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Topic: Car optocoupler, resistors, lifetime and heat (Read 743 times) previous topic - next topic



I have a personnel project in which  I read the car CAN bus to detect reverse signal. When I catch the reverse signal, I want to activate a switch as shown in the attached picture.

I plan to use an optocoupler but I do not know which one to use, which resistors I need.
Can you help me please ?

What I know is that I can use 5VDC from arduino digital pin to control an optocoupler which can switch on the 12VDC of the car. I assume that the car radio only see the difference between its GND and the orange wire so I suppose that current can be extremely low. Thus I guess that I can use a low current to activate the optocoupler which can improve its lifetime (from what I read on Internet). Lifetime is important because I do not want to remove the autoradio each year to replace optocoupler. I also see that optocoupler can heat but I need to control that in order to prevent to burn my car. Moreover I also read that I need to care about the CTR value ...

Thank for your help !


If the current required is low enough (MEASURE IT!) you can use a opto. Because you don't need speed a simple PC817 will do just fine.

But you have to insert a resistor in line with the led in the opto. Just see the datasheet for Vf and the max forward current. Just calculated it like any other led.

If a opto becomes hot you're in the process of killing it aka you're out of spec ;)
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Your car should have a fuse for the radio, can you check what amperage it is please?

Thanks.. Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....


It's naive to think a simple Opto will control the ~5A your average radio consumes.  This is why TomGeorge asked you for the fusing value to get an idea of actual current draw.

You can BUY a ready-made solution in the form of a DC-DC Solid State Relay, or you can "roll your own" which will take some time and thought.  The Opto DOES however give you the level shift you need to run a P-Channel FET and switch the "high side".  They are cheap enough and you don't need a logic level one because your Opto can switch the 12V to turn on the gate.

Let us know which option you want to pursue.   Thanks, Bob.


Given the noisy environment and fairly high current I'd be tempted to consider a auto-type relay.

They're cheap and tough



I've had absolutely ZERO problems running the ProMicro boards,  two of them, in a Dodge Grand Caravan.  I should qualify that I NEVER let the Arduino's on board regulator be the first line to the vehicle.  I usually use a 9V regulator to step the 12V down to 9V and then go into the Arduino's regulator.

That does two things, it limits the dissipation of the on-board regulator on the Arduino and gives another level of noise isolation because you now have TWO regulators swallowing spikes.

Your concerns regarding noise are valid, however I doubt the switching device will be the problem.

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