Making an Ammeter using Arduino Uno

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As part of a hobbyist construction of an Arduino Uno voltmeter, I used a potential divider of 10K ohms and 470K ohms to step down the voltage to a level which the Arduino Uno analog pin can sense (0-5V). Then, the limit of the permissible detectable voltage would be 0-240V, I want to know if there are any precautions that I should be wary of before implementing this.
Also, for an Arduino Uno Ammeter, I tried extending the above idea, of introducing a small series element of resistance into the path whose current is to be determined. Now, using the above idea, making two voltage measurements (using the same circuitry) on either side of the test resistor, we can calculate the current by

I = (V1/480K) + (V2/480K) + (V1-V2)/r

where r -> test resistance.

Lastly, for an AC Voltmeter, I was thinking to do the usual AC to DC conversion using bridge rectification followed by filtering it with a suitable capacitor, and then followed by stepping down the DC level (as in 1. above) and then feeding it to the Arduino. Could someone guide me as to any yardsticks/limitations of my idea?

Here, too, I would like to know any drawbacks of my idea, or any guidelines I should be following. Also, I am not bent on getting some specific specs, I would also be happy if I could get t detect some range of current as well. I am concerned whether the Arduino has enough resolution to detect such small voltage differences, especially when its (0-1023) corresponds strictly to (0-5V). I would also apologize for any wrong notions and would be glad to be corrected.

Best Answer

One thing to bear in mind:

240V IS DEADLY - USE EXTREME CAUTION

If you are thinking of directly sampling mains voltages, DON'T! (unless you know what you are doing, in which case why are you asking questions here?)

Sampling a range of 0-240V with a 12-bit ADC is not really ideal. You get 0.23v per bit, and as you can usually count the LSB as noise, that is effectively about half a volt resolution. Not really enough to sense anything meaningful from a small current shunt. It'll be like "troll counting" - none, one, lots, many.

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Electronic – arduino – Detect car +12V using Arduino

Knowing that all sorts of weird stuff can happen in automotive power circuits, and not being especially knowledgeable in those systems, I'd err on the side of caution and use an opto-isolator.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Pin 1 = Car 12v R1
Pin 2 = Car Ground
Pin 3 = NC
Pin 4 = Arduino Ground
Pin 5 = Arduino 5v R2
Pin 6 = NC

With this scheme, your Arduino and the car aren't connected electrically at all. At worst, the optoisolator is destroyed, and you can replace it for less than a dollar. Put it in a socket and you won't even need a soldering iron to perform the repair.

R1 was selected such that input voltage transients up to 120V won't exceed the maximum forward current of U1. D1 avoids exceeding the maximum reverse voltage of U1 if the input voltage is inverted. The value of R2 isn't especially critical, so it might as well be the same value as R1.

You won't need any heat sink. Heat is the result of electrical energy being converted to heat, and power is the rate of energy conversion. Power \$P\$ in an electrical system is the product of current \$I\$ and voltage \$E\$:

$$ P = I E $$

So, the voltage itself doesn't make heat: it also depends on how much current is flowing. In both these circuits, the current is low enough that the power is small and no heatsink is required.

Electronic – arduino – Problem with Arduino UNO and RC servo

Your servo may behave erratically, and you may find that this only happens when the Arduino is plugged into certain USB ports. This is because the servo draws quite a lot of power, especially as the motor is starting up, and this sudden high demand can be enough to drop the voltage on the Arduino board, so that it resets itself.

If this happens, then you can usually cure it by adding a high value capacitor (470μF or greater) between GND and 5V on the breadboard

http://learn.adafruit.com/adafruit-arduino-lesson-14-servo-motors/if-the-servo-misbehaves

As others have commented, if your servo requires more current than the Arduino (or your USB port) can supply, you should provide a separate power connection for the servo.

Your USB port may be limited to 100mA or to 500mA, the Arduino may be able to provide slightly more current at 5V if it is connected to a power source through the barrel-jack connector. It depends on the specifications of the 5V regulator on your particular Arduino (or clone).

If you power the Arduino from a regulated 5V supply, you can draw higher currents from the V_IN socket.

I note that some retailers of your servo suggest it be used with a motor controller board.

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