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Wanting to build a simple circuit for CDL trucks

Discussion in 'General Electronics Discussion' started by Clinton McPhee, May 6, 2015.

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  1. Clinton McPhee

    Clinton McPhee

    May 6, 2015

    I am a safety manager in WA state and I am trying to find out a good way to build a simple device for our fleet of trucks.

    I want to create something that will remind our drivers to look at their rear view mirrors before driving off.
    What I want it to do is this:
    Using a gap sensor on the drivers side of the truck, a light mounted on the rear view mirror(s), and a button wired into I assume some controlling unit on the dash somewhere, I would like to see the light on the mirror (and perhaps an audible sound) come on once the DS door is opened. When the door is shut, the light on the mirror and sound stay on. It will turn off once a button is pressed on the dash.

    I can find no such product out there. Any thoughts?

    Feel free to shoot me an email clint (at)
  2. Gryd3


    Jun 25, 2014
    I won't shoot you an email, but will respond for you here.
    The device in question will require a method of detecting an open door:
    - Tap into the pre-existing door switch.
    - Build/Buy a proximity switch, light sensor, or contact switch to add to the door.

    The door detector will trigger a 'latch' that could be:
    - built from transistors or other ICs.
    - built from a simply relay that loops back to itself to latch.

    The latch will power any additional indicators you want, lights, buzzers, etc.
    You can buy 'blinking leds' that don't require any additional hardware. Very similar to the blinking XMas light bulbs.

    Ideally, the button would be a 'normally closed' momentary push button that sits between power and your latch. When the button is pushed, it cuts off power to your circuit and resets the latch turning off the indicator.

    Food for thought:
    -Do you want this triggered if the vehicle is on or off?
    -Do you want more than one button to be pushed?
    -Are you concerned with drivers tampering with the circuit?
  3. Clinton McPhee

    Clinton McPhee

    May 6, 2015
    I would like to run it through the switched power. So off when the truck is off
    Just one button to turn off the mirror lights
    I am really not too concerned about driver tampering. But I would like to be able to make it look OEM.

    I love your idea of tapping into the existing door switch. You kind of lost me in the way the unit could be built. I would love to find some existing unit for cross application.

    I really appreciate the help!

  4. Gryd3


    Jun 25, 2014
    It's doubtful you will find 'an existing unit' that you can use without a lot of modification.
    The door switch in most automotive applications is a normally closed switch that is 'held' open when the door is closed. When the door opens, the switch returns to it's normal position and closes.
    So, if you want to use the existing switch, use a multi-meter and ensure there is 12V on the wire (usually one) that goes to the switch when the switch is depressed. When you release the switch, it connects that wire to ground completing a circuit and you will measure 0V on the wire.
    If this is the case, you simply need:
    - any 12V relay with 'Normally Open' contacts. (Which is pretty much most of what you will see)
    - a Diode
    - a normally Closed Momentary Button. (Which will be the 'off' button for the driver to turn off the check mirror indicator)
    - an indicator. (Which you can make anything you want. Light, Buzzer, squirt gun, etc.)

    The wiring may catch you a little off guard immediately, but what ends up happening is that a relay has two halves.
    An input side, and an output side.
    The input side is two pins that will activate the other side when you attach power to it.
    The output side is 2 or more pins that will either internally connect to each other, or disconnect from each other when the input side gets power.
    They are almost always used to trigger something big with something little, because a little AA battery can trigger a relay to activate, and the relay can handle things like a starter or high power lights. We will be messing with that a little. We are going to make the relay trigger itself.
    Wiring is quite easy, once you realize you only need to connect two of the relays pins together to make it work.

    So, switched power goes to the relay. (Pin 1)
    You can use a tap-in on the door switch in the truck. You will connect the relay (Pin 2) to this wire with a diode. (This is to allow the door to activate your circuit, but to disallow your circuit from activating the door)
    Now, you build your indicator, and connect it to switched power, but instead of connecting it to ground, you will connect it to the relay. (Pin 3)
    Then, you connect the relay to ground (Pin 4)

    At this point you have a working 'momentary' indicator. When the door opens, it completes the circuit for power to flow across pins 1 and 2 (The input side)
    This then connects pins 3 and 4 together, and considering your indicator is waiting for pin 3 to connect to ground, your indicator now turns on.
    To make this 'latch' we have another step.
    Connect the Normally Closed button to pins 2 and 3... This switch will end up powering the relay by itself.
    Remember that the Door Switch connects to ground when the door opens. This allows power to flow from pin 2 to ground. which triggers pin 3 to connect to ground, which is also connected to ground! So now the door closes, and pin 2 is no longer connected to the ground through the Door switch, but it's still connected to ground through pin 3 because it was activated! It now latches :)
    To turn it off, when the button is pressed, it disconnects pin 2 from pin 3. Which means that pin 2 is not grounded anymore and the relay turns off. (As does your indicator)
    You can also turn the key off in the truck. It won't matter if pin 2 is grounded if there is no power. The relay will turn off and the door will need to be opened to trigger your indicator again.

    *There is a flaw in this design though. If the door is closed before the power is switched on, the indicator will not light.
    **If you have issues imagining the the wiring, I can try to draw a picture.
  5. Clinton McPhee

    Clinton McPhee

    May 6, 2015
    Yeah, that might not work then. I couldn't insure that a driver would close his door before the power is switched on. What I am trying to do is to have a specific reminder for drivers to check their mirrors before pulling forward. There is a particular company that we work for where we have had several incidents where they have a green/light red light system in place for when our trailers are loaded. It seems that this green light has them fixated looking forward. A few times a ladder did not retract and thousands of dollars of damage was done. This idea would at least pull their eyes somewhere than something that looks like a traffic light for an "all go."

    What about this? Every time they go to engage first gear LED's flash on the rear view mirrors? I would love to just have it happen when the close their door, but they could sit waiting for the light for an extended period of time.
  6. Gryd3


    Jun 25, 2014
    I think that is a valid option, but would trigger quite frequently during city driving I'm sure. If it gets annoying the driver may tamper with it.
    We can modify the circuit so it starts in the on position.
    The indicator would turn on when the switched power does, and when the door is opened after that.

    Is there any other thing we can use perhaps? This indicator is only for leaving a docking area?
  7. Clinton McPhee

    Clinton McPhee

    May 6, 2015
    As far as the frequency, not necessarily. First gear wouldn't be used unless they are at a complete stop. And they will be loaded, so it's a pretty sure bet they will start in 1st. Some are automatics and some are 18sd manual.
  8. Gryd3


    Jun 25, 2014
    Then you could simply but a small microswitch in the housing for the shift lever, but I'm not sure how to make that work with an automatic.
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