Low voltage speaker driver

Discussion in 'The Projects Forum' started by Roto, Jul 17, 2011.

  1. Roto

    Thread Starter New Member

    Jul 13, 2011
    Hi folks,

    This is my first post and I guess it’s a fairly long one but I wanted to try to cover everything.

    I’m mainly a digital guy mostly running motors. I use transistors strictly as on/off devices normally.

    In this case I'm reading an SD card with a PIC microcontroller so it’s a 3v circuit.
    The card contains audio files (voice) that I will be sending out via the PWM pin.

    My problem is adding the best amp/speaker combination that can be heard clearly at about 3 feet or so in a quite room.
    I would like to use a 3v lithium battery (CR2032) to run everything.
    The speaker can be up to about 2” diameter but needs to cost less than $3.00 (if I only bought one at a time).
    I would like if it were less but I know speakers are the most important part.
    Amplification should be less than $2.00.

    I know bass is going to start at about 350hz. on the speakers (8 ohm) that I’ve looked at.
    The circuit will remain in sleep mode almost all the time. Once a day it will play a message.
    I need about 4 hrs. of total battery(s) life.
    A CR2032 looks like a little less than 0.7 whrs?
    From the limited testing I’ve done I doubt 3v by itself can produce enough sound.
    Besides just an op amp I’ve considered (without any real knowledge):
    Transformer coupling to the speaker.
    Voltage boosting circuits.
    Piezo drivers.
    A simple darlington pair or discrete fet’s etc.

    Or perhaps the most obvious going to 6v using 2 lithium batteries.
    But then what’s the best way of running the 3v PIC/ SD card combo?
    It would seem that adding this extra (small) load to one of the batteries might be OK unless this circuit can somehow be made to float between 1.5 and 4.5 volts?

    The housing for the speaker is a hollow plastic 2 part casting. The inner dimensions are not set in stone at this moment but will be roughly 4.0 x 2.5 x 0.5 inches.
    A port could be added to the casting to create a miniature bass reflex speaker enclosure?
    But operating only in the mid range because of the poor speaker.
    Also the speaker is in the bottom of the enclosure which is supported on ¾ to 1 inch legs sitting on a table top.
    I know at this point I’m going beyond electronics into acoustics etc. But maybe one of you folks out there has some knowledge on the subject of enhancing the output mechanically as well as electronically?
  2. MrChips


    Oct 2, 2009
    Personally, I cannot see this working off a CR2032 battery. I would use a regular off the shell PC amplifier/speaker that is powered from a wall adpapter. Use the PIC to generate a 600mV or about and feed into the audio amplifier.
  3. Roto

    Thread Starter New Member

    Jul 13, 2011
    As I said I also doubt that 3v is enough. That's why I ask about using 2 CR2032's for 6v.
    The PIC itself puts out a 0-3v signal up to 25ma.
    I've tried a NJM2113 op amp (at 3v). It's just enough with a 4" speaker but not enough using the small speaker that the project requires.