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Low Voltage VOX circuit question

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Low Voltage VOX circuit question
« on: April 12, 2013, 01:36:12 AM »
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 maybe even less as possible. Mosfet output.
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 I miss 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 3V, I assume there are other NPN SMD transistors for lower voltages. Any ideas?
I'll be thankful for any suggestion, and I really appreciate a schematic circuit.
Thanks in advance,