Help with 555

Discussion in 'General Electronics Chat' started by sage.radachowsky, Apr 12, 2011.

  1. sage.radachowsky

    Thread Starter Member

    May 11, 2010

    I have been on this forum a lot and love this place. I'm pretty experienced with lots of circuits but one thing I have never used is a 555 timer.

    I know where to come for advice, because I know that some great people here are experts on the 555.

    I just put one on a board as a power-on indicator, and I meant it to blink about every 2s, for a brief blip. But it is not working. Attached is the circuit I used.

    I am using an ICM7555 from NXP for low current draw as compared to other 555s. My supply is over 9V (six AA cells in series).

    What is happening: the LED is always on but very very dim. I am using an LED with 1.8V for Vf and 2mA rating. I used the 5K resistor so it should get about 1.6 mA when OUTPUT is low. However, OUTPUT is not going low. It is always staying high but about 0.1V lower than V+. This is enough for the LED to glow just very dim.

    Any ideas what is wrong? Does the circuit look right? As far as I can see, it is the same one in the data sheet for astable operation. I checked the pinout many times, and it seems right to me.

    Any clues? If you 555 experts say it looks good then I will try another chip, perhaps this one is fried somehow.

    Thanks in advance.

  2. sage.radachowsky

    Thread Starter Member

    May 11, 2010
    Okay, so i figured it out. It was working, but way too fast... I changed the capacitor to 1 uF and it is working perfectly. I just did the calculations wrong for the time constant.

    In hindsight, I can see my own stupidity, which is a nice feeling!

    Namely, it is obvious in hindsight that no LED would *ever* produce light with only 0.1V from anode to cathode... what was really happening was that it was cycling so fast that I could not see it, and my multimeter was averaging out the voltages of the square wave.

  3. SgtWookie


    Jul 17, 2007
    Change the 2 MEG resistor to 2.7 Meg, the 20k resistor to 100k, and the 5k resistor to 3.9k Ohms. That, along with the change to a 1uF cap, will give you just about the timing you want.

    If you want the flash to be longer or shorter, increase or decrease the 100k (20k) resistor.
    sage.radachowsky likes this.