Trouble with 555/556 timers,

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  • #1266
    Avatarmelcus
    Participant

    Hi,
    A while ago I remember you posted a video about using a 555 timer to create a beacon, I couldn’t find it but I though I would have a go anyway. As the local electronics stores was charging the same amount for a 556 timer as a 555 timer I decided to just get the 556.
    So after a bit of goggling I got the pinout for the 556 and a few example circuits for a flashing led a long with a calculator to work out the flashing rate, everything was going well until I actually tried to make it :).
    So according most websites I have looked at the high time for the circuit = 0.693 * (R1+R2) * C
    I have tried a number of different values but for now lets just say
    R1 = 10K ohms
    R2 = 4.7K ohms
    C = 0.1uF
    which should give me up time of approx 10 seconds and down time of approx 3 seconds.
    In my circuit all I get is up time the led never goes out.
    So I played around with different resistors but the led never flashes and then by accident I found out the resistors and capacitor aren’t having any effect on the led at all, infact the only pins that are having any effect are the power and reset pins. In the example circuit I am using the power and rest pins ate both mean to be connected to the battery, well I found when I build the circuit as long as one of those is wired up the led connected to the output pin will light up.
    Any ideas on where I have done wrong here?
    I have tried both side of the 556 timer.
    My battery pack is 4 AA bateries.
    The circuit I am using can be found at
    http://www.csgnetwork.com/ne555timer2calc.html

    Thanks
    Mel

    #1267
    RC Tractor GuyRC Tractor Guy
    Keymaster

    Hi, I put those values into this calculator and it say’s that frequency would be around 750 Hz which I’m pretty sure is too fast for the human eye to perceive so it would probably just look like the LED was on all the time. Try to chose some values that would be under 20 Hz and you should be able to see the LED flashing, maybe a 10uF cap if you had it. I think that’s the problem anyway.

    #1271
    Avatarmelcus
    Participant

    Hi,
    Thanks for that, slightly embarrassing that I don’t know where my decimal point goes, but to make matters worse I realised I was using a .01 uF capacitor rather than .1. Anyway change of capacitor and resistors and I now have a flashing led.

    Thanks
    Mel.

    #1286
    RC Tractor GuyRC Tractor Guy
    Keymaster

    An easy mistake to make, glad it’s working for you.

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