# Meet the LM3915

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

## 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 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 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. 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. 1. Pedro Ney Stroski says: