0.1 sec delay to off

Discussion in 'General Electronics Chat' started by RogerRoger, Dec 8, 2012.

  1. RogerRoger

    Thread Starter New Member

    Dec 8, 2012
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    I'm not a great wizard with electronic circuits but I'm trying to learn. :)

    I would like to build a 12v delay circuit. It would need to delay the off cycle by 0.1 sec.

    I drag race and sometimes use a pro tree (.400 sec) and sometimes a sportsman tree (.500 sec). If I had a .100 sec delay circuit, my reaction times wouldn't need to vary based on tree type.

    The trans brake button holds the circuit closed (12v goes to a solenoid at the transmission. When the button is released, the current is cut from the solenoid and the car launches. I'd like a switchable circuit that would allow a hard wired direct connection to the solenoid and a second circuit that would allow a .100 sec delay in cutting the power to the solenoid.

    Is this something that can be done by a novice?

    Thanks,

    RogerRoger
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes. Google "555 one-shot".
     
  3. RogerRoger

    Thread Starter New Member

    Dec 8, 2012
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    Thanks for the hint. I'm sure the 555 ic is the answer but I'm not sure how to apply it. I've been reading about it but most seem to use the timer to make something cycle on and off. What I want to do is delay the off, once. (Thus your instructions to look at one-shot.)

    I'm sure I didn't provide enough info. Here's some more:

    switch on - 12v hot constant as long as the switch is on
    switch off - 12v hot for an additional 0.1 sec and then stay off.

    Thanks for helping a novice!

    Roger
     
  4. RogerRoger

    Thread Starter New Member

    Dec 8, 2012
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    I should add that I need 10 amps at 12v.

    Thanks!
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    488
    I assume that one terminal of the solenoid is connected to ground (chassis or "-" pole of the battery), is that right?

    Do you know how to work with a soldering iron?

    The most difficult is not the circuit but the filters and possible protections you need (in case it fails).

    In a car you can have very short high voltage transients on the 12V line (from the ignition), these need to be filtered because they could either cause malfunction or total failure of such a circuit.

    I can draw up a circuit around a 555 tomorrow.

    Last question: What happens if for any reason the main switching element, e.g. a MOSFET shorts and causes the solenoid to remain energized constantly, even when the button is released?

    PS: some questions may have obvious answers to you but I don't understand a thing about race cars. :)
     
  6. RogerRoger

    Thread Starter New Member

    Dec 8, 2012
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    praondevou,

    Thanks for the interest and the help! No worries about any "obvious" questions. I'm thrilled that you're helping.

    I'm pretty good with a soldering iron and have done some electronic work.

    I'm not sure how much spike and noise the system will have. It's an altered dragster so the only electronics on it are the ignition systems. (No lights, radio, fuel injection, etc.)

    If the solenoid locks on (energized) the car won't leave the line and I'll lose. No crash or breakage. Now, once the circuit turns off, it absolutely can't turn back on unless I hit the button. That would cause a lot of damage.

    Thanks again,

    Roger
     
  7. #12

    Expert

    Nov 30, 2010
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    Prandevou, here is something to start with.
     
  8. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I would have preferred not to use the 555 because I fried some of them in noisy environments due to voltage spikes more positive than VCC on Trig or Thres. But they are readily available. It depends a little bit on where you live and where you can buy your parts.

    To make this work on a workbench you'd only need the 555 and what is directly connected to it + Q1 and D1.
    All the rest is to handle transient voltages, overvoltage etc.
    I'm not sure if this is enough, or if it is too much. :) This will also depend on your layout. The 16V TVS diodes near Q1 are for your peace of mind. D2 is also a TVS but it's voltage can be a little lower than the breakdown voltage of Q1. D1 discharges the solenoid when it's switched off.
    The examples for Q1 and D1 can be what #12 suggested, there are many other choices though.

    Then, "would cause a lot of damage" doesn't sound good. MOSFETs (Q1) often fail by shorting source and drain, i.e. solenoid is energized.

    I would probably put a relay in series with the MOSFET (or the whole circuit) that makes it impossible to reenergize the solenoid once you turned off the switch and the delay is over.


    [​IMG]
     
  9. #12

    Expert

    Nov 30, 2010
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    There are a hundred ways to do this...well, maybe only a couple of dozen ways...but I was getting tired and left it to Praondevou to work out the finish details.

    Mount C2 and C3 really close to the chip, like practically soldered to its feet. Use a heat sink for the big transistor.
     
  10. RogerRoger

    Thread Starter New Member

    Dec 8, 2012
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    Thanks guys! This is a lot of info and I'll need to work through it. It's WAY nice of you guys.

    Thanks again,

    Roger
     
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