Analog electronics, Electronic components, Instrumentation, Integrated circuits, Projects, Semiconductors

Meet the LM3915

The LM3915 is an integrated circuit that uses many operational amplifiers to measure voltage level or audio volume.

Post about the working principle of operational amplifier.Click here

How does the LM3915 work?

This chip detects voltage analog signals on pin 5, the analog signal passes through a buffer and then passes for 10 comparators with amp-ops and a voltage divider with different values of resistors in series, in a way that each comparator’s non-inverter input has a different voltage. Therefore, each LED turns on with a different minimum voltage.

inside LM3915
Inside of LM3915, this schematics also is on the component’s datasheet. Source: Stack Exchange.

To power up LM3915, pin 2 must be linked to the ground (GND) and pin 3 to the source’s positive pole. Pin 7 provides voltage comparators’ resistors, therefore, pin 4 must be grounded and pin 6 linked to pin 7. If pin 9 is disconnected, only one LED will be turned on, this is dot mode. If pin 9 is connected to pin 3, will appear a bar of turned-on LEDs, until reaches the maximum limit, where all are on.

Similar integrated circuits

Also exist LM3914 and LM3916. The main difference between the three: LM3914 turns on the LEDs on a linear scale, making it useful for measuring voltage levels. While the LM3915, turns on LEDs in a logarithmic scale of 3dB/LED, therefore, has a 30dB range. While the LM3916 has a logarithmic range of 70dB. The two latter are suitable for audio measurement.

Using the LM3915

LM3915 application
This is an application of this chip. This type of circuit is called VU-meter. Source: LM3915 datasheet.

You can supply LM3915 with a voltage between 3V and 25V, in this project, a 12V power source supplies the circuit.

For what serves the voltage reference (V_{REF})? Determines LM3915’s sensitivity and the voltage where the 10 LEDs will be on. I made many measurements, using a 10kΩ to transfer an adjustable voltage signal between 0 and 12V on input and modifying R2, to observe which voltages turn on the LEDs, using 3 different values for R2. In addition to measuring V_{REF}. The table below shows the relation of turned-on LED number when input voltage on chip’s pin 5 is gradually increased.

measured V_{REF}0.052V3.75V9.73V
LEDs onR2=47ΩR2=3,3kΩR2=10kΩ
The VU-meter schematics. SW1A key adjusts the circuit to dot mode (only one single LED on) or bar mode (LED bar on). SW2B selects if the input signal will come from a 10kΩ potentiometer or from an electret microphone. Here shows the LM3916, but the pinout is the same as LM3915.
The complete VU-meter.

Circuit operation video.

About Pedro Ney Stroski

1 thought on “Meet the LM3915

  1. A detail that I forgot to add. The R1 resistor has the function to make input to pin 5 “0” when the microphone doesn’t receive sound. To avoid fluctuating signals.

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