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  • Control circuit design review

    I'm starting work on my Car PC in a 2012 Honda CR-Z, and I've run into an issue that I thought would be common, but lots of searching hasn't found anything useful. The background here is that I want to have my reverse camera work even if the computer is off. So if I pull the PC out to work on it, or it breaks down, I'll still have rear visible. However, if the computer is up and running, I want the camera to be displaying through the PC so I can put an overlay on it and I want to have the camera powered at all times when the PC is running so I can record the video in case I am in an accident or something.

    Click image for larger version

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    Here is the logic:
    LCD Power is ON when the M4-ATX has power OR if I am in reverse
    LCD is showing Reverse camera (directly) if the PC is not yet ready AND I am in reverse
    Reverse camera has power if the PC is ready OR I am in reverse

    The blocks with 'NO' are Normally-Open relays, 'NC' is a Normally-Closed relay, and 'NO-10S' is a 10-second Delay-On, Normally-Open relay. I've omitted diodes, LEDs, and ground connections in the diagram. Input is on the left, Output on the right, and Control is either top or bottom.

    My crappy way of sensing the PC being ready is to use a time-delayed relay - eventually that will be replaced with a USB controlled NO-relay that is set/released on Windows startup/shutdown.

    Anyone have criticisms or recommendations? I'm not an electrical engineer, I'm a software engineer, so if it sucks don't be shy.

  • #2
    For the video side you just need a 1 to 2 composite splitter and power you could use the minibox picoUPS 100w along with a small SLA> Excuse the stick drawing, hopefully you get the idea behind the drawing. SNO
    Last edited by SNOtwistR; 06-18-2012, 01:26 PM.

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    • #3
      I think you misunderstood - I'm looking to be able to use the backup camera when the car PC is completely turned off - for example if I disconnect it. For the video side, you're completely correct - I'll just split the signal. Things are more complicated on the power side. Here are the scenarios I'm thinking:

      1) Turn on car, LCD turns on immediately (shows BIOS, loading, etc), put in reverse immediately, PC not yet in Windows, I want the backup camera displayed on the LCD directly
      2) Driving down the street, I want to check my blind spot, press a button on PC, backup camera should be displayed on the PC
      3) Find a parking spot, put in reverse, PC is up, backup camera will be displayed on the PC
      4) If I pull the PC (and power supply) from the car and it's on the bench at home, driving around the LCD is off - if I put the car in reverse, LCD comes on viewing reverse camera

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      • #4
        I have adjusted the drawing to show the power from computer is only to power the camera when PC is on. When pc is off the camera draws power from the sla. you should also put a couple of diodes in those lines from the computer. Hope that a little clearer as to what i was trying to show. Also thats not a power supply in my picture its a UPS type unit, here is the link http://store.mp3car.com/Mini_Box_pic..._p/pwr-040.htm

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        • #5
          OK, I see what you mean now, the "Computer" on the diagram isn't mean to be the computer, it's meant to be the power supply for the computer.

          The trouble I see is that the camera would be powered 24/7, whether the car was on or not, and whether or not the car is in reverse.

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          • #6
            You could instead of using SLA just feed that from another line off ignition power that way when computer is on its feeding power on the left side of the UPS and if computer is off, the ignition power is feeding the right side of UPS and will shut camera off when key is turned off for the night. And only on when you are in the car. only 30 bucks to solve your problem as I see it. SNO
            Last edited by SNOtwistR; 06-18-2012, 02:23 PM.

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            • #7
              Thinking more about your goals why not feed the reverse light directly to the monitor trigger wire and power the camera off ignition line NO ups just direct feed when car is on camera is on and when you back -up the monitor will switch over to cam view and when you want to view camera while driving it will be on without out the need for reverse trigger to turn camera on. SNO

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              • #8
                Running the monitor trigger directly from the reverse light is something I considered (may do this as a 1st step while I build out the PC), but I want to be able to make an overlay - like the little curved lines showing where your wheels are headed, or integrated parking sensor information, automatic gamma/contrast correction, etc. Maybe even fish-eye distortion correction.

                Regarding the previous response - that's essentially what I'm doing with the relays. Just OR'ing the Reverse lights and PC-Ready lines, but two relays costs less than $10 and they are easy to replace locally. I'd have to do the same thing for the LCD power circuit and the trigger wire, too, so I'd be spending $90, whereas with relays I'm probably going to spend maybe $35. I expect to be able to have a control box with the relays, diodes, maybe an LED for the 'PC ready' line, fuses, linear regulators for the LCD & camera, and still be less than $100.

