Help please - 12V 555 timer - trigger with 3V signal

Discussion in 'The Projects Forum' started by dageonyar, Mar 8, 2013.

  1. dageonyar

    Thread Starter New Member

    Feb 19, 2013
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    Hi. I'm trying to trigger a 555 timer setup to output a single-shot 12V pulse. The problem I have is, the trigger is a 3.3V output, which isn't high enough to pull the trigger input high, so my 555 is always triggered. This circuit works perfect for me when I use a 12V trigger, but not the 3.3V. I don't think my 3.3V output is open-collector. A 12V 10K pull-up didn't work. Any help appreciated.

    [​IMG]
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You know that the 555 trigger operates on a low-going pulse, right?

    A logic level MOSFET should work to switch the 555 trigger low, but you may need an inverter in front of it.

    I just noticed: the reset pin on the 555 should be tied high.
     
  3. dageonyar

    Thread Starter New Member

    Feb 19, 2013
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    Ok, I'll tie RST high. I think the datasheet mentioned that was optional.

    My 3.3V device actually drives 2.95 when inactive, and drives low (0.33V) when active. It's a IQS127D capacitive switch

    http://www.mouser.com/ds/2/42/iqs127_datasheet-7419.pdf

    Not sure if I have any MOSFETs in my kit. I have some NPN and PNP transistors, and some JFETs.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Yeah, I don't think the reset pin is the cause of your problem, but it's better to keep it from floating.

    You might be able to do what you want with a transistor inverting switch circuit. Google around and see what you find.

    Or maybe someone here has a working circuit, and will post it.

    I'll look, too.
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Two NPN's should work as you need a double inversion. Signal to first base thru 10k, collector thru 10k to +12V, & base #2 , #2 collector to +12V thru 10k & pin 2 555.
     
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  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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  7. dageonyar

    Thread Starter New Member

    Feb 19, 2013
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    Does this look right? Doesn't appear to be working. When I turn it on, I have ~3V at the output of my controller, but 0.03V at pin 2 of the 555. When I activate my controller, I see 0.0V at the input.
    [​IMG]
     
    Last edited: Mar 8, 2013
  8. dageonyar

    Thread Starter New Member

    Feb 19, 2013
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    Ah, got it! Looks like the 10K at the front was a bit too much. By removing that seems to have fixed it. My controller output is limited to 1mA anyway, so I think I should be safe?

    I do however have a power-on glitch that enables my relay on first power up, and then it de-activates properly after 3 seconds. Not sure if it's a glitch on the RST pin, or output at my controller. I added a 6.8nf cap on pin 4 to GND. Perhaps not enough?
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Try a 100k from pin 4 to Vcc and a 1uF cap from pin 4 to ground.
     
  10. Evil Lurker

    Member

    Aug 25, 2011
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    Why mess with a bunch of transistors when all you need is a trim pot going off pin 5 to ground and simply compensate by increasing the resistance going into your timing capacitor?

    Remember 555's got an internal 5k-5k-5k voltage divider going to its two internal comparators and you have direct access to the upper one via pin 5. By adding a trim pot coming off pin 5 going to ground you can reduce the voltage on pin 5 and change the upper comparators normal trip point of 8v running off a 12v supply to say, 6 volts. Slap two 5k resistors in series on 6 volts, what do ya get? 3V at the lower divider/comparator. That means your input source will now be capable of pulling the pin sufficiently high after generating a trigger pulse to initiate a new timing cycle. The only catch is it changes your RC time constant, so you will need charge up your timing capacitor slower, hence the need for more resistance.
     
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