Hello,
I'm new of the forum. I have an old 1967 Volkswagen Beetle and I'd like to add some electonics on it. The car battery is 6 volt, can i connect arduino and shields/sensors to it without any protection? I'm afraid of overvoltage and, in particular, overcurrent at start.
Thanks a lot
6V as such will do, but as it's automotive you need to be able to handle transients of roughly -20 to +200V, and significant noise on the power supply. In this case, as it's an old vehicle, well, you say it's still 6V so maybe I should say ancient, there'll be LOTS of noise.
Those were the days ...
The battery voltage can range from 5.25 to 7.2 V in theory. In practice the car voltage can be much less (on start) or somewhat more (on full speed), with ugly spikes in both directions. In neither case you can feed an Arduino Vcc directly, but as unregulated (Raw) voltage it may work, depending on the minimum voltage drop on the on-board regulator. For use with more modules a separate voltage regulator is preferable, with a diode and a fat capacitor (gold cap?) on the input side for bridging the start time of possibly several seconds.
I never used a µC with my Beetles, you may find more substantial advice in a Beetle or automobile forum.
DrDiettrich:
The battery voltage can range from 5.25 to 7.2 V in theory. In practice the car voltage can be much less (on start) or somewhat more (on full speed)
Or use a buck/boost module with 5V output to Vin (plus appropriate transient filtering of course).
3.3volt Arduino...
Thanks a lot for your answer. Should i build this voltage regulator or can i find/buy something yet ready?
If i would add a relay which cut the alimentation to the engine (for example connecting an rfid to the relay, so i can have a theft protection) which relay should i use? I have this one but i think it's not appropriate (30V 10A), what should i search?
Thanks again
A buck converter certainly is the nowadays power supply solution. Find one with an input range of at least 4-16V and 5V output. Or choose a slightly higher output voltage (>6V) and feed it into the Arduino unregulated supply input, for additional protection.
The relay should be sufficient for cutting off the ignition coil. If the relay is switched on before start and switched off after stop the contact volatage should not matter, i.e. 30V is enough.
I agree with Dr. Diettrich- Make sure the power to the Arduino is disconnected during cranking. I also had a mid-60's Volkswagen with a 6-Volt generator. (It ran on three cylinders, but that's another story).
Here is a buck voltage converter from Aliexpress that you can try. (I have a couple but I haven't tried it yet).
The spec is
Current output: 1.5A, Voltage Input: 4.2 ~ 40 V
Ok, thanks
Because also you, SteveMann, have a 1960 beetle and perhaps knows the electrical system, where would you put the relais? It should be in a hidden and not easy to access place for theft protection.
I was thinking about this: because my battery is 6volts, if I use a buck converter I should put the output voltage to 5V and so power on Arduino directly from the 5V input, if I would use the Vin pin I should have a bigger voltage (about 7V) can I use a step up converter (like XL6009)? Is this safe?
If you want to power the Arduino with essentially nothing else connected to it, "Vin" may be usable - if the voltage suits (no less than about 6.5 V at any stage). Using "Vin" and then wanting to connect other things such as relays is asking for pain.
Otherwise, simply forget about "Vin" completely, it is just an ornament.
For your application, given that you will not be using the Arduino while the starter motor is running ("cranking") (i.e., it is connected to the "Accessories" line), then a switchmode "buck" converter is what you want. If it needs to be on all the time, then you want a "buck/ boost" converter. This will protect the Arduino, at least in regards to its power supply.
And you do not build switchmode regulators yourself, you buy them as assembled modules, Seriously!
And for "real world" projects, use a Nano or Pro Mini - not a UNO.
Surely I don't want to build a regulator myself, but Arduino and its hardware must be connected to battery also during cranking (the first thins I would like to do is an anti theft device which allow or not cranking), so I could use one of this converter, should it fit? Or could it be fried during cranking voltage peak and should I choose a more solid converter?
Another thing that is not very clear to me is that output voltage is dependent on input voltage, so if with the screw I set 5V out with 6V in and during cranking voltage input reaches, for example, 30V the output remains at 5V or becomes greater?
I thought also not to use directly car battery but I could use a battery pack (for example two 18650) which can be charged by the car battery or car charging system during car running, could this be an alternative?
I would use not Uno but Mega because I need more than 13 digital pins, is it a good choice?
caesar753:
I would use not Uno but Mega because I need more than 13 digital pins, is it a good choice?
The Mega suffers from the same major problem as the Uno: no solderable headers. Use a Nano or Pro Mini instead.
Both the Nano and Uno have 20 digital I/O pins, by the way (numbered 0-13 & A0-A5), two of which are used for software upload.
About permanent connection I was reading on the forum to use flat ribbon cable and IDC connectors, could it be a good solution for mega? Can a single row male IDC connectors be inserted in mega/uno headers?
Look for Proto or Prototype Shields.
Thanks for your advice, but, because i have to use long wires to reach car parts, do you think it's safe enough to use a screw shield like this and on the sensor solder screw terminals like this?
You have said "long wires to reach car parts". Now you have us worried. 
It is not just a matter of connecting A to B. The whole discussion here is about how you connect a somewhat fragile (in automotive electrical terms) microcontroller to distant devices and of what sort. You will likely need to mount the Nano on a custom board - stripboard or even a PCB - in order to connect all the protection components, not just a "screw shield". 
