555 timer trigger question

Discussion in 'General Electronics Chat' started by skinner927, Jan 24, 2008.

  1. skinner927

    Thread Starter Active Member

    Dec 31, 2007
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    ok, I realize that my original question is very large so I've decided to break it down into many simple question and i'll just put it all together in the end.


    I have a 12v source

    I'm trying to figure how to trip the trigger on a 555 timer when the power is applied and when it's turned off.

    I was thinking of putting a cap in between the source and the trigger.

    Here's the question.

    1) The trigger has to be 1/3 of the Vcc? so my trigger would be 4v on a 12vcc?

    2) If I reduced the power down the the correct amount (as of this time 4v) then put a cap inline that would charge in 0.01 seconds. Correct me if i'm wrong, but once it charges, it will block all flow of current to the trigger UNTIL the power is pulled, then it will send the charge to the trigger letting it go again when the power is pulled.


    Thanks for the help, you've all been really helpful and I'm happy I've found this forum. I really hope to actually learn this stuff and not have to ask a question every time I'm trying to do something, but right now time is somewhat of an issue for this project.

    again, thanks :)
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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  3. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Trigger requires a maximum of 1/3Vcc to work . Try connecting a 10nF cap from trigger to 0V and a 100K to V+. It will trigger at switch on and the trigger circuit will stay dormant when trig level goes above 1/3Vdd.

    The cap will discharge again during switchoff. If you want a quicker discharge the fit a diode (reverse polarity) across the 100K.
     
  4. skinner927

    Thread Starter Active Member

    Dec 31, 2007
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    so, is this what you're saying will work?

    [​IMG]


    I'm a little confused since the two of you contradicted yourselves.

    Every datasheet i've read says that the trigger needs to be MAXIMUM 1/3 Vcc.

    Can I get a second opinion. I know 555s are cheap but if this one decides to blow later on it'll be a pain in the... you know, because this is going behind my dashboard in my car.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The trigger voltage of a 555 typically must be 1/3rd of the supply voltage or less. So just ground it and it will work fine.

    You show a capacitor to ground at the Trigger input which will cause the 555 to trigger when the power is applied and you have a 100k resistor to charge it so the trigger will be at 63% of the supply voltage in RC seconds (1ms).
     
  6. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Yes, that's right. Assuming your +12V is the Vdd of the 555, then your IC wil come to no harm.

    To confirm, the Trigger pin operates whenever its voltage is between 0V and 1/3Vcc. The output then goes high. The output is reset back to 0v when the Threshold pin is between 2/3Vcc and Vcc. Taking the Reset pin low will also reset the output.
     
  7. skinner927

    Thread Starter Active Member

    Dec 31, 2007
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    ok, this clarifies a lot. I thought it was opposite.

    so how could this be achieved when I need it to go when power is applied. Would I have to do a transistor or relay or is there a special way of doing it I just don't know about?

    I'm very new to all of this, but the project i'm needing this for doesn't have the time for me to learn all of this.

    Again, thanks for the help! :D
     
  8. pebe

    AAC Fanatic!

    Oct 11, 2004
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    I'm trying to figure out what you want. I've done a search and came up with this from one of your other postings.
    It doesn't make sense to me. What, exactly, are you trying to do? Can you explain further?
     
  9. skinner927

    Thread Starter Active Member

    Dec 31, 2007
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    I'm putting a radio from a 2003 Jetta, into a 2001 Jetta. Normally you'd think it would just be plug and play, but it isn't. In 2003 and on, Volkswagen made a different radio. I want this radio in my '01 Jetta. The problem is, is that this new radio gets it's on/off signals from the Engine Control Unit (ECU). In 2001, the radios would turn on and off from a separate ignition line. There is no way to "reproduce" these signals to turn the radio on and off.

    I want to create a circuit that I would wire up to the power switch in the radio that would turn it on and off with the car.

    The radio has no problem running as it is, it's just that I have to turn in on and off every time I leave or enter the car.

    Here's an idea of what I want to do.

    [​IMG]

    This circuit needs to trip the relay for a fraction of a second once power is applied to ignition, but also trip the relay again once power is removed.

    Thanks for the help :)
     
  10. pebe

    AAC Fanatic!

    Oct 11, 2004
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    So the radio is powered all the time and just needs +12V pulses to toggle it on and off?
     
  11. jvjtech

    Member

    Jan 26, 2008
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    My first post: I will toss out some things to consider. The 555 has a wide tolerance for its operating voltage and is well suited to the wide range that the nominal +12 V supply in an automobile will run through, e.g. from below 9V to over 16 V. It would be possible to design a circuit that will trigger the 555 as a voltage across a Cap, e.g. through an RC circuit, crosses a trigger theshold as the Cap charges on power on and also as the Cap discharges on power off. My concern is that the in the power off case, there might not be enough residual power in the system to reliably power a 12V relay. Regards.
     
