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Circuit Description 1) The electret microphone is powered by and has a load of R1 from an LM2931 5V low-dropout regulator. 2) The 1st opamp stage is an audio preamp with a gain of 101. 3) The 2nd opamp stage is a single-supply opamp which works fine with its inputs and output at ground and is used as a rectifier driver with a gain of 1.8. It is biased at ground. Since it is inverting, when its input swings negative, its output swings positive. 4) Three 2N3904 transistors are used as emitter-followers: a) Q1 is inside the negative feedback loop of the 2nd opamp as a voltage reference for the other two transistors. Hopefully the transistors match each other. b) Q2 emitter-follower transistor quickly charges C8 which discharges slower into R13 and is used as a peak detector. c) Q3 transistor is the automatic gain control. It is also a peak detector but has slower charge and discharge times. It drives the comparators’ resistor ladder in the LM3915 to determine how sensitive it is. R15 from +5V is in a voltage divider with the ladder’s total resistance of about 25k and provides the top of the ladder with about +0.51V when there is a very low sound level detected. Loud sounds cause Q3 to drive the top of the ladder to 5.1V for reduced sensitivity. (责任编辑：admin) |
