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555 timer

Discussion in 'General Electronics Discussion' started by Senaph, Mar 29, 2020.

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  1. Senaph


    Mar 29, 2020
    I need to use a 555 timer (astable) mode to keep a LED off for 60 seconds than on for 2 seconds, a bit smoke detectors that turns a red LED on about every 60 seconds for 1-2 seconds then repeats. If it is easier to turn the LED on for 60 seconds and off for 2 seconds it won't be problem as I can invert pin 3 from the 555. I'm using a breadboard. a circuit diagram would be greatly appreciated.
  2. AnalogKid


    Jun 10, 2015
    What is your desired LED current?

    A bipolar 555 (not CMOS) can source or sink enough current to drive a normal little LED directly, so there is no need to invert the output. Any online 555 calculator will give you component values.

    If the LED must be driven by pulling the anode up to Vcc, then adding one small signal diode to the standard 555 astable circuit will give you the narrow duty cycle you want.

  3. bertus

    bertus Moderator

    Nov 8, 2019
  4. bertus

    bertus Moderator

    Nov 8, 2019
  5. Nanren888


    Nov 8, 2015
    From memory, the idea of charging and discharging the timing capacitor from the output, pin 3, is a good way to get virtually symmetrical on and off timing. As the normal charge path can be controlled with resistance to the rail and the discharge path through pin 7 can have resistance inserted to control the discharge time. You are working quite hard using charging and discharging from pin 3 to make it asymmetrical again. Does it work out that the normal 555 value calculators can't get the on and off times needed with the normal astable configuration and threshold pin?
    Last edited: Mar 30, 2020
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