VoltagePin...R2...GND
so it means R1 is between 12V and VoltagePin, R2 is between VoltagePin and GND

[fastegg]

Originally Posted by meelis11

Hi,
has this idea been discussed before? At least I haven't seen it.
Anyway, I would like to see battery voltage in MPGuino and started searching how to measure 12V battery voltage (max should be 15V I think) and found this link and some other voltage divider pages.
Also I am going to add coolant temperature to this screen (needs extra input pin) along with liters/hour and RPM
new screen will be "Car sensors" or similar.

Meelis

Yep... having battery voltage is an excellent idea, so is coolant temperature...

[lamb.chop]

Why not just display PID 42 Control module voltage ??