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                • #9
                  i think you can accomplish 90% of this with one or two standard relays by using their normally-closed switches (and of course splitting the video signal too).

                  but for clarification:

                  - how does your monitor work, does it auto-switch to a rear-view video input whenever there is a signal there? or does it use a +12v trigger wire to switch to the rear-view video input?

                  - for the goal "Find a parking spot, put in reverse, PC is up, backup camera will be displayed on the PC", how do you plan to trigger a software event when in reverse, are you using fusion brain, or joycon, etc?

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                  • #10
                    The monitor is a Lilliput 869 with the 12v trigger wire - should work exactly like the 669.

                    I will be using a Joycon EXR (for now) for sensing when the car goes into reverse (this will be directly tied to the reverse lights).

                    In the future, I will probably be buying/making a USB relay board with 12V inputs, and it will handle triggering the camera when the reverse lights come on and will replace the 10-Second Delay-On relay.

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                    • #11
                      ok, so not sure if this is excatly what you area already doing, just explained in a non-visual way....


                      have the screen powered by an IGN/ACC +12v

                      split the cam video signal

                      feed one of the cam signals into the lilliputs rearview video input
                      run a power wire to the camera from your reverse lights
                      also from the reverse lights, run a wire to the liliput +12v rearview input trigger
                      so far we have a camera that comes on with reverse

                      next run the other cam signal into your PC with whatever video capture card you are using
                      from the power supply of the PC, get +12v and run it to the camera (leaving the reverse light power connected to the camera still as well)
                      now we have a camera that still works on reverse no matter what, but also has power whenever the PC is on and you can manually switch to see it using your PC software

                      get another +12v feed from the PC power supply, and run it to a standard automotive relay (to the coil)
                      cut your original reverse light feed that is going to the lilliput rearview input trigger and splice in the normally-closed switch portion of the relay
                      now anytime the PC is on, the lilliput triggered input doesn't happen (but if PC is missing or off, the normal lilliput rearview input works with reverse still)

                      finally, run an additional feed from your reverse lights to trigger the joycon input that tells the PC to switch to the rearview cam screen

                      and so in the end, whenever the PC is on, the camera is on, and reverse triggers the PC to view the cam (or you can manually select it).
                      and whenever the PC is off or missing, the camera is only on with the reverse lights, and the monitor uses it's rearview video input.

                      the only problem i see with this simple solution is that if your startup controller starts booting the PC immediately, then you won't have rearview right when you start car because the PC is on, yet not ready to show video (bootup). this could be solved with an additional relay with delay circuit.

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                      • #12
                        Running the Reverse +12V -AND- the PC power supplies +12V together is not going to work - you can't splice them together because it means when the reverse lights are on, the PC is going to be getting unfiltered power, and when the PC is on, so are the reverse lights. It is also potentially very dangerous, depending on what else is hooked up to that line. In my circuit I solve this with two normally-open relays that have the Inputs both spliced to always-on +12V, and the outputs are spliced. This prevents current from going the wrong way up the triggering wires. You could possibly use diodes to accomplish the same thing, but you have to keep in mind they end up dropping the voltage, which may screw up your ability to use a cheap linear regulator on the LCD (and on the reverse camera).

                        Using a Normally-Closed relay to skip the reverse-trigger when the PC is on is already in the circuit (The middle NC relay)

                        My circuit also has a delayed-on relay for the exact reason you specify.

                        I think the only other thing you mentioned was powering the screen via IGN/ACC - That was my original plan, and looks to be the typical setup, but I found out that if the PC is off, I get a bright blue "NO INPUT" screen on the LCD, so if the PC was off/disconnected I'd get that for the whole drive (except while in reverse) - so I've rigged it up to provide power to the LCD when the PC is on or if the reverse lights are on - both connected to two relays to act as OR logic.

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                        • #13
                          FYI - I am moving forward with this design with some small modifications. First is that the timer circuit will be tossed out from the get go - I've got about $400 worth of Arduino parts and accessories on the way. This will also replace the Joycon EXR, too. I am hoping that after I get my system polished up I can sell ready-made boards to make life easier for everyone else.

                          The next modification is the addition of rotary switches on each output that let me set different modes of operation. Modes will be "Always On", "Always Off", and "Normal", with the LCD screen having an additional mode so that I can select whether it is on when the M4 power supply is on or when the PC is up and in Windows.

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