RFiD car lock. Feed arduino with 12V from vehicle.


I am currently working on a rfid car lock and have two questions.

I need to supply the arduino with power. Can I just hook it straight up to the battery? I assume I need some regulator and some capacitors to smooth out the power? For how long can I run the Arduino on a car battery before the battery goes flat?

I have a power lock (not remote) system in my car. How do I interface with this? I assume i need some kind of relay for the arduino to trigger?


The Arduino can be powered off a 12V car battery.

When in the car, though, the battery is only piece in the circuit. The alternator is putting out AC current, which the voltage regulator converts to DC. While the car is running, the voltage is likely to be 13.8 volts, or higher.

The acceptable input voltage, when on external (non-USB) power is 6 to 20 volts, but, anything over 12 can cause overheating, so you might need to design your own voltage regulator that drops the 12 to 14V to about 7.

The idle current draw of the Arduino is pretty low - around 50 mA or less, if I remember correctly. This would require a long time to drain a car battery which is rated in hundreds of amp hours.

Yes, you would need a relay to trigger the existing locks.

Thanks for the reply :-)

Will the arduino survive the voltage spikes in the long term?

If any power circuit expert could come up or refer to a very simple DC - DC circuit to feed the arduino with power. I need very limited power, just enough for the arduino and the rfid reader chip.

You wouldn't need to power the Arduino or RFID reader when the vehicle was running, would you?

If not, you could use a normally closed relay triggered by the ignition circuit. Turning the key on would open the relay, cutting power to the Arduino. Turning the key off would close the relay, powering the Arduino again.

I DO need it to be running after I turn the engine on. One reason is that I wan't to start the engine and leave it running while I lock the doors from the outside of the car.

There are other choices, but here's a nice Switching Voltage Regulator, this will keep the power-loss to heat at a minimum. (this will also help from draining the car battery) These have the decoupling caps built in, and also run at about a 85% efficiency, compared to the 50% ish from the 7805. But yeah, they're spendy... you can get the variable version for the same price, has a potentiometer built on it for the variable output. http://www.dimensionengineering.com/DE-SW050.htm

Basically, plugging 12v into the Duemilanove itself... results in a fairly warm board which means all the energy that isn't used, is being wasted in Heat. (depending on the load, and the supply) I'd suggest building your own board for this purpose. (doesn't need to be complex at all)

But yeah, interfacing to the power-locks should be semi trivial if you have experience with tearing cars apart. You could look at some diagrams from the Remote Start kits to give you an idea of where to start, basically it will be the same principal, just substitute the Arduino for the Remote system. (Minus all the extra stuff, like the remote-start itself, haha.)

a DC - DC converter will work fine for your purpose.

Or, if you wanna be cheap... just get a car charger for a phone that outputs 5v... cut off and splice it to an Arduino connector. Then wire the other side to the 12v battery. This is probably the easiest way... and the cheapest... I'm sure you have plenty of old phone chargers lying around.

ANOTHER way, is to get a car charger for an Ipod... that has a USB connector... then connect the USB to the Arduino. Then you'll have 5V power to the Arduino.

If you search the archives, you will find many more ideas on this subject.