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Thread: Electronics people, help me with a custom circuit

  1. #1
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    Electronics people, help me with a custom circuit

    Newb here, tried searching, all the circuits I found either required +5VDC or were ATX specific...alas, I need someone to draw up a circuit for me cause I don't know enough about designing my own to do this:

    I'm helping a friend put a laptop-based carputer in his '97 Dodge ram, complete with a touchscreen LCD. The laptop is a modified dell LS, which I have pulled the screen off and soldered wires in. There are 4 wires coming out of the laptop:

    2 for the power on indicator LED
    2 for the power switch

    Since the laptop itself is going to be hidden from view, we have the power switch wired out to relocate so it can be turned on without actually getting to the laptop (which will probably be put underneath the bench seat)

    Now we also have a Jensen 300W power inverter, that takes 12VDC in and puts out 120VAC. The laptop's transformer goes into there to produce 18VDC or whatever obscure voltage the laptop takes.

    Now I have wires running out of the laptop for the power LED, because what I would like to have is a system where I can hit the power button for the laptop, and by extension have the laptop and the power inverter turn on. Conversely, when I shut the laptop down (either by button or windows) I want the power inverter to turn off, to save electricity. Eventually, to make the whole assembly a little smaller, I'll probably get a dell 12V laptop power supply, so that I can connect the switching supply directly to the transformer and sell/forego the power inverter and make things a little smaller. But I'll still need this circuit to turn on/off whatever converter is taking 12V truck power and turning it into whatever voltage the laptop needs, and I'll still need it if I get an SDC to turn the laptop to "shutdown" or "standby" at the "off" key position. Basically, when the power LED is on, I want the inverter on, and vice versa.

    So, what I need to make is get a circuit that will take roughly 2VDC at very low amperage (the LED power) and get it to at least 4-5 volts at a fairly low amperage in order to switch a solid state relay that turns the inverter on. I don't mind including a 9V battery or a couple AA batteries in order to keep the laptop isolated from 12V at considerable amperage (the truck battery), but I would prefer to avoid making this circuit run directly off of the 12V truck power for safety reasons (don't want it to all go up in smoke in the event of a short circuit).

    Can one of the electronics masters on the board help me out?

  2. #2
    Variable Bitrate Saab9-5's Avatar
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    Have you thought this out? The reason I ask is because in order to utilize the laptop's LED to turn on any circuit, the laptop will need its own independent power source, for example the internal battery. That's fine until the laptop's battery goes dead. Then you won't be able to power on the inverter.

    That said, I'll have a circuit for you in a few minutes.

  3. #3
    Variable Bitrate Saab9-5's Avatar
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    Here's the circuit. U1 is an optoisolator that will keep the laptop isolated from the vehicle's power. The laptop's power LED will be replaced by the optoisolator's LED. That switches an internal transistor that switches another transistor, Q1, to handle the relay. The relay can be any standard 12VDC relay as long as the contacts are rated to handle the inverter's maximum load.

    Parts List
    U1 4N27 Optoisolator
    D1 1N4001 diode
    Q1 2N3904 NPN transistor
    R1 4.7K resistor
    RLY1 12VDC relay

    The circuit is powered by the vehicle - no need for AA or 9V batteries.
    Attached Images Attached Images  

  4. #4
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    Thanks a lot.

    I have given thought to the laptop battery dying - so there are actually going to be two buttons on the dash - one for laptop power and another for a bypass relay that will turn the inverter on without laptop power - thus, hold the bypass button, hit the power button in the event of a dead laptop battery.

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