VOX Problem

Discussion in 'The Projects Forum' started by Sumsum, Apr 11, 2013.

  1. Sumsum

    Thread Starter New Member

    Nov 10, 2012
    2
    0
    Hello all,
    I have to build a low power VOX, and I need your help.
    There are many designs on the web, however the one I need to build has
    to work from 3V maximum(!) and as less as possible.
    The delay should be approx. 3 minutes.
    I've done many experiments using transistors and 2 diodes "pump", but I had no success so far. As I am limited by low voltage, I cannot use CMOS IC.
    As well, all the circuit I've tested are not switching off sharply, just depending on the RC, so they are not too effective for me.
    Another problem is the charging delay of the RC timing. It takes too long charging, from the time it gets the first sound, so it cuts the beginning of the word.
    Using 555 or a CMOS from a 9V battery circuits work very well, but from 3V it is impossible finding a decent circuit.
    One more question: I am using mmbt3904 transistors for the AF, and even I can see on my oscilloscope nice amplification from 2 to 3 V, I assume there are other NPN SMD transistors for lower voltages.
    I'll be thankful for any suggestion, and I really appreciate a schematic circuit.
    Thanks in advance,
    S
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    Why can't you use CMOS ic's? You are using a 555 cmos timer. The delay time is the problem. I have used the cmos 555 on 3V many times, but to get the 3 minutes of delay can be a problem because of leakage current of electrolytic capacitors. One cmos divider like a 4060 could do the trick.
     
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