Hooking up the Arduino to a car's 12v

Question

I am planning an uno based tachometer for my truck, which doesn't have a tachometer. With nothing in place for me to get a reliable signal, I started looking at other sources to read the rpm from the engine.

It seems to me that an engine should affect the power every time a cylinder fires --the power should spike in some way. So in theory I should be able to hook up to the 12v cigar lighter, count the spikes, and calculate the rpm. It would only need to be accurate within 500 rpm or so.

This is a two part question.

1. How would I hook up the arduino to the 12v system? As in, what should I do to protect the io of the arduino?

2. How would I gather the test data to see if this is possible? Is there a way to hook the arduino up to the truck via the 12v and my computer via usb and send data back that I can record or monitor?

Answer 1

How would I hook up the arduino to the 12v system? As in, what should I do to protect the io of the arduino?

First you're going to need to do some investigative work. You're going to need to beg, steel, borrow (or use your own if you have one) an oscilloscope. You can then use that to examine the 12V power coming from the cigarette lighter. Armed with that you can then tell:

1. If there is any power fluctuation from the engine, and
2. What form that fluctuation takes.

If there is any fluctuation it will appear as an AC component imposed on top of the 12V DC voltage. Removing the 12V DC offset is simple enough. All you need to do is pass the power through a high pass filter. That's simply a capacitor and a resistor. You need to ensure that the cut-off frequency of the filter is below the lowest frequency you want to measure (idle frequency) yet high enough to cut out the DC component effectively.

Once you have done that you can then clip the waveform using a diode to protect against negative voltage, and a resistor and zenner diode to limit the positive voltage.

For instance, something similar to the following (it's not exact - the source isn't creating spikes, but an offset low duty cycle PWM, which is kind of similar for demonstration purposes):

enter image description here

The first point measured is the 12V signal with big high voltage spikes. Next the power has been high-pass filtered by the RC network and the lower portion of the waveform clipped by the reverse biased diode. Following that the zenner diode clips the upper portion of the waveform to within safe limits for the Arduino. The 100Ω resistor with the zenner is there to limit the current through the zenner.

Don't take any of those values as gospel, since the signal I am filtering there is not your signal. You need to know your signal first before you can work out exactly how it should be filtered, but that gives you the basic idea of how it can be done.

How would I gather the test data to see if this is possible? Is there a way to hook the arduino up to the truck via the 12v and my computer via usb and send data back that I can record or monitor?

Connect the filtered and clipped signal to an interrupt input on the Arduino. Count the pulses within a given time period (1 second, 5 seconds, whatever you choose) and multiply up for number of pulses in a minute. Don't forget to then divide by 4 for a 4-cylinder engine, since I would expect you would see 4 sparks per revolution. How you then gather the data is up to you. It might be simplest to add an SD card shield to your Arduino and record the data onto an SD card. You can then place that card into your computer to process the data at your leisure.

The best way to power your Arduino is, of course, direct from the truck's power. The safest way, because of all those nasty spikes in the power, is to use car USB phone charger adapter. Plug that into the USB of the Arduino and all should be nice and happy.

• Frank Robert Anderson: I was hoping I wouldn't need to acquire an oscilloscope. Is there any way to figure out the filter's values without one, or to build a beefy way to hook up the arduino to 12v without the filters in place to get the numbers?