Beginner needs circuit design help for 555 timer project

Discussion in 'The Projects Forum' started by RigidCollision, Dec 19, 2018.

  1. RigidCollision

    Thread Starter New Member

    Dec 10, 2018
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    Hi Guys,
    I am embarking on a new project to design a circuit for an automatic vehicle signal actuator.

    The goal of the project is this:
    The problem.
    I have a car that has indicators operating on a standard 12v on-off-on toggle switch. As you can imagine this switch gets left on occasionally. Both misleading and potentially dangerous.
    The solution
    Replace the toggle switch with a 2 way momentary switch and create a circuit that supplies 12v to the correct circuit for 5 seconds. In addition, the timer is continually reset when 12v feed from the brake circuit is applied. As soon as the brake this feed stops, the 5 seconds starts again.

    Effectively, this allows a a lane changing signal as well as a solution for waiting at junctions or to cross a stream of traffic.

    I would imagine a 555 timer would be the way to start with this (or a 556 dual timer).

    From what I can tell I need to use 'Monostable' mode.

    Where do I go from here!?
    Is the 555 (556) the right choice?
    Is it a robust solution?
     
    Last edited: Dec 19, 2018
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Before you start on this project, have you checked that modifying the car's electrics in this way is both legal and insurer-approved where you live?
     
  3. RigidCollision

    Thread Starter New Member

    Dec 10, 2018
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    Yeah, we're cool. :)
    As you can imagine, a car with a toggle switch indicator system is not a standard thing anyway.
     
  4. RigidCollision

    Thread Starter New Member

    Dec 10, 2018
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    I have got a basic wiring diagram for a Monostable circuit (included below) and I will be starting from there.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Ok. Can you confirm that the brake pedal actuation gives you 12V and not 0V?
     
  6. RigidCollision

    Thread Starter New Member

    Dec 10, 2018
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    Hmm, good point. It's a 12v positive at the switch.
     
  7. brezel1111

    New Member

    Dec 19, 2018
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    You could divide the brake signal to ttl level and lead it to the reset pin. You'd have a pullup/puldown resistor depending on high active/low active brake signal. This way you would have one timer for the blinking - astable - and a timer set for 5 sec or whatever you want - monostable. There might be a better way but I this way you would need 4 555 timers: 2 for left and 2 for right.
    The monostable will enable the astable timer.
     
  8. brezel1111

    New Member

    Dec 19, 2018
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    Something like this. Of course you might need something on the output if you're driving the lightbulbs/diodes.
     
  9. RigidCollision

    Thread Starter New Member

    Dec 10, 2018
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    Hi, Thanks for the diagram.
    I was going to use a 556 (effectively a doubled up 555 timer) and inject the new circuit to 'hijack' the current toggle switch and provide a power feed to the existing blinker in the wiring loom. I would also imagine that the sensible thing would be to use a couple of relays to handle the vehicle current?
     
  10. brezel1111

    New Member

    Dec 19, 2018
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    If you hijack the toggle switch, you dont need the astable timer or realys - mosfet might be better here anyway. There are many many ways to go for it, you just have to make up your mind: )
     
    Last edited: Dec 20, 2018
  11. RigidCollision

    Thread Starter New Member

    Dec 10, 2018
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    I assumed a 555 timer was the only real solution, so any advice is always appreciated. The MOSFET is a real simple solution but I am concerned about heat. Any other options?
     
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