Construct a LED VU Meter using LM3915 ICs

This LED VU Meter (volume-unit)is capable of monitoring and displaying power levels present at the speaker terminals ofan stereo audio power amplifier. The levels are displayed in ten discrete steps using 10 LEDs for each channel. This projectis designed to give an approximate visual indication of the audio power output of each channel.

LM3915 IC

This is a monolithic Dot/Bar Display Driver IC made by National Semiconductor. It takes an analog voltage input on pin 5 then drives 10 LED's providing a logarithmic 3dB/step analog display. When measuring power, a 3dB increase means that thepower input has doubled. As the power doubles, an additional LED will be lit until the maximum is reached. The display can be bar or moving dot depending on the connection of pin 9 to the positive supply. The LED drive current is regulated whicheliminates the need for current limiting resistors. The supply voltage can be between 3V to 25V.

The IC contains an adjustable voltage reference. A nominal 1.25V is developed across pins 7 and 8. Twoexternal resistors (R2 & R3) programs the full scale from between 1.2V and 12V applied to pin 5. 10.5V is used to turn on all 10 LED's. The voltage required to turn on all the LEDs is set by R2 and R3. The IC develops a nominal 1.25V referencevoltage (Vref) across pins 7 and 8. Since this voltage is constant then the current through R3 is also constant. This current also flows through R2. The total voltage across R2 and R3 is given by

V = Vref. ( 1 + R2/R3) + Iadj.R2

The last term is due to a small adjust current (75-120uA) flowing out of pin 8. For the values shown the voltage is approximately 10.5 volts. Since pins 6 and 7 are joined then this is the voltage applied across the internal voltage divider network. Therefore when the voltage on pin 5 equals 10.5V all ten LEDs will be lit. Vref is independent of the supply voltage.

Internally this chip consists of ten voltage comparators. The non-inverting (+) input of each comparator is connected to an accurate ten-step voltage divider network. Each comparator will therefore trigger on a different comparison level. Theinverting (-) inputs of each comparator are commoned together and connected to an incoming DC signal via a high impedance input buffer.

The resistance values of the voltage divider network are such that the comparators progressively turn on their LEDsfor each 3dB increase of input signal level. There are ten LEDs so the total range indicated is 30dB. The output of each comparator drives an LED via a current-limited npn transistor. Outputs may be run in saturation so that logic inputs on other IC's may be direct driven.

The signal to be measured is fed to the input of IC1 (pin 5) via a voltage doubling network consisting of C1, C2, D1and D2. This gives the circuit more sensitivity to low level input signals.

Capacitor C2 is charged towards twice the peak-to-peak value of the input signal (less 1.2V for the drop across D1 & D2). However, resistor R1 tends to discharge C2 between the signal peaks. Hence the DC voltage at pin 5 is equal to approximately twice the RMS value of the input signal.

The actual value of the power level displayed depends not only on the voltage across the speakers but also theresistance of the speakers themselves. The equation for calculating power is P = E2 / R. The following gives therange of power levels displayed for common speaker resistances of the LED VU Meter :

8 ohm speakers 5.6 milliwatts to 2.87 watts
4 ohm speakers 11.2 milliwatts to 5.75 watts
2 ohm speakers 22.4 milliwatts to 11.48 watts

Note that the combination of R1 and C2 makes the circuit non-linear. The power values given above are for a frequency of 1 Khz. Frequencies below 1 Khz tend to be displayed lower than their actual level while frequencies above 1 Khz tend to be displayed higher.

LED VU Meter Parts List

The parts list of the project is as shown below.

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