Ok bit of a silly question but I am power the UNO of my motorcycle. Should I use the 7812 or the 7805? Will it run ok of the 7805?
If you use the DC plug of the Uno (or Vin) you need 7...12V.
Using 5V as input would result in a voltage that is too low for the Arduino board.
When you supply 12V, the voltage regulator on the Uno board could get hot, when you draw current for a number of leds and so.
A voltage regulator in a car or motorcycle is a good idea for extra safety.
A safe value would be 7.5 or 9V.
So a 7809 would be a good choice.
Ditto to what Erdin said. And use some nice big caps on the regulator
to help filter out engine electrical noise, etc.
I'm curiousas to what you are making?
Erdin:
A voltage regulator in a car or motorcycle is a good idea for extra safety.
A safe value would be 7.5 or 9V.
So a 7809 would be a good choice.
And add plenty of filtering. The power will be connected to an alternator so the 12V line will be awful.
Thanks all, i will use the 7809. I was going to remove the dc jack, but i have just noticed the Vin pin. Can i use the 7809 into that or should it be 5v?
RobvdVeer:
I'm curiousas to what you are making?
I'm building a GPS datalogger and tachometer/ shift light for my 125 race bike.It will replace the instrument cluster. The datalogger will write to an sd card my long/lat, speed, heading, revs, brake on off etc. Due to the polling time of the gps module i have I'm using a second primarily to be a stand alone tach and shift light.
Phase 2 will have an accelerometer/gyro but still working on the MPU6050. Still a lot to do and 2 weeks till race day EEK
You can use the 9V to the Vin.
Erdin:
You can use the 9V to the Vin.
Thank you Erdin!
Sorry for hi-jacking this thread but I need to do the same, i.e. connect an arduino + GPS + GSM to a car's cigarette lighter socket and would appreciate some help with the input voltage filter(s).
Does anyone have a good wiring diagram to use in such a case?
For example:
Note:
- use ie. 5W zener - it has to absorb large peaks, if any
- the 100uF cap could be bigger - ie 470uF..1000uF
- the power dissipated at the 7808 regulator is P=(14V-8V) * I, for example with 100mA it is 0.6Watt - so you need a cooler, or you may use a dc-dc converter to keep the stuff cool..
No warranties of any kind 
The 78xx series of regulators are not a good choice for automotive applications.
You should use an "automotive grade" regulator.
Voltage spikes on the 12V rails on a car or motorcycle caused by back-EMF from the coil / ignition system can reach voltages as high as 60 to 70 volts even when filtered. These spikes are about double the absolute maximum of the 78xx series of regulators. There are regulators available with maximum input tolerances of in excess of 70V specifically for use in automotive situations.
Best would be to use a switching 5V regulator in the form of a cigarette lighter adapter for a USB mobile phone. Connect that either direct into the USB socket on the Arduino, or break out the 5V / GND wires and connect it to the 5V pin on the power header.
They can be bought for a couple of $$ from eBay / amazon.
majenko:
The 78xx series of regulators are not a good choice for automotive applications.You should use an "automotive grade" regulator.
Voltage spikes on the 12V rails on a car or motorcycle caused by back-EMF from the coil / ignition system can reach voltages as high as 60 to 70 volts even when filtered. These spikes are about double the absolute maximum of the 78xx series of regulators.
Well the zener diode and L1 / L2 in the circuit above addresses the spike issue I think...
sonnyyu:
This might be not the case, OP solve the 5v problem and facing 12v only.few plans for OP;-
plan A
Power Supply Filter, L-C Type. It is cheaper and simpler, and worth to try first. ( credited by db2db )plan B
Isolated DC-DC converter. offer additional protection and higher noise reject rate.plan C
Wireless power technology, it might be worth to note on calendar since you might need to wait few more months. It offers the possibility of "cutting the last cord", thereby removing the last remaining wired connections required to power and recharge. It is base on similar technology as Isolated DC-DC converter and noise reject rate is great. Now you need to make sure Arduino design is wireless and power cable is last cord.
Wireless power module is available now.
majenko:
The 78xx series of regulators are not a good choice for automotive applications.You should use an "automotive grade" regulator.
Voltage spikes on the 12V rails on a car or motorcycle caused by back-EMF from the coil / ignition system can reach voltages as high as 60 to 70 volts even when filtered. These spikes are about double the absolute maximum of the 78xx series of regulators. There are regulators available with maximum input tolerances of in excess of 70V specifically for use in automotive situations.
Best would be to use a switching 5V regulator in the form of a cigarette lighter adapter for a USB mobile phone. Connect that either direct into the USB socket on the Arduino, or break out the 5V / GND wires and connect it to the 5V pin on the power header.
They can be bought for a couple of $$ from eBay / amazon.
Which regulator do you recommend for an automotive project?
fungus:
Erdin:
A voltage regulator in a car or motorcycle is a good idea for extra safety.
A safe value would be 7.5 or 9V.
So a 7809 would be a good choice.And add plenty of filtering. The power will be connected to an alternator so the 12V line will be awful.
To give you an idea of how awful: "automotive" MOSFETs are typically rated at 55V for use on 12V car
electrical systems.