Monostable 555 Timer Switch Question

Discussion in 'The Projects Forum' started by jmacusaf, Dec 11, 2014.

  1. jmacusaf

    Thread Starter New Member

    Jun 11, 2014
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    Hello Everyone,

    I am very new at electronics and was hoping someone can give me a quick solution or a place to look since my research has come up short.

    Is there a way to trigger a one-shot infrared LED in a 555 Timer circuit with a momentary switch continuously depressed?

    I have read that the amount of time a switch is depressed has to be shorter than the pulse. In my project, I would like a magnetic connection to press and hold a momentary switch to trigger a one shot infrared LED flash. However I do not want the battery to drain if the pulse is just going to keep happening while the switch is depressed. Not really sure where to look. Every schematic and tutorial does not mention a continuous momentary switch press.

    Thank you for your help
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If I understand your requirement, you want a short pulse every time a switch is operated, the duration of the pulse being a short pulse, even if the switch stays depressed for a long time?

    What are the current requirements for your LED?

    How long a pulse do you need?

    What is the supply voltage?
     
  3. jmacusaf

    Thread Starter New Member

    Jun 11, 2014
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    Thats correct. A 2 second pulse operated by a 3-5v battery that only pulses 1 time even while the switch remains depressed. Its mainly to reserve battery life.

    I did not buy the infrared LED emitter yet but it will be a typical one from radio shack - Forward voltage (V): 1.28V, rated at 100mA
     
  4. DickCappels

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    Aug 21, 2008
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  5. djsfantasi

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    Apr 11, 2010
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    Wouldn't you omit D2 and R3?
     
  6. jmacusaf

    Thread Starter New Member

    Jun 11, 2014
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    Thank you! I am going to give it a try!
     
  7. Alec_t

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    Sep 17, 2013
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    Reduce R2 to 100 Ohms to get ~ 70mA through the D1 LED.
    BTW, that circuit starts when S1 is released.
     
  8. DickCappels

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    Aug 21, 2008
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    You have sharp eyes; thank you for catching my error. R3 should be removed instead of R2.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    No, a 555 starts when the input trigger goes low.
    But the circuit shown in post #4 will retrigger and the output will stay high as long as the switch is depressed.
    You need to capacitively couple the input trigger to pin 2.

    Another problem is that a standard 555 will not operate at 3V.
    You could use a CMOS LMC555 which can operate down to 1.5V. But since the output current capability of that is limited to a few mA, depending upon the supply voltage, you would also need to add a transistor buffer to drive the LED.
     
  10. Alec_t

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    Normally, yes. But in the circuit shown the button S1 also holds the reset pin 4 low while pressed.
     
  11. MikeML

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    Oct 2, 2009
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    Here is my idea. Works on 3V. Drives a grounded-cathode IR LED. Uses parts available from RatShack.

    99.gif

    D1-D2 represent the IR LED. I didn't happen to have a model that had the 1.45V Vf, so two Si diodes in-series comes close. R4 sets the LED current, but with only a 3V battery, there is not much headroom for good current regulation.

    Time delay is primary determined by the time-constant (R2//R3)*C1. Leave R2 and R3 alone, and change C1 to make it different.

    D3 and R1 discharge the timing capacitor while S1 is open, so that the one-shot is ready for a new cycle.

    There is zero standby current drain with S1 open.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Yes, I missed that it was also connected to the reset pin. :oops:
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Stick with 5 V supply.
    Go to Textbooks, top of psge, VOL VI, Experioments, Ch 8, 555 Mono----
    Re read post # 9
     
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