My first circuit

Discussion in 'The Projects Forum' started by bbucky, Oct 25, 2007.

  1. bbucky

    Thread Starter New Member

    Oct 16, 2007
    Here is my first circuit. Does it look like it will work? I have no formal training so please be gentle.

    R1 is supposed to adjust the sensitivity of the mic. When there is noise the mic triggers Q2 which activates the play button on the mp3 player. The Q2 collector-emitter circuit then "steals" current from the base electrode of Q3. This shuts off the Q3 transistor which shuts off the mic. That way repeat noises will not trigger the mic too soon.

    With the mic blocked Q2 turns off and Q3 is reactivated. The RC combo gives an adjustable time up to 10 sec before the mic will come back on. This allows the song to finish playing before the mic is activated again. Circuit restarts. At least that is how I intended it to work. I know this could be done with timers, etc, but I was trying to learn a little about electronics.

  2. thingmaker3

    Retired Moderator

    May 16, 2005
    As soon as your 100uf cap charges, Q1 will be cut off. This will happen about 1 to 10 seconds after power-up, depending on the setting of R3.

    Try putting the cap across Q1 base and ground, in parallel with R2.

    Also, the 3.3K emitter resistors don't make sense to me. Is there a reason you chose that value? Q3 will always be conducting if you stick with the 3.3k values. (Q3 Vb = 4.6 if Q2 cutoff, and will be 4.3v or more when Q1 conducts.)
  3. bbucky

    Thread Starter New Member

    Oct 16, 2007
    I see, should it look like this? Does a capacitor in parallel draw away the current from the base of Q1 until it is fully charged? If so do I even need R2? Then when Q3 is blocked will the capacitor discharge to ground?

    I wasn't real sure about the resistor values. How did you calculate Vb for Q3? If Q2 is shut off do we evaluate the base circuit as having R4, R6, and R8 in series?