Sound Level Indicator
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 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
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.
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
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.
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.
1) You could use a switch to change the brightness instead of a pot,
or leave it bright.
2) You could use an LM358 dual opamp (I tried it) but its output
drops above 4Khz. The MC33172 is flat to 20kHz with this high gain.
3) You could add a 1uF to 2.2uF capacitor across R5 so the indicator
responds only to bass or “the beat” of music. Then an LM358 dual
opamp is fine.
1) The stripboard layout was designed for a Hammond 1591B plastic
box with space in the lower end for a rechargeable 9V battery. One
bolt holds the circuit board and a second bolt was cut short as a
2) A second piece of stripboard was used on a diagonal to space the
LEDs closely together. A few LEDs needed their rim to be filed
slightly to fit.
3) A third piece of stripboard was used as a separating wall for the
battery and it interlocks with the LEDs stripboard to hold it in
4) 11-wire flexible ribbon cable connects to the LEDs.
5) Use shielded audio from the microphone and a rubber grommet
R2, R3, R5, R7, R8, R10--100k
P1--10k audio-taper (log) pot
C1, C4, C8--330nF
LEDs--MV8191 super-red diffused
Electret microphone--two-wire type Box--Hammond 1591B
Battery--9V Ni-Cad or Ni-MH