  12. Søren

    Senior Member

    Sep 2, 2006
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    Hi,

    This should do then:
    [​IMG]
    When turning the ignition on, Q1 pulls C2 down triggering the 555 and turning the ignition back off pulls C1 down triggering the 555 once more. Should it ever "get out of synch" due to fiddling with the ignition (could happen although highly unlikely), just press the button on the radio once manually.
    D1 and D2 keeps C1 and C2 from swamping each other.

    A CMOS (LMC)555 is specified to keep quiscent drain down, since it has to be on at all times. An ordinary LM555 would drain around 7.5 to 15mA, while the LMC555 only takes around 1/50 of that.

    No values are too critical, C1 and C2 could be anything down to around 1nF if needs be.


    Personally, I would find a way to emulate the behaviour of the ECU towards the radio, insted of having to wire into the button on the front of the radio - couldn't be that hard after all.
     
  13. Søren

    Senior Member

    Sep 2, 2006
    472
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    Btw. You might dispence of the relay and just use a transistor to "push the button".
    You can test it by bridging the points of the button with a diode (if it doesn't work the first time, try turning the diode the other way round).
     
  14. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    This alternative circuit to the 555 uses a 4077 XNOR gate array. If the inputs of a gate A are similar, then its output goes high. That makes the inputs of B dissimilar and its output goes low. The relay is then off.

    When you switch on the ignition, the top input of gate A goes high immediately but the bottom input is held low until the 220nF has had time to charge up. The result is a pulse that switches on the relay for around 1/3 sec. You can increase the pulse length by increasing the cap. At switch off the inputs to A are again dissimilar, pulsing the relay on again.

    Gate B can output about 4mA and the resulting transistor current should be enough drive for a small relay. If required, you can parallel the two spare gates with gate B to increase the available drive. D2 and the 10µF cap are to reduce negative spikes from the battery supply. You can download the datasheet from here to get pinouts.

    http://www.datasheetcatalog.com/datasheets_pdf/C/D/4/0/CD4070B.shtml
     
  15. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Yes, it's a common edge detector (and with the non-inverted version - XOR) only one gate would be needed ;)

    However, the 2 remaining gates could be put parallel to gate B as well, increasing the output current threefold (although not needed with a transistor to drive the relay).
    However, it occured to me that the OP wanted a 555 circuit?

    Besides, the leftmost resistor is not needed, as the ignition switch is low impedance to ground (through the ignition circuit) and you should probably use a much lower value than 2M2 (and a proportional higher value cap) for the timing, or you will most likely get the radio to switch on/off at random times - the automotive environment is a very noisy place, electrically speaking.
     
  16. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    As I said
    True, but all the time it was being installed in the vehicle the gates would be floating - in a hostile environment.
    I had thought about that. But going much larger than the 22onF would be going into the realms of electrolytics - with their higher ESR things could be worse.
     
  17. Søren

    Senior Member

    Sep 2, 2006
    472
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    Sure, slipped my eyes :eek:


    It ain't hostile while you install stuff, at least I hope it isn't, since it would be a bit unsafe to install anything with the motor running - and floating gates are ok when unpowered, as long as you take minor ESD precautions (i.e. don't walk up a nylon carpet and then touch the circuit to something grounded).


    The timing instability would be a higher concern than ESR in that circuit, but thinking of the purpose, that shouldn't be too bad - 470k and 1µ should do nicely IMO.
     
  18. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    Exactly. I doubt whether someone carrying this from a workbench to install it in a vehicle, with gates floating, would be thinking of ESD precautions. It would be an accident waiting to happen. It's not worth the risk for the sake of one extra resistor.

    In fact with hindsight I should have included a 100K series R in the feed to the top input of gateA, too.
     
  19. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    I think you have a pessimistic view at autoelectri installers since you have to behave rather ugly to ditch a 40k, but better safe than sorrow of course.
    Personally I have never burnt a 40k IC this way... 30V Vdd does bad things to them however ;)
     
  20. skinner927

    Thread Starter Active Member

    Dec 31, 2007
    36
    0
    I only said 555 because I figured it needed some type of timed relay and a 555 timer is cheap. I'm not electrically savvy. I can hook relays up apply resistors to LEDs but when it comes to ICs and complicated situations like this, it's way over my head. I'd like to learn exactly what everything does but at this time I need to get the circuit done and in my car.

    I'm really excited with all the responses. I'm going to try out that first idea from Soren as I can get most pieces at RadioShack.

    I wanted to know if a 100nf is == 0.01 uf.

    and if BC547 == 2N3904

    I know nothing about transistors so I tried matching everything up as close as I could. emitter-base is 6v and max volts is 60v, those were the same I though they'd work the same. Can I get a confirmation?

    again, thank you for everything
     
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