I built this circuit back in June 2010, but it wasn’t stable, and tended to motorboat (yeah, the output went putt-putt). I had to do some troubleshooting to find out what it needed to cure the problems. I finally got it running, and was I in for a surprise! This thing is super sensitive, it easily detects a whisper at 12 feet (4 meters) away. But it has to be in a very quiet area, or else the blue LED just stays lit from all of the ambient noise. As it is, I had to turn the TV off down the hall in the living room, because it just sat there and blinked with every barely audible sound from the TV more than 25 feet away.
I originally put the 100k adjustment pot to adjust the turn on threshold, but it has so much gain that the I set the adjustment pot to minimum and it still lights up really brightly. So all I have to do is ground the 33k resistor end that goes to the pot. The 100k pot could instead be used for a sensitivity or volume control by putting it between the stages where it says ‘not used’ on the schematic. [There was a 12 hour pause during which something went wrong in the website…]
The BC550C NPN transistors are high gain, low noise for audio preamplifier use, but the ubiquitous 2N3904 should work instead. The BC337 supplies vey little current so it coule be replaced by any NPN, such as the 2N3904. The BC327 has to supply a very high peak current to light the LED, so it should be a high current PNP transistor. I would use a PN2907A, 2N4403 or similar. Others such as the 2N3906 or BC557 will sacrifice brightness, but the rechargeable batteries will not be discharged so fast. But this detector should be very bright and colorful, so put a few more different colored LEDs in there. Some of the lower voltage LEDs such as the red or orange may require a resistor in series to prevent current hogging.
I brought this to work. At home it works very well, with a quiet evironment. But at work with the ambient noise from air conditioning, equipment, and people, the blue LED tries to stay on most of the time. I can go into a small room or storage room and the noise is low enough to let the LED go out. Then it detects the whispers easily from several yards away. But the other places are mostly too noisy to allow the LED to go out. And I didn’t give it a volume control, so I can’t reduce the sensitivity and the LED lights up steadily.
Back to experimenting…