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 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 increases 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’s 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 corresponding to the audio level, 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. 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.