I’m working on a keyfob system for my car, and given looking the arduino directly to the car battery pretty much kills the arduino when you start the car, I figure I’ll hook it up to a high mah li-ion or lipo battery in line with a charger, and hook the charger to a usb powered lighter plug - that way when the car is off it runs off battery, and when its on the batteries can top up. Its the best solution I can come up with - as I’ve seen nothing anywhere on how to keep blowing it out.
Not sure how a car stereo/head unit runs without blowing out when the car starts, or if the 12v line powering the head unit is filtered somehow (and thats safe to use?)
Either way - I havent found a good answer anywhere. So the in-line battery issue seems to be the best bet.
Problem: those provide 3.7v and the receiver for the keyfob needs 5v. I’d like to power both off the same source.
Any good ideas there?
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Lots of things are connected to the 12V line in a car and live a long time. You can just use a 3 terminal regulator and hope for the best (which would probably be OK for 90% of the world). If you want to worry about spikes and transients etc, you can build in some over voltage protection. There are lots of ways to do this. The best solution I’ve seen uses a TVSS for the fast spikes and an SCR/zener combination for slow spikes or a continuous overvoltage (like a failed power supply). Both were set for 15V clamping.
There were a couple of threads from people wanting to put Arduinos on motorcycles that had the same questions, you may want to search for them.
Here’s a link with part numbers etc for very simple but effective OVP:
http://www.ad5x.com/images/Articles/Vprotect.pdf
Well let me state - I’m pretty good with an Arduino, but pretty noob with electronics. Half of what you just said I’ve no clue what it all means.
I was ready to plug the arduino directly into the 12v from the cigarette lighter but then read somewhere that when you start the car, the power surges are pretty nasty. Someone somewhere posted their Arduino worked great - until they started the car - and the surge baked the Arduino. Is over-voltage the issue I’d be dealing with?
I’d also need to somehow reduce the 12v from the car to 5v somehow to power the FOB receiver. Tho - I can use the 5v out from the arduino for that I’m sure.
InfernusDoleo:
Well let me state - I’m pretty good with an Arduino, but pretty noob with electronics. Half of what you just said I’ve no clue what it all means.
I was ready to plug the arduino directly into the 12v from the cigarette lighter but then read somewhere that when you start the car, the power surges are pretty nasty. Someone somewhere posted their Arduino worked great - until they started the car - and the surge baked the Arduino. Is over-voltage the issue I’d be dealing with?
I’d also need to somehow reduce the 12v from the car to 5v somehow to power the FOB receiver. Tho - I can use the 5v out from the arduino for that I’m sure.
Part of the problem is the misunderstanding that the 12vdc system operates at 12vdc. It operates at nearly 15vdc while running due to the alternator. It may very st times. Some cars regulate it down at idle. Also if your battery is low in charge, it will draw more current, reducing the voltage.
There is also a lot of noise from the ignition system.
Yes, it’s overvoltage that is the worry in an automotive application. Less than $10 in parts will give you all the protection you’ll need as well as filtering for the noise and smaller spikes.
You have to look at the schematic for the Arduino, and find the spec sheet for the regulator. Figure out how much current you’ll need for the Atmega and whatever else you have connected to the board, then see if you have any headroom left in the regulator to power the fob.
If you want or need a separate regulator, check out the 7805 (and compatible) 3 terminal regulator. The 7805 is a linear regulator, it converts excess voltage into heat so is inefficient unless you only need small currents (a few mA), which should be the case if you’re leaving it connected to the car battery all the time. There are newer, switching regulators that are more efficient, but have other limitations (maximum current, maximum input to output differential).
An Uno has a voltage regulator onboard. Due to poor heatsinking (aka none) it won’t support a “high” input voltage and “high” current draw at the same time. The product page says it’s good to 20v (12 recommended). If all you’re running off the Arduino’s 5V is the Arduino, you may be OK. I might be tempted put a series resistor btw the car “12V” and the Arduino barrel jack to drop some voltage across that resistor. If starting transients are a concern, add some of the OVP mentioned above btw the “12V” and the Arduino.
As for the FOB … a lower clocked Arduino can run off lower voltages. The 8 Mhz Arduino Mini runs off 3.3V and I might guess would run off the “3.7V” from a Lipo directly. I can check on the rest of the board parts but I’m sure the MCU part number is the same from 3.3v to 5v models.