555 PWM Question

Discussion in 'The Projects Forum' started by malkos, Apr 14, 2010.

  1. malkos

    Thread Starter New Member

    Apr 14, 2010
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    0
    I'm just starting on learning electronics, and made a 555 astable oscillator/pwm based off
    h**p://www.sentex.ca/~mec1995/circ/pwm555.html
    Rather than the transistor, motor R2 and Schottky diode i have 2 leds hooked up. One from V+ to pin 3, and one from pin 3 to ground. This flashes them alternately. But the led sourced from pin 3 fades up, rather than blinking on...
    The only other change from the above circuit is a 10μf cap for C1, to adjust the frequency.
    Basically, how can i get the second led to turn on sharply, rather than fade up?
    Thanks for any tips...
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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  3. roltex_rohit123

    Member

    Jan 1, 2010
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    put a capacitor of 47uf 16 volts across +ve and Negative cell terminals. did you get what u want?
    if yes then every circuit of 555 needs this when you make use of both +ve and -ve from the outpin 3.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    It sounds like you may have a CMOS 555. What is the exact part number on the package?

    A standard 555 switches very cleanly. It does have one drawback though, check out chapter 4, figure 3.1 of LEDs, 555s, Flashers, and Light Chasers
     
  5. malkos

    Thread Starter New Member

    Apr 14, 2010
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    0
    Thanks for the tips folks. Got it all working as expected. it is a standard ne555, i just think i had something wrong on my breadboard. i don't have an oscilloscope, but it seems the way i had it put together was making a sawtooth, rather than square wave. After rebuilding the circuit a few times it seems to be behaving its self.
    -Malkos
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Pins 2 and 6 on this chip are a sawtooth. This design uses the fact to make the LEDs throb.

    [​IMG]

    This article is still not finished, but it had everything, including the protoboard layout...

    555 PWM Oscillator

    Bill's Index
     
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