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i have to mount a reverse camera on my car. The problem is that the car voltage is not stable, probably the engine makes some interferences. Infact when i turn on the car but not the engine it works. When i start the engine instead it doesn't work. I searched online and found something like this filter: https://www.ebay.com/itm/12V-Car-Camera-Power-Relay-Capacitor-Filter-Rectifiers-for-Audi-BMW-VW/202242499757

I can buy that filter but the shipping time is very long, about two months.

I'm wondering if anyone of you knows how to make it homemade?
I think it's made with a capacitor but what type of capacitor?

Thanks in advance

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When i start the engine instead it doesn't work.

This can mean either: -

  1. The noisy 12 volt power feed to the camera is affecting the camera and making it behave incorrectly or,
  2. The camera video signal is getting corrupted by the car's interference but the camera is working correctly.

To solve #1 you need to filter that 12 volt feed and quite possibly use a 12 volt regulator like this: -

enter image description here

It is an "automotive" rated design but it will still need an RC low pass filter between the car's raw 12 volts and the input above. The R is in series and might need to be a few ohms and the capacitor is in addition to Cin shown above and should be about 470 uF or greater. I would also use a 20 to 30 volt transZorb across the capacitor in case there are voltage spikes that might exceed the maximum voltage rating for the chip (42 volts).

But this will not prevent interference being picked up on the camera's video cable. To solve this you might have to use an isolation device to "break" any chassis link that might be forcing ground currents along the video cable. Or you might just get lucky by re-routing the cable. Success will depend on what the video output stage is and what the input circuit is for the device receiving the video input. Differential is best. Single-ended has problems.

Not knowing what current the camera requires also throws another unknown into the problem.

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    \$\begingroup\$ Yep, the real problem is not fixing things – as you said, that's just a matter of applying known methods – but figuring out where the problem is actually caused. \$\endgroup\$ – Marcus Müller Jul 13 '18 at 12:29
  • \$\begingroup\$ Thank you for your exhaustive reply but i'm not so good at being able to create a circuit like that. I created instead an LC filter (i posted an answer about that). Anyway I bet the circuit you sent me works perfectly so +1 for that. \$\endgroup\$ – Francesco Re Jul 15 '18 at 19:11
  • \$\begingroup\$ The trouble is that I can’t convince you that under certain automotive situations, particularly when the engine is running and the alternator is spiking pulses of voltage, there will come a time when the energy of those pulses will likely produce excessive voltages due to resonating the LC filter you used and think might be a panacea. It might last a day, a week or a year but hopefully for you it will last a lot longer. Trouble is, if it never fails, your solution might be seen as useful to others. Because of this, be wary about marking it as the correct answer because it isn’t \$\endgroup\$ – Andy aka Jul 15 '18 at 19:48
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I have found a very simple solution: an LC Filter. I have tried it out and it works.
This is the schematic specific for Volkswagen/Audi/BMW cars:

enter image description here

Thank you for your help.

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