Detect car +12V using Arduino

Question

I am looking to detect 12+V from a car wire using an Arduino.

I have found the following schematic:

I know how crazy automotive voltage can get so I just want to make sure the schematic I found above will accommodate the crazy random currents that the car could produce.

Also, wouldn't I need some type of heat sink taken that I am stepping down a +12V to 5V or less? That, in my mind, would produce a pretty good amount of heat?

Answer 1

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.

^{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.

Answer 2

You are on the right track, never hurts to have the reverse diode in there although you can belt-and-braces it by sticking an 18-22v Zener in its place.

This is a similar version, widely used and works fine: http://www.msextra.com/forums/viewtopic.php?f=101&t=40330

Some good background info on the nature of transients in automotive applications Harris appnote , for starters this graph says a fair bit:

Here's the circuit from the forum link above:

Answer 3

As noted in the comment to the question: "A simple resistive voltage divider would work best in this case." The voltage divider detects presence of 12V (HIGH @3V3 or 5V) and converts the voltage of the car circuit down to Arduino levels. http://en.wikipedia.org/wiki/Voltage_divider

• Vout = R2 / (R1+R2) * Vin
• 3300/(10000 + 3300) * 12.8V = 3.17V
• 6800/(10000 + 6800) * 12.8V = 5.1V

Be aware that the voltage of the car may be higher than 12V, but at the lead-battery it should not exceed 13.8V for charging reasons. I have used this for detecting a 12V circuit. You can also try to use this to detect charging levels of the battery if you connect it to an Analog input.

Answer 4

It may be a bit late, but what I did is very simple. Understand this, optocoupler is the way to go to make your entire circuit simple and isolated from the car/motorcycle power system. But, instead of buying one, you can build one within 10 - 30 cents. Using a 3.2V white LED, ~620 Ohm resistor and a LDR. Now its just a simple LDR voltage reading on your Arduino side to detect whether there is any Battery (+) Positive is connected or not (engine on/ battery connected).

Connect 620 ohm resistor to the negative pin of the LED. Then connect the 12V negative with the other end of the resistor. Connect the Positive 12V terminal with the positive pin of the LED.

The good part is, it can handle voltage up to 19 Volts.