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RR LED flasher (alternating)

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  • Nicolae Sfetcu
    RR LED flasher (alternating) (From Darren Leigh) You can get a 50% duty cycle if you use a CMOS version (a 7555 for example). CMOS has a rail-to-rail output
    Message 1 of 2 , Nov 22, 2001
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      RR LED flasher (alternating)

      (From Darren Leigh)

      You can get a 50% duty cycle if you use a CMOS version (a 7555 for example). CMOS has a rail-to-rail output swing which lets you get away with this:

                  +--------+
                  |        |
                  |  7555  |
                  |        |
             +----|2       |
             |    |       3|--+---> output
             +----|6       |  |
             |    +--------+  |
             |                |
             +-----/\/\/------+
             |       R
           -----
           ----- C
             |
             |
            gnd
      

      To make little railroad lights alternately flash, hook the output to a pair of LEDs -- one via a 3904 (small NPN) and the other via a 3906 (small PNP).

                   ^ Vcc
                   |
                   \
                   / R
                   \
                   /
                   |
                   |
                  ---
                  \ / LED   (glows when 555 ouput is high)
                   V
                  ---
                   |
                   |
                   /
                 |/
      in -\/\/\--|  3904
            R    |\
                   V
                   |
                   |
                  gnd
      

      and:

                   ^ Vcc
                   |
                   /
                 |<
      in -\/\/\--|  3906
            R    |\
                   |
                   |
                   |
                   \
                   / R
                   \
                   /
                   |
                   |
                  ---
                  \ / LED   (glows when 555 output is low)
                  ---
                   |
                   |
                  gnd
      


      (From rschramp@...)

      I think the easiest way is to build a relaxation oscilator with an opamp (e.g. 741), some resistors and an capacitor. Use two LEDs as lights.

      0V--cap-+--resist--+
              |          |                 +5V
              | |\       |                 _|_
       +5     +-|-\      |                 \ / LED
        |       |  \     |     +----resist--+
       resist   |   >----+-----+
        |       |  /     |     +----resist--+
        +-------|+/      |                 _|_
        |       |/       |                 \ / LED
        |                |                  |
        +----resist------+                  0V
        |
        |
      resist
        |
        0V
      

      This is a smitt-trigger (a circuit with hysterysis) which alternating charges and discharges a capacitor through a resistor. If the output voltage is low the upper led is lighted the current throug the led is limmited by the series resistor. If the output is high the lower led is lighted.
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