Three Fives 555 behavior

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  • #20382
    nuggetz
    Participant

    I put together a test circuit using a real 555 timer and then used the same resistors, caps, and LED to test on the 555 kit I just assembled and the LED flashes weird. Shouldn’t the time that the LED is off = the time that it is on? The kit 555 was producing more of a blip. I dont want to attempt breadboarding anything more complex until I know whether or not this is normal or not. 

    • This topic was modified 2 years, 3 months ago by Windell Oskay.
    #21944
    Windell Oskay
    Keymaster

    The values on the example circuit were picked for being easy values to find, rather than for uniformity of the output signal. 

    Plugging the values (100 k, 100 k, 1 uF) into a 555 calculator gives times of:
    .14 seconds high
    .07 seconds low
    4.8 Hz overall frequency.
    So, look carefully at your LED blinking, and see if it looks like that. If it doesn’t, first check your *external* circuit very carefully.  It’s pretty hard to get the 555 itself to oscillate repeatedly if there is a major error inside, so it’s more likely that the problem is outside.  Next, go through the 555 and check that resistor values are in the right places, and that the soldering is all secure.
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