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Thread: Monitor ACC 12V Line

  1. #1
    Variable Bitrate
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    Monitor ACC 12V Line

    I'm wanting to monitor my ACC line so I can trip a relay and turn my AMP's off when the car goes off. Reason being is my CarPC is set to stay on for a period of time for short trips etc, but I don't need all the AMP's on when I'm in the gas station.

    Since ACC is 12v and the inputs only accept 5V, what is the best way to do it?

  2. #2
    Raw Wave
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    An opto coupler.
    Or relay.
    Or transistor or FET etc.

    Have the <device> switch GND and have a pull-up resistor to your 5V input.

    (aka "Open Collector" interfacing as used on computers, car ignitions (points, ignitors etc) and just about anywhere interfacing to "an unknown" (voltage) is used....)

  3. #3
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    Quote Originally Posted by OldSpark View Post
    An opto coupler.
    Or relay.
    Or transistor or FET etc.

    Have the <device> switch GND and have a pull-up resistor to your 5V input.

    (aka "Open Collector" interfacing as used on computers, car ignitions (points, ignitors etc) and just about anywhere interfacing to "an unknown" (voltage) is used....)
    Appreciate the reply. Unfortunately I don't really follow. Your understanding is a little over mine clearly.

  4. #4
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    Hmm. Your amp doesn't have remote on/off, eh?

    Just use a linear regulator since it's such a small power draw. Find one that regulates 12 volts to 5 volts.
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  5. #5
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    First the "level" translation:

    wikipedia Open_collector where the first fig Fig. 4.1 Open Collector Schematic in its referenced Chapter 4: Circuits is better.
    [ There is also a cute brief at acroname.com/robotics/info/concepts/opn_clct which covers both open collector/drain AND wired-OR connections. Just substitute ignition points, relay contacts, transistor or FETs etc for the logic gates(s) shown, and realise that the input and output voltages can be different and anything. The input determines protection required - eg, I used Mega-Ohm resistors and a zenor & capacitor to convert a high-tension ignition spark signal to an open-collector transistor that signalled whatever (EMS, tacho, strobe, counter, etc).]

    wikipedia Pull-up_resistor for the concept of keeping the output high except when the open-collector output is on & low.


    And then wikipedia Opto-isolator for an isolated method where safety or unsuitable input voltages and grounding is an issue. (Wow - opto-isolators are open-collector! Fancy that!)
    Devices like 4N28 are still common.

    But a simple transistor will probably do in your case....


    EG - a relay with its coil connected to the ACC line (#85 to GND, #86 to ACC) and contacts from GND to your pull-up resistor from your 5V supply (eg: #30 to GND, #87 is the output to your device and a pull up resistor (maybe 10k) to the device's +5V etc).
    Note that the output is LOW when the input is +12V - ie, it is floating (+5V) when ACC is off.

    If a relay is too primitive or noisy or slow etc, then a transistor should be easy.... that requires two resistors and a transistor. But that's for later...


    PS - the above references contain a lot of extra or complex stuff. Try to keep to the basics only.... (Yeah - none is basic at first. And then, what isn't part of the basics? Iteration....)
    Last edited by OldSpark; 03-01-2011 at 02:35 PM. Reason: PS...

  6. #6
    MySQL Error soundman98's Avatar
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    i would just use a 5v vreg.. just connecting it to a switched 12v, and ground should be enough for this.. just be sure to have a large enough heatsink for the power the vreg is burning off..


    or your amp is using the 5v rail of the psu? in that case, a relay is fine-- pins 30& 87 are common and N.O., and 85 &86 are your coil contacts.
    but your sure that the amps require 5v switched input? this isn't a car audio amp right?
    Last edited by soundman98; 03-01-2011 at 05:27 PM.

  7. #7
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    Think I might have confused you guys a bit.. Of course my amps have remote on..

    Right now I have my FB tripping a digital out everytime my CarPC boots that trips a relay to turn my amps on. This was done to avoid any "thump" by having the AMP's on before the CarPC.
    The problem with that is I have my CarPC set to stay on for 15min after ACC is cut. During that 15min MDX is still running therefor the digital output is still tripping the relay for my AMPS. I want to cut that relay off.

    Realistically, it is just to save some drain on the battery. I want the CarPC on during those trips so it doesn't have to boot again, but I really don't need the AMP's sucking power.

  8. #8
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    I think Mitch's USB-IO-1 would work for your situation. Plus, it has a plug in that works with CF (don't know if it's updated for 3.5 though).

    http://www.rush2112.net/mkportal/mod...products_id=43

  9. #9
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    lol, i didn't even realize which section this was in.. with the Fb in mind, you should be able to connect a 12v acc signal to one of the FB inputs, and use that to trigger the amps for a off signal--though the next part would involve programming that i can't help with..

  10. #10
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    Quote Originally Posted by SKiTLz View Post
    Think I might have confused you guys a bit.. Of course my amps have remote on..

    Right now I have my FB tripping a digital out everytime my CarPC boots that trips a relay to turn my amps on. This was done to avoid any "thump" by having the AMP's on before the CarPC.
    The problem with that is I have my CarPC set to stay on for 15min after ACC is cut. During that 15min MDX is still running therefor the digital output is still tripping the relay for my AMPS. I want to cut that relay off.

    Realistically, it is just to save some drain on the battery. I want the CarPC on during those trips so it doesn't have to boot again, but I really don't need the AMP's sucking power.
    Ok, it's very simple. You just use two resistors and be done with it. Divide the 12v into a 5v signal. Realistically, divide 14.4v into 4.5v just to be safe. 2 resistors at a cost of a couple cents and you are done.

    If you want isolation (the optocoupler that spark was talking about), then there is an interface board specifically made for the FB that passes through an analogue input isolated. The board even has dividers for 12v, 6v, and 3.3v built in. So one side is 12v, the other side is a 5v signal. Easy peasy and isolated.

    For something as simple as the IGN line, just use a couple resistors from radio shack.
    Fusion Brain Version 6 Released!
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