Discussion in 'The Projects Forum' started by tracecom, Jul 17, 2014.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    Attached is a PDF of a circuit designed and published on the web by VK4ADC. I am interested in using a simplified version for a noise maker that I need. My need is only for the PICAXE and LM386 portion of the circuit, and I have worked with both the PICAXE and the LM386, but there are a few things in the attached schematic that I would like some input on, expecially in the interface between the PICAXE output and the LM386 input.

    1. There seems to be three .1uF caps in the circuit. Are all these required?
    2. Are 1N4148 diodes suitable for the two diodes with opposing polarities?
    3. He apparently is using a 9V battery to feed a 78L05 to obtain 5V for the PICAXE, but why would he not connect the LM386 directly to the 9V battery?
    4. In the RC network, he doesn't identify the second cap. What value should I try?

    Here is the link to his project page. http://www.vk4adc.com/web/index.php/picaxe-projects/38-audio-detection/165-picaxe-08m-tone-gen

    Thanks in advance.
  2. MrChips


    Oct 2, 2009
    1. The first 100nF at the Pin2 output is to AC couple the square wave to the next stage, i.e. the signal will be clipped to -0.7V to +0.7V with the two diodes.

    The second 100nF cap in series with the 1kΩ resistor and the 47nF serves as a band-pass filter. This is followed by a second low pass filter.

    You don't need the second and third 100nF cap.

    You may choose to eliminate all of these depending on the sound you desire.

    2. 1N914 or 1N4148 small signal diodes will be fine.

    3. Powering the LM386 directly from 9V is fine. Make sure you put a filter capacitor across the battery, 470μF - 1000μF/16V.

    4. Modify the PI network to suit your taste.
    tracecom likes this.