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555 timer one shot question

Discussion in 'General Electronics Discussion' started by juscar, Oct 16, 2017.

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  1. juscar

    juscar

    2
    0
    Oct 16, 2017
    Hi,

    I need a 555 timer chip to control a switch so that when a momentary switch is activated for whatever amount of time the timer 555 gives a short one shot and doesn't repeat.


    Here's the back story..

    I'm using a Bluefruit EZ-Key Bluetooth HID keyboard controller to control the page turn on an ipad. It's a foot switch for hands free page turning while playing piano.

    As seen here:
    https://learn.adafruit.com/bluefruit-bluetooth-wireless-foot-pedal-switch/overview

    So the problem is when I press the switch it sometimes turns a few pages rapidly. The EZ-Key acts like a computer keyboard. When you hold a key it does multiple shots. So, I need one shot even if the switch is held for longer. I have googled one shot 555 timer chip and found a circuit that does pretty much the opposite. When you press the momentary switch the 555 timer makes the switch stay on for a set amount of time like 5 seconds or something. It depends on what resistor and capacitor is used and is calculated with a formula. t = 1.1 *r *c.

    I can make this a short amount of time but if the switch is still active after the shot it re triggers it. So, I need some way of triggering one shot and not resetting until the switch is released and pressed again.


    Also the circuits I have seen work for supplying a LED with power, but I need it to connect a switch to ground - like the way the EZ-Key switch works. I think I can this with a transistor but I need help with this also!!


    Any ideas?
     
  2. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Welcome to EP.

    What does the Bluefruit EZ-Key activate, a PC?
    For windows you can control the repeat rate and repeat delay of the keyboard.
    By setting them to low you may solve your problem.
    .
    Here is a screen shot of WIN XP.

    key-xp.jpg
     
  3. kellys_eye

    kellys_eye

    4,276
    1,146
    Jun 25, 2010
    The 'repeat delay' can't be disabled (AFAIK) so keeping your foot on the switch would eventually result in the page turning at some inconvenient time.

    As you surmise you need a monostable - non-retriggerable (or one-shot) - that requires the footswitch to be lifted and pressed AGAIN for the page turn option to be initiated.

    Google will reveal schematics if you use the appropriate search words.
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    You can "Edge Trigger" a 555 like this. It will not re-trigger until S1 is reopened and then closed again.

    Chris
    upload_2017-10-16_22-25-45.png
     
  5. AnalogKid

    AnalogKid

    2,361
    659
    Jun 10, 2015
    How short of an output pulse do you need, max and min values? Mechanical switch contacts "bounce". Then the switch goes click, the contacts come together, bounce off of each other, and come together again, many times. There can be dozens of on-off cycles in a few milliseconds. So there is a possibility that with a very short output pulse (say 1 to 10 milliseconds) you still would see multiple pulses. My guess is that your desired pulse width is long enough for this not to be a problem, but without more information it's just a guess.

    ak
     
  6. juscar

    juscar

    2
    0
    Oct 16, 2017
    Thanks everyone!
    Good advice here. I'm looking into the settings on the ipad to see if I can slow down the repeat rate but it doesn't seem possible. That would be the handiest solution.

    'Monostable non-retriggerable' as a google search has brought up some good ideas. I might find what I'm looking for there.

    I like that 'Edge Trigger' idea, but I couldn't get it to work. It seems to retrigger. I tried the circuit with an LED. If I keep my foot on the pedal the LED just stays on. I'll look more into this, see if I can sort it out.

    That's a good point about the mechanics of a switch. It might not be a clean one shot connection happening. But I guess I'm looking for a 10ms or so one shot which would turn the page and not two!

    I'm keep working on this.
     
  7. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Setting repeat parameters on OS here ,might help.
    You can also contact Apple support with a direct question on how to do it on the iPad here
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    What AG pointed out (switch bounce) is true but a 555 monostable should eliminate that. The edge trigger example I posted is not a complete circuit. It only demonstrates what the waveform at the Trigger Pin should look like vs the S1, R1 voltage node. The Output Pin characteristics you're describing should not be. I suspect that you took that schematic at face value and failed to add the RC timing components.

    Please post your full 555 schematic.

    Chris
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
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