Newbie, need to build a circuit for my car, please help.

Discussion in 'General Electronics Chat' started by maks-g60, Jul 8, 2010.

  1. maks-g60

    Thread Starter New Member

    Jul 8, 2010
    Hi and thanks for taking the time to view my post.

    basically i want to make a circuit to power my electric mirrors on my car when i select reverse gear. i need the motors to go down so i can view the kerb/wheels as im reverse parking. but also need them to return to normal when i de-select reverse. ie 4 sesc down on trigger 4seconds back up when trigger is released. hope this makes sense.

    im new to all of this so please bear with me.

    purchased and assembled one of these now im not sure its correct for my application

    any help or pointers would be much apprecitaed.

  2. debjit625

    Well-Known Member

    Apr 17, 2010
    I have no idea about your car and how the mirror is assembled ,but for the stuff that a motor will control the angular position of your mirror you could use a "Servo motor"
    .If you dont know about servo motor then google about it you will get a lots of info.

    Good Luck
  3. maks-g60

    Thread Starter New Member

    Jul 8, 2010
    just to be clear my mirrors are already have motors, i just want to tap into the up and down functions with a timer.
  4. SgtWookie


    Jul 17, 2007
    Hello Mak,

    I don't have a clue how your mirror motors are powered, as you have not mentioned what manufacturer/make and model of auto you have.

    The usual arrangement for US made vehicles is a pair of geared-down DC motors to control the X/Y axis of the mirrors that can run in forward or reverse. No feedback is given as to the actual position of the mirror; but there are limits so that the motors won't get burned out by being operated in a stall condition.

    Some vehicles have options that remember preferences for different drivers. These can automatically set mirrors, power seats, steering wheel tilt/extension, lumbar support/etc. to the drivers' pre-set preferences simply by pushing a button or using a specifically coded ignition key.

    However, if your vehicle simply has X/Y motors, you won't be able to set the absolute position of the mirror simply by time; as there are too many unconsidered variables.

    You would need a means of determining the mirror's travel limits, and then use something like a microcontroller to measure the displacement from the limits, using some method to provide feedback from the mirror.

    It would be a somewhat complex project, and would require moderate skill levels. It would not be a good project for a beginner to take up.

    However, you could start along that path by exploring the world of microcontrollers.
  5. tom66

    Senior Member

    May 9, 2009
    I think all the OP needs to do is just wire in two 555 monostables to the control circuit.

    Here's my idea:
    - Connect up two 555 monostables with a delay which you can set using a variable resistor or similar device.
    - One monostable triggers on the rising edge of the reverse light or other indicator (like on the dashboard). The other triggers on the falling edge. You can do this using a flip flop I think.
    - When the first triggers, it sends a signal to the control motors to move up. When the second triggers it sends a signal to move down. You will need to adjust the delay to get it right for you. You will also need to figure out how the control signals are wired to the motors, whether there is some kind of H-bridge which is easy to tap into or if there is some complicated double change over switch involved.
    - If the mirrors are in a different position than normal, you will need to adjust the settings, or it may move them to the stopping point, and will not be able to move them back to the original point.

    For example if you set it up to move the motors for 2 seconds, then it will fairly consistently put them in the same position. As you will not be doing much reversing, you might need to move the mirrors back to put them into place every few months if they drift slightly (which is inevitable.)

    This is the simplest, cheapest, and best discrete system I can think of. No need for micros. Let us know if you have any difficulty with the individual parts of this circuit...
  6. maks-g60

    Thread Starter New Member

    Jul 8, 2010

    thanks for the input, i nearly gave up on this when i heard the word microcontrollers, i knew it could be done quite easily its does not need to be perfect. anychance of a digram and list of bits as i will need?
  7. tom66

    Senior Member

    May 9, 2009
    Yeah, I'll get back to you on that... might take me about an hour. Happy to help, it gives me experience too.

    (Please note: I haven't built this circuit, but all the important bits should work. You will still need to have some electronics knowledge.)
  8. tom66

    Senior Member

    May 9, 2009
    Ok, here it is. v0.01 preliminary. Have a look, see if you understand it, maybe it'll give you some ideas.

    The power supply will probably need to be more rugged in an auto environment, large spikes are common on the 12V circuit from what I understand.

    Also the TIP31's will probably need to be heatsinked.

    The delay is calculated as:
    t = 1.1RC
    where t = delay, R = potentiometer value, C = 4.7e-4.

    For example with the potentiometer at 10K (i.e. full scale) the delay is 5.17 seconds. At 1K it is 517 milliseconds. The circuit is about ±20% accurate, due to the limitations of the electrolytic capacitor. Because of this, the potentiometers need to be individually set to get the delay you want.

    I recommend you built this on a solderless breadboard. It is easy to change. However it might not be rugged enough if you are driving along, components could fall out which could cause malfunction (It's unlikely that it would force the mirrors to an extreme position, but it could happen. More likely is it will just stop working.) You could try epoxying everything on, but you won't be able to reuse the breadboard (a breadboard is £4.36 from Maplin) and it'll be difficult to repair. Still it's the easiest for a beginner and you can easily change it before you commit.
    Last edited: Jul 8, 2010
    shard likes this.