Posted on Feb 5, 2014

Power for the circuit is provided by B1, a 9-V battery. Total battery current is 14. 5 mA, so an alkaline battery should last about 250 hours. Audio signals are picked up by microphone MIC1. The output of MIC1 is buffered by Q1 to maintain the 3000- © load of resistor R2. The input impedance at pin 16 of the NE614 (U1) is 1600 © to reduce the gain

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on the high end and preserve the RSSI linearity. The RSSI signal is a current source that flows through R6 to establish a voltage at pin 5 of U1;the RSSI output voltage is a function of the input SPL. Capacitor C10 removes high-frequency components. The slope of the RSSI line is nominally 0. 084VCC/10 dB. Op amp U2-b is configured as a non-inverting voltage buffer with a gain of 1. 2; multiplied by the RSSI slope, this produces a slope of 0. 1VCC/10 dB. The display, DISP1, is a 10-bar LED that is used to indicate sound level in 10-dB increments from 30 to 120 dB(A). The display is driven by an LM3914linear bar-graph driver, U5. The internal voltage regulator of U5 functions by establishing a constant current through R15 because the voltage between pins 7 and 8 is maintained at 1. 25V. The resulting voltage at the emitter of Q2 is about 5V, which powers U1, U3, and the internal voltage-divider string of U5 at pin 6. Each segment of the display will increase by 10 dB, independent of the exact magnitude of VCC While the slopes of the display driver and the RSSI signal are the same, they might not have the sanit offset. For that reason, an LM334, U4, is used to source a constant 10- A current through R17 to generate a fixed off-set voltage on top of the composite RSSI voltage at the output of U2-a. By adjusting the value of potentiometer R17, differences in microphone sensitivity can be accommodated so that the display reads...

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