The LM3915 is an integrated circuit that uses many operational amplifiers to measure voltage level or audio volume.
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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.
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
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} | 0.052V | 3.75V | 9.73V |
| LEDs on | R2=47Ω | R2=3,3kΩ | R2=10kΩ |
| 0 | 0V | 0V | 0V |
| 1 | 0.05V | 0.28V | 0.53V |
| 2 | 0.10V | 0.33V | 0.73V |
| 3 | 0.13V | 0.52V | 1.02V |
| 4 | 0.18V | 0.67V | 1.39V |
| 5 | 0.25V | 0.92V | 1.92V |
| 6 | 0.33V | 1.52V | 2.72V |
| 7 | 0.49V | 1.72V | 3.80V |
| 8 | 0.71V | 2.48V | 5.36V |
| 9 | 1.01V | 3.62V | 7.65V |
| 10 | 1.46V | 5.19V | 10.73V |
Circuit operation video.
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.