I'm new to electronics and need your help.

I have retrofitted a Chromecast to my car and have powered it using a cigarette lighter power adapter to USB. Currently the power adapter is hidden and powered from the cigarette lighter power source itself - hardwired. It takes about 1+ minute for the Chromecast to power up after I switch the key to 1st position. I would like to power it up somehow prior to turning the key to 1st position. The vehicle is a MY2015 Mercedes W222 and has ambient lighting which always lights up once I unlock it. It is controlled by the SAM Module which gets a wake up signal from the electronic ignition switch.


simulate this circuit – Schematic created using CircuitLab

I was wondering if I can use it somehow to power a 12V relay which will supply power to the adapter before I enter the vehicle - this way it will be ready to cast once I'm in or at least faster.

Today I measured up the voltage at one of the ambient lighting lamps, which has 3 wires for the LEDs and one common GND. The ambient lighting can be any one of 6 different colors, so the voltage to the LEDs will be different in every color. Today I measured it in one color and one of the LEDs had 9V and other had less than 3V.

I know there are 9V, 5V and 3V relays, but I'm not sure what will happen if I connect a 3V relay and all of a sudden there are 9V because the color has changed.

It would be great if somebody here has an idea how I can use the LEDs as a power source (not consuming energy in standby) so that I can power a 12V relay with them and use it to power the Chromecast power adapter.

Another important thing is that the cables to the led are pretty thin - as per the wiring diagram of the vehicle - 0,18mm^2. There are a couple of lamps connected to one splitter, and between the splitter and the control module the cables are a bit thicker - 0,35mm^2. I don't know how much amps are going through these wires too. I have amper clamps but I'm not sure if they will capture the low LED consumption.

Also, this "prestart" circuit should leave the job to the cigarette lighter power source, once the key is in 1st position, so that the relay don't stay energized on a long trip.

  • \$\begingroup\$ Do you know where to find permanent (non-switched off by ignition) 12V in the car? Do you know if the lighting LEDs are switched from the +12V wire or the ground wire? I assume you don't want to run an ugly wire to the light, so you know where to connect to the controller that drives the LEDs? \$\endgroup\$
    – bobflux
    Apr 18, 2022 at 16:44
  • \$\begingroup\$ Yes, I know where the permanent 12V is, and yes the LEDs are switched from the +12V. I have the wiring diagrams of the car and see that the GND is a constant one, and the positive voltage is changed by the Front SAM Module. \$\endgroup\$ Apr 18, 2022 at 17:00

1 Answer 1


You could try something like that:

enter image description here

D1,D2 take some power, either from the LED lamps or from the cigarette lighter. If you want more inputs, add more diodes. R2,C1 form a lowpass filter to get rid of the PWM that will probably be used to dim the LEDs, and turn that into DC. Q1 flips the polarity to drive the MOSFET X1, turning it on, and it powers the DC-DC converter that feeds the chromecast. D3 is a 12V zener to protect the MOSFET gate in case of transient overvoltage on the 12V line.

With a MOSFET, there's no need for a relay, this will consume about 0.5mA as base current for Q1 when it's on, and zero when it's off.

The MOSFET should be a PMOS, say 40V Vds, with suitably low RdsON, depending on how much current the load draws. Basically, set the search criteria to 40V, RdsON<0.1 ohm, easy to solder package, and sort by price.

Do not forget the fuse

  • \$\begingroup\$ Thank you very much. I have used everycircuit to simulate it and it seems to work as described. For the dc-dc converter I used a lamp with voltage 12V and power consumption 2.5W(as I have measured about this much of consumption with the ampere clamps). When I run the simulation I get 2.78V coming to the lamp and 46.6mA and am wondering if this will be enough to power up the dc-dc converter? Or maybe I have designed something wrong for the simulation? \$\endgroup\$ Apr 18, 2022 at 21:09
  • \$\begingroup\$ The problem is the P channel MOSFET in the simulation. It has enough Vgs to turn on, but its resistance is too high. Unfortunately there are no part numbers on the schematic. Try to find another P channel MOSFET with RdsON below 0.1 ohms. Where will you buy the components? I can have a look... \$\endgroup\$
    – bobflux
    Apr 18, 2022 at 21:51
  • \$\begingroup\$ Here is a link to the simulation. After I have setted up the property Length to 50 nm it is working as expected. I can buy this P MOSFET with 0.02ohm RDSon, and also found the 2N3904 from the same site here in Bulgaria. What fuse should I set? When engine running the voltage goes to 14V from cig lighter and permanent. Will this be a problem? \$\endgroup\$ Apr 19, 2022 at 11:01
  • \$\begingroup\$ Shopping list: 1xMOSFET,1x2N3904,, 4x1N4148(because I want 4 inputs), 1xcapacitor, 3x10kohm, 1x100kohm, 1xZener12V. What do you think? \$\endgroup\$ Apr 19, 2022 at 11:44
  • \$\begingroup\$ Looks good. MOSFET should work fine. Could use lower power resistors to make it smaller and easier to solder. If you use an automobile lighter plug 5V DC-DC converter it should be designed to work over the whole range of voltage, so no worries for 14V. By the way, don't use counterfeit ebay LM2596 modules! As for the fuse, it's there to protect against fire, so you should use slow blow, rated for a bit more current than the DC-DC will use. For example if it's a 5V 2A output, that's 10W, with 80% efficiency that's 1A on 12V, so 1.5-2A fuse. Don't forget the fuse holder and enclosure... \$\endgroup\$
    – bobflux
    Apr 19, 2022 at 12:02

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