Sound Level Indicator

Posted on Feb 6, 2014

This project uses an LM3915 bar-graph IC driving two sets of ten LEDs for a 30dB range. The circuit is unique because it has an additional range of 20dB provided by an automatic gain control to allow it to be very sensitive to low sound levels but it increases its range 20dB for loud sounds. The LEDs are operating at 26mA each with the brightness

Sound Level Indicator
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control at maximum, which is very bright. The circuit has a switch to select the modes of operation: a moving dot of light, or a bar with a changing length. My prototype has a little 9V Ni-Cad rechargeable battery in it to be portable and the battery is trickle-charged when the project is powered by a 9V AC-DC adapter. 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. 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. 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. 5) The LM3915 regulates the current for the LEDs so they don`t need current-limiting resistors. In the bar mode with all LEDs lit then the LM3915 gets hot so the 10 ohm/1W resistor R16 shares the heat.

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