3.3V Fading LED Circuit ?

Discussion in 'The Projects Forum' started by Oregon Boy, Jun 1, 2015.

  1. Oregon Boy

    Thread Starter New Member

    Jun 1, 2015
    Hello, I am trying to design a circuit (non-555 timer) that will fad an LED using a coin battery - 3.3V. I assembled this circuit attached but it does not work...the LED stays on..but there is no fading. I am puzzed about this circuit since it claims to work. Any suggestions? or is there a better way. Thank you
  2. DickCappels


    Aug 21, 2008
    Welcome to AAC!

    I do not see a VDD connection to the chip. Connect pin 14 to +3.3 volts and see if that fixes it.

    Also you would want to ground all of the unused inputs to keep the chip from dissipating power unnecessarily.
  3. bertus


    Apr 5, 2008

    What color of led are you using?
    A red or green already need about 2 volts. a white one will need 3.5 - 4 volts.

  4. Oregon Boy

    Thread Starter New Member

    Jun 1, 2015
    Yes pin 14 is connected to 3.3V - actually I have been trying 6V to start. I am using a RED LED and it is on..but there is no fading One question is if you look at the left Capacitor I am assuming that is a 22uF ?? I am thinking this is a typo 2u2 is actully a 22uF? I will double check the connections since I am using a bread board. Thank you
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    The time constant for oscillation is 2.2sec, with 1meg and 2.2uf, its probably the time delay thats wrong for the 22uf cap and 47k,

    when its running, try shorting pin 1 to pin 14 and then see if the led dims
  6. Oregon Boy

    Thread Starter New Member

    Jun 1, 2015
    OK looks like this circuit is a no go. I have double checked all connections and resistor values. I disconnected the BJT and here is what I get at the output at what would be the Base of the BJT:
    Pin 14 - VDD is 6.7V
    Pin 1 - is 1.67V
    Pin 2 - is .09V
    Output of 22k - Base of BJT side is .12V
    If I short pin 1 to gnd then the output of the 22k will go to about 5.3V after about 6 seconds - remove the gnd and it will discharge in about 2 seconds. In any case this circuit will not toggle on its own, and now that I am looking at this circuit I think it is working as intended..that the ground should have a manual switch on it to switch between fade in and fade out. Overkill if that is the only intent. I am just looking for a 3.3V fading LED circuit - non 555 timer because of the 3.3V
    thank you
  7. ScottWang


    Aug 23, 2012
    You can using a resistor and led to replace the relay and diode as below.

    The led will be fade in and fade out about 3~4 seconds when the sw1 turn on and turn off.
  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    Of course, it should cycle automatically; that is the the whole point of using the Schmitt inverter configured as an oscillator.