change polarity when power is off

Discussion in 'The Projects Forum' started by etminasm, Mar 24, 2011.

  1. williamj

    Active Member

    Sep 3, 2009
    180
    33
    etminasm,

    That particular camera appears to be made for the exact purpose you intend for it and the price is right. Drawing only 90mA a relay probably won't be necessary. Just power directly from brake light, but I do suggest an inline fuse.

    Without a relay or any other circuitry a project box won't be necessary either. Just take some flat bar and bend it in a U shape with tabs and cover the camera to protect it from items bouncing arround in the trunk. (see attached) Bear in mind you'll most likey have to modify the trunk liner to fit around your installation.

    Question for you... What monitor do you intend on using? This project is very practical and handy to boot, and it looks like a lot of fun. I am concidering doing the same project myself.

    good luck,
    williamj
     
  2. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    2,491
    389
    One point on rear view cameras. I have one on my truck and when first installed, it was quite confusing. When looking in a mirror, your brain does an automatic left/right reversal. If you use a standard monitor, you WILL be confused unless either the camera or the monitor flips the image. On my monitor, I was able to find a data sheet for the video chip and it had two leads available for either vertical or horizontal flip. Hard wiring the proper pin to the proper source provided a proper "mirror" view.
     
  3. etminasm

    Thread Starter New Member

    Mar 24, 2011
    12
    0
    Hi,

    thanks a lot for your ideas and drawing. So you think camera woun't disturb backup light, so I could connect it stright to it (trough fuse of course)? But what size of fuse do I need to use? 0,5 A or less or more?

    Thanks BillB3857 for your comment. I haven't thought about that. Really, picture in monitor will be not like in mirror..

    I will use my player's monitor for the camera. It is KD-AVX44. Now I hope it will have the function to flip the image or it will do automatically. Because there is 'rear view camera ready'. So I hope JVC have presumed that... :)

    I agree with you williamj, it is lots of fun with this project. I bought player, it has option for rear view camera, so I can't to not use that option.. :) Just like small boy with his small toys. In this case toys are little bigger :)
     
  4. williamj

    Active Member

    Sep 3, 2009
    180
    33
    etminasm,

    Size of the fuse depends entirely on the size of the load you need to carry. If all you're trying to carry is the camera at 90mA then all you'd need is a fuse for .09 amps.

    What I've suggested so far is for the camera only, with the monitor being powered elsewhere. If however, you wish to power the camera and the monitor as a single circuit then you would need to reconsider everything.

    For this new arrangement I would suggest going back to the brake light, relay, trunk light setup. I don't know what kind of current draw this setup would require but you do not, repeat, do not want to overload the brake light circuit.

    This arrangement would be...

    brakelight powers relay,
    relay powers camera/monitor circuit from trunk light (or any other ignition controlled voltage source) circuit

    Current draw would be current draw of camera plus current draw of monitor, so do the math and size appropriately.

    good luck,
    williamj
     
  5. CDRIVE

    AAC Fanatic!

    Jul 1, 2008
    2,219
    105
    Sorry, but a fuse must be rated higher than the maximum current being drawn. Surge current at turn on should also be considered. FYI.. Hens teeth may be easier to find than a 90mA fuse. PICO makes very low current fuses but I don't think 90mA. A 500mA to 1A fuse should suffice.

    I also don't understand the references to the brake lights. Isn't it the backup lights that we're talking about?
     
  6. williamj

    Active Member

    Sep 3, 2009
    180
    33
    CDRIVE,

    I must admit I'm not very good at solid state circuitry and I thank you for your corrections. I know that line voltage equipment (motors, large relays and such) have a larger start up current draw than a run current draw. I was, however, under the impression that solid state and intergrated circuitry was more susceptible to higher current and voltage variations and that a closer fuse tolerance was required. Thanks again for the correction.

    As for the "brakelight" refference, chalk that one up to a random "brainfart" event. It should have been as you stated "back up light".

    That's what so really great about this forum... Whenever a mistakes happen, and they do happen, the corrections are only a posting away. Keep up the good work!

    Thanks again,
    williamj
     
  7. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    2,491
    389
    The fuse may experience additional current at turn on of even solid state devices due to the fact that most circuits contain filter capacitors on the incoming power bus. Charging these caps causes a momentary current surge. Likewise, fuses are not usually installed to only protect the device being fused, but to protect the wiring going to it. For the application in discussion, the fuse should be placed close to the power source. That will protect from any possible cut insulation on wire rubbing sheet metal. The fact that a properly sized fuse blows says something is already wrong in the load or the wire feeding it, after the fuse. In you home, if in the United States, you probably have a 15 amp circuit breaker protecting a circuit that has a 40 watt light bulb.
     
  8. williamj

    Active Member

    Sep 3, 2009
    180
    33
    BillB3857,

    Thanks for the post.

    In this instance I was figuring that the "BACK UP" (got that one right!) light was already fuse protected from the lighting circuit as a whole and was only concerned about protecting the camera itself. Well, any way, that was my thinking.

    thanks again,
    williamj
     
  9. etminasm

    Thread Starter New Member

    Mar 24, 2011
    12
    0
    Hi,

    thank you for your posts and info. However I am still not sure what size of fuse do I need to use. Would it be ok if I go for 1A? Little bit confusing example about 40W light bulb with 15A fuse.. Do I need maybe bigger fuse then 1A?

    But... I can try with 1A. If it will blow - will try little bigger

    Thank you all guys for your thoughts!
     
  10. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    2,491
    389
    Sorry to have confused you with the 40W light bulb and 15A circuit breaker. The point I was trying to make was the the fuse or circuit breaker feeding a wire is to protect the entire circuit (wire) no matter what may be connected to it. Each device, if protected, will have a fuse rated to provide for opening the circuit if there is a failure in the device. Usually, if the device fuse blows, the device has a real problem and replacing the fuse may or may not fix it. Often, the new fuse will blow as soon as power is applied or shortly after.
     
  11. etminasm

    Thread Starter New Member

    Mar 24, 2011
    12
    0
    BillB3857,

    Thanks again. Ok, I will see with the fuse and my small circuit. I don't really think, that I could do something wrong with such a simple wiring. I will check that all my wires will be isolated, will put small fuse and will be ready.. If it will blow, then I'll check my wiring again..
    Biggest question for me now is if camera or player will have mirror picture. I could not find anything in KD-AVX44 specs about mirror pic function. Could not find cameras specs at all.
    Maybe you know how could I be sure? where to find that info?
    I will receive the player just in the beginning of may, so it is long time to wait..
     
  12. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    2,491
    389
    I really can't help you with the mirror capability of your unit. I only know that I had the problem with the simple LCD monitor I used for my system. One other concern did come to mind, though. Somewhere along the line you asked about how the caps across the relay coils could charge instantly. The reply was that they would charge through the diodes. The concern that came to mind is that if your car uses electronic control modules for functions such as lock/unlock, the high surge current needed to charge capacitors that large could damage the control module. Usually an automotive manufacturer will only provide a given level of overhead in their driver circuits and the large capacitors MAY move the current beyond what is allowable. Maybe someone else can chime in with more exact information.
     
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