VU Meter (LM3915) – B2P20 (BBox 2)
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The LM3915 is another popular IC that can
be used to build interesting circuits. It is designed to drive 10 LEDs depending on
the analog voltage signal applied to the input. The LM3915 has an adjustable voltage reference
and each of the LEDs represent a 3dB step, this makes it a perfect chip to build a volume
unit meter or VU meter. It can only handle one input at a time, so
we use two LM3915 ICs to handle both the channels. The audio signal from the Bluetooth receiver
is of a low amplitude and we use an LM358 op-amp to amplify this. Since the LM358 contains
two individual op-amps, a single IC is sufficient for both the channels. The circuit for the
right and left channels are identical so we can simply look at the right channel for now. The audio input is first fed into the op-amp
which increase the signal amplitude by approximately 8 times. The signal is then passed through
a capacitor to block the DC component and is then passed through a trimpot, the output
of which is fed into the LM3915 IC. As we will see later, the trimpot allows us to adjust
the output signal and this would only need to be done once. Resistors R10 and R11 set
the reference voltage which is about 450mV when the circuit is powered with 5V. R12 is
used to set the LED brightness. This is what the assembled PCB looks like.
Let us connect the audio output from the Bluetooth speaker PCB to the VU meter and
let us switch ON the power supply. We can adjust the audio level that is fed to the
LM3915 by adjusting the potentiometers. By default, the LM3915 represents the audio level
by illuminating a single LED, but since the actual audio
signal level changes rapidly, we not only see the peak, but also the lower audio levels
that build up to the peak. Adding a jumper to the mode pin will switch
the LM3915 into the bar mode, which illuminates the LED corresponding to the peak audio signal
level and all the others below it. Here we have one LM3915 in the dot mode, with the
other in the bar mode. One thing to note is that even though we have
the same amount of current flowing through the LEDs the green LED appears to be much
brighter than rest. One way around this would be to add individual current limiting resistors
for the green LEDs. I will be creating a Bluetooth speaker system
to use with my computer complete with an enclosure, subscribe to the channel if
you would like to see that and more DIY builds in the near future.

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