In this project, we implement the LM3915 bar-graph IC to operate a set of twenty LEDs, effectively covering a 30dB range. What sets this circuit apart is its inclusion of an automatic gain control, extending its sensitivity to low sound level while simultaneously expanding its range by an additional 20dB for louder sounds. This unique feature makes it a versatile Sound Level Indicator capable of accurately measuring sound levels across a broad spectrum.
The LEDs are set to a high brightness level, each drawing a current of 26mA when the brightness control is at its maximum setting, resulting in a very bright display. The circuit includes a mode selection switch that allows you to choose between two modes of operation: one where a moving dot of light is displayed, and the other where a bar of light changes in length.
For the sake of portability, my prototype is equipped with a small 9V Ni-Cad rechargeable battery. When the project is powered by a 9V AC-DC adapter, the battery is kept charged through a trickle-charging mechanism.
Circuit diagram
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 ladders 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.
Options
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 the music. Then an LM358 dual opamp is fine.
Construction
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 guide.
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 place.
4) 11-wire flexible ribbon cable connects to the LEDs.
5) Use shielded audio from the microphone and a rubber grommet holding it.
Parts
- R1–10k
- R2, R3, R5, R7, R8, R10–100k
- R4–47k
- R6–1k
- R9–56k
- R11–4.7k
- R12, R14–100
- R13–330k
- R15–220k
- R16–10/1W
- R17, R19–390
- R18–22k
- P1–10k audio-taper (log) pot
- C1, C4, C8–330nF
- C2–47uF/10V
- C3, C9–100uF/10V
- C5–100nF
- C6–470uF/16V
- C7–10uF/16V
- IC1–MC33172P
- IC2–LM3915P
- 5V reg–LM2931AZ5.0
- LEDs–MV8191 super-red diffused
- Electret microphone–two-wire type Box–Hammond 1591B
- Battery–9V Ni-Cad or Ni-MH
- SW1–SPST switch
- Adapter jack–switched
