dev builds
themes manual
device status forums
mailing lists
IRC bugs
dev guide

Wiki > Main > CarPowerSupply (compare)

Difference: CarPowerSupply (r21 vs. r20)

How to build up very easily a simple power supply to get your beloved Archos Player or Archos Recorder v1 charged in your car.


Quite well known problem. The Archos Car Adapter Kit costs a fortune compared to the costs of a selfmade power supply. Furthermore, it has to be plugged to the cigarette lighter plug, which gets then quite big (can't shift the gears properly anymore), furthermore you have to unplug it when stopping the engine on some cars because some cars deliver permanent power on the plug. My small circuit will be connected to the car radios output to external amplifiers. From now on, Archos will solely get current (and charge) if the radio is switched on, and most probably playing from the Archos, otherwise I would not have bought the unit, eh ? My car radio delivers a quite decent system for automatic switch on and of the radio, which I can take advantage now of: - Engine stop automatically switchs off the radio. Archos will then pause for 10 minutes (see Car Adapter Mode), then switch off too. - After engine stop, I can switch on the radio back again. Archos will then power up and resume if it was off, or simply resume. - Engine start starts the radio if it was on last time the engine was stopped. Archos will get up then too. As the radio takes a while to boot up, it may be that the rockbox is faster smile Beware that there is a hard coded delay of several seconds inside the code for the case where rockbox resides charge-less in pause (these 10 minutes after engine stop). This has been implemented to not yet start the hard disk as the power voltage may be flaky due to engine starting.This is virtually not needed anymore with this regulator, furthermore, if you connect it to the radio as I described this is only getting an additional delay. To get rid of this delay, you have to build your own version out of the cvs and modify the code. Exactly that code that I can't find now. Strange, I'll have alook at this later.

Note: There is a known problem with non-flashed players/recorders. In their nature, when getting on by power on the charging plug, they will boot into the charging screen. They will charge, but not be able to play music. I recommend to install an additional switch to be able to first switch on the device by it's normal "On" key, then 10-15 seconds later (after rockbox booted) switch on the power supply. There is another trick that could be taken advantage of (specially intresting with non-flasheABLE units): With non-flashed players try the following: Switch off your unit if it's not yet. Plug in the charger to the mains power supply. Then, stuck in the charger plug for about one second. Get it out again (If your unit shuts down again try with longer time). The unit will now continue starting. In a few seconds, the original archos firmware is trying to find out by which event the unit has been switched on. Due to the fact that there is no charger anymore connected, archos firmware is thinking you switched on the unit by the button. He will continue normal boot up, load into the rockbox that lays on the hard disk. rockbox will come up, and after that, you can easily reconnect the charger. It will start to charge the batteries in a few seconds. The thing is: If you get this On-1sec-Off-10-secs-On into asmall circuit, you're done !

As the Frequently Asked Questions about Batteries and Charging describes, voltage should be around 10 and 12 V. The real challenge is the very fluctuating car power voltage, especially on old cars, as I have. When starting (cranking) or driving with headlights and rear window heater on, voltage may be VERY low (even below 10V), or VERY high when driving a long ride over the motorway without lights (please, only if the sun is shining !) and without any other big power consumers such as the rear window heater (up to 14V). As you can see in the FAQ, below 10V would work but take too long. Above 12V could heat up the unit, which is certainly not what we want. So we are goint to use a regulator to limit power to somewhere between 10 and 12V. You'll discover the real challenge as soon as you learn that normal power regulators drop about 2V. So with normal power regulators, we cannot get happy. Either we chose a 12V regulator which will need 14V of input and ensure that power will not get above 12V, but it will only deliver 12V in the very special case of a new car (haha) or a battery and a car power budget in a good shape. Or we get a 10V regulator, which will need 12V of input and still not charge if power drops below 12V. Now, there are low drop regulators and even very low drop regulators. I choose for a 10V of the latters as it already has a few features build in:

  • precise 5, 8.5, 10, 12V outputs
  • low dropout voltage (500mV Typ. at 1.5A)
  • very low quiescent current
  • thermal shutdown
  • short circuit protection
  • reverse polarity protection

Now, let's get the hands dirty after all that talking.


Parts, components

  • 1 Power regulator: L4940 V10 (Price about 0,75 € at
  • 1 Capacitor: 0,1 uF at least 15 V (Price: hilarious)
  • 1 Capacitor: 22 uF at least 12V (Price: don't think about it)
  • 1 Power plug to the unit (Price: I think that was about 1,5 € when I had a look at
  • some wiring cable
  • some heat shrinkings
  • about half a meter of solder.
  • the data sheet about the L4940 ( or


  • A soldering iron (a thin one. Don't ask your daddy if he's a roofer, he'll get you a griller)
  • A nipper/edge cutter
  • Maybe some tweezers
  • Coffee, rather some relaxed, steady hands.


let's see how we can assemble it.


The wiring has to be done as on the figure 1, page 3/16 in the data sheet. Left side is input (car / radio), right side is output (archos power plug). The lower track is the minus (-, ground, mass, 0V). The upper track is the plus (+12V, power, hot wire, ...). The box in the middle is the power regulator. On the left and the right side of it, you'll see these two capacitors. Please notice that the right one has a polarity. The black bar means "goes to the ground". There will be a very similar marking on the capacitor as well.

Personally I took some cables from an old floppy cable. I don't think it's to thin, and it's nice, because they are already stuck together.

I soldered it in a very compact manner as you can see on the following pictures, but there is no electronical purpose. You can uncompress it as much as you want or need.


Img_2724.jpg Img_2725.jpg



Img_2735.jpg Img_2737.jpg


For testing, get a multimeter and a power supply (worst case your car). Don't connect your archos, measure the power supply power (ie your car's power). It should be at least 11V, else the testing makes no sense. If it's below 11V, start teh engine and proceed in a few minutes. Connect your wiring, measure it't output. If it's around 10 V (I mean very close to 10V -+ 0,1 V most probably), you can connect the archos without danger. It should get on automagically and show up with the charging sign (the tiny plug next to the battery).



Connecting to the Car Radio

As I havn't done that yet, I don't fill out that section. It will come later.


Contact ? For what ? I got no clue ! Well, I assume I should drop my name: ManuelDejonghe, lImbus in irc

r23 - 10 May 2006 - 20:57:07 - ManuelDejonghe

Revision r21 - 28 Jan 2005 - 12:11 - ManuelDejonghe
Revision r20 - 28 Jan 2005 - 11:33 - ChristiScarborough
Copyright by the contributing authors.