Newbie 555 question

Discussion in 'General Electronics Chat' started by KrakenFan, Dec 6, 2008.

  1. KrakenFan

    Thread Starter New Member

    Dec 1, 2008
    So I'm playing with a 555 chip, you know making an LED flash, working toward a KITT/Cylon sequenceing LED array for, er, um, my son. Yeah, that's it! My son. Anyway, I set up the first part of this circuit and after getting the 555 portion set up I followed the suggestion to put an LED on it to verify that the circuit was working. I started wondering how to vary the flash rate and duration. Am I understanding it correctly that you can adjust the flash rate and duration with R1 and R2 respectively? With R1 setting the time "High" or "on" and R2 setting the time "low" or off? If that is so, what does the cap control?

    Thanks for indulging me,

  2. bertus


    Apr 5, 2008
  3. eblc1388

    Senior Member

    Nov 28, 2008
    The changing of these resistor values only give you a limited range of High/Low time. However, you cannot lower the resistor values to less than a few KΩ or raise the value to over MΩ. So the value change only give you a certain timing range. Let's say 0.5 seconds to a few seconds.

    Changing the capacitor value would move this adjustable range higher or lower. e.g. you can have 20 second High time and 15 second Low time if a larger capacitor is used, or 0.05 second High and 0.05 second Low if a smaller capacitor value used.
  4. Wendy


    Mar 24, 2008
    Check this out...

    The last entry, the oscillator, should be of special interest for your application.

    The CD4017 is a useful chippie. I used it over 20 years ago for a light chaser, my latest entry using it is here.
  5. mik3

    Senior Member

    Feb 4, 2008
    The 555 detects the voltage on pin 6 to switch from a high output to a low output and the voltage on pin 2 to switch from a low output to a high output. If the output is initially low and the voltage on pin 2 goes below 1/3 of Vcc then the output goes high and the capacitor starts to charge through the resistors. The output will remain high until the voltage on pin 6, thus voltage across the capacitor, goes over 2/3 of Vcc. When this happens the output goes low and the capacitor discharges through R2 and pin 7 (internal transistor). As you see, the output stays high until the voltage across the capacitor goes over 2/3 of Vcc. The the time the voltage across the capacitor reaches 2/3 of Vcc depends on the values of the resistors and the capacitor. For example, if you increase the resistors value then the capacitor will charge slower and thus the voltage across the capacitor will reach 2/3Vcc in a bigger time interval. Also, if you increase the value of the capacitor it will need more time to charge and thus the time the voltage across the capacitor will reach 2/3Vcc in a bigger time interval.
    This was a brief explanation.
  6. Metalfan1185

    Active Member

    Sep 12, 2008
    Good luck with your circuit, be sure to use a LM78xx series Voltage regulator to protect your circuit from the variable voltage from the alternator.

    I do automotive and novelty lighting as well, you may be interested in some other LED driving IC's that are pretty versatile,

    also check out the

    4017 (10 step sequencer)
    LM3914 (10 Segment Dot/Bat Display Driver)

    and if your really serious, there's Programmable IC's, that's where im about to venture in to.

    I have some circuits myself that may interest you, you can email me at if you want to take a look.