How Does a LM3915 IC: Visualized Audio Level Display Using LM3915 - LEKULE BLOG


Saturday, 20 August 2016

How Does a LM3915 IC: Visualized Audio Level Display Using LM3915

Why is Audio Level display needed?

Often in many applications, it is required to measure the level of the audio signals, for example in discotheques to get an idea about the loudness of the sound signal, in other areas to measure the noise signal.

How audio levels of signals are displayed?

Often there are many ways to display the audio level of the signal, one of them being the use of sound pressure level meters, which simply calculate the change in pressure of the sound signal. They are based on the fact that sound signals at different frequencies have different pressure levels.

Another way is having a visualized display of the loudness of the audio signal. Normally the display is in form of a sequential glowing of LEDs to indicate the level of the audio signal

By the term loudness, we mean the magnitude of sound signal, perceived by each individual such that the person can bear the sound pressure. For the same pressure, the loudness of the sound varies for different frequencies.

The above two methods measure the loudness of the sound or the volume in decibels. One decibel is equivalent to 10 times the logarithm of the power of the sound signal. Normally it is not feasible to have a large scale for the direct measurement and hence the scale is shown mostly in decibels or dB.

Before going to details about the visualized audio level meter, let us have a brief idea about LM3915
LM3915 is a dot/bar display driver which drives a series of LEDs based on the analog input. It basically drives each adjacent LED in 3DB steps logarithmic way. It operates from with a supply voltage of 3 V to 25 V.

LM3915 Pin Description
Pins 1, 10 to 18: Each of these pins is connected to the cathode of the output LED. The anode of the output LEDs are connected to the 3V to 20V supply.
Pin 2: This pin is the negative analog voltage supply and is usually connected to ground.
Pin 3: This pin is the positive voltage supply and usually the supply voltage is at min 3 V to max 20V
Pin 4: This pin is usually grounded
Pin 5: This pin is the signal input pin and the audio signal input is given to this pin.
Pin 6 and Pin 7 are shorted together. The current through pin 7 decides the current drawn by each LED.
Pin 8: It is the pin used for adjusting the reference voltage. There is a resistance of 1.2kohms between pin 7 and pin 8 such that a voltage of 1.25V is between the pins. A potential divider is connected to the resistor which is used to adjust the reference voltage.
Pin 9: This is the mode select pin and is used to select either the dot mode or the bar mode. For the bar mode, the pin is connected directly to pin 3 , i.e. to the positive voltage supply. For dot mode, the pin is left open, without any connection.

How does LM3915 works?

LM3915 basically receives analog voltage as input in the range between 0 to 1.5V and this is given to the Buffer amplifier which drives a series of 10 comparators.  There is a reference voltage source which can be programmed. This reference voltage is given to each comparator using a 1:10 Potential divider arrangement. Each comparator compares the input voltage with a reference voltage and accordingly drives the corresponding LED connected to it.

It can operate in dot mode or bar mode. In bar mode, the LEDs are driven in continuous mode, i.e. the glowing of LEDs appears as if in continuous form. In dot mode, a single LED glows at a time.
Using a LM3914 / 15 IC U2  – Dot/Bar Display Driver along with signal amplifier IC LM324 IC U1 to 4 a nice dancing light as per audio signals are feasible. High power lamps can also be used by adding opto coupler in series with the LEDs and the diac of the opto driving  the triac for 230 volt lamps. The entire display system can operate from a single supply as low as 3V or as high as 25V. Numerous mechanisms could be cascaded for a dot mode display or bar mode display with an extent of 60dB or 90 dB. LM3915 can also be cascaded with LM3914 for a linear/log display or with LM3916s for an extended-range VU meter.

The LM3915 displays are used in audio applications, power meter and RF signal strength meters.  And these displays are suited for signals with wide dynamic range, for example audio level, power and etc.

LM3915 as an Audio Level Meter

This circuit utilizes only one IC and few external components. It displays the sound/audio level as far as 10 LEDs. The input voltage can vary from 12V to 20V, but we using only 12V of voltage. The chip holds a flexible voltage reference and exact ten-stage voltage divider. The high-impedance input buffer acknowledges signals down to ground and up to within 1.5v of the positive power supply. Further, it needs no protection against inputs of ±35V. The input buffer drives 10 distinctive comparators referenced to the accuracy divider. Accuracy is normally superior to 1dB.

Using the dot mode, the LED illuminated represents the instantaneous value of the audio wave form. Both peak and average levels can be easily observed. Since the dot will be constantly moving, the LED’s are run at 30mA for sufficient power. The full-scale reading is 10volts; that is easily adjusted by changing resistor R2. The LM3915 signal input can with stand signals up to 35 volts. If there is a chance that the audio input could exceed this range, either attenuates it or include enough series resistance to limit the current to 5mA.
Audio level meter

Application – Visualized Audio Level Meter

Let us have a detailed description about the visualized audio level meter
VU Meter free circuit diagram
In the above given circuit, the audio signal is first converted to electrical signal and this electrical signal is further processed using a set of comparators to get the serial input. Audio input signal voltage can be from a cell phone, TV audio or a radio is maximum 6 volts.

The analog voltage input is given to the IC LM3915. Initially the mode is set to be in bar mode. A switch is used to set the mode in bar or dot mode.  The IC accordingly performs manipulations using the analog input and the LEDs are driven accordingly. The LEDs start glowing after every 3dB change in the audio level. Thus the audio level is displayed through the glowing LEDs at the decibel level.

Now you have got an idea about the IC LM3915 if any queries on this topic or on the electrical and electronic projects leave your ideas section below.
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