Go Down

Topic: Safely detecting 12v on/off in car (Read 866 times) previous topic - next topic


Hi Guys

I need to detect whether several wires in a car are "on" or "off". I'm concerned about connecting up to 15v (max I measure on my car) directly to an analog input. What's the safest/simplest way to determine if wire is hot or not




Apr 15, 2013, 03:19 am Last Edit: Apr 15, 2013, 03:21 am by Osgeld Reason: 1
opto-isolator is the best bet, or a resister + zener diode

have the current come in and hit a resistor (1k?) so there is not much current, then use a 5 volt zener to drop the voltage to safe levels and feed into normal digital input

dont feed the arduino anything over 5 volts, especially the analog channels. There is the argument that the AVR has input protection diodes, but I made a device that sniffed 24 volt lines to see if they were on or off, and I grabbed the wrong diodes off the rack, got 24 volt zeners instead of 4.7 volt zeners (the part number on the reel was 1 number difference)

it lasted about 2 days before acting weird, then about a week later it was stone dead, kind of suprised it didnt sneak back up the ftdi cable and mess with the USB port


+1 for optocoupler

you may also google for ruggeduino...
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)


Thanks Osgeld

Sounds like a good plan. Would rather use digital if it's easy to do, and definitely don't want to risk blowing anything up.

I calculated this as about 120mA, is that correct. Anything specific I need to look for in the diode specs other than 5v and 120mA?

Would these do the trick?


One last thing, do I need a load resisitor? I read somewhere that you need a seires and a load resistor, or can I just connect the top to the digital input and ground to ground?




Thanks Rob

Think I'll pick up a ruggeduino for testing!

Any suggestions on a part no/source for an opto coupler? (Not too sure what attributes I should be looking for)



Go Up

Please enter a valid email to subscribe

Confirm your email address

We need to confirm your email address.
To complete the subscription, please click the link in the email we just sent you.

Thank you for subscribing!

via Egeo 16
Torino, 10131