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Volume Unit Measurement with LM3915 and LM3916
The circuit was designed by two monolithic integrated circuits to measure the level of audio signals whose unit of measurement is known as volume unit.Volume Unit – an entity equal to a decibel for expressing the magnitude of a complex audio; it states the level of complex, nonrecurring, and non-periodic signals such as music and speech
- TL072 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
- LM3915 – a monolithic integrated circuit that detects analog voltage levels and powers ten LEDs, LCDs or vacuum fluorescent displays, supplying a logarithmic 3 dB/step analog display which can be calibrated logarithmically and is suitable for log measurements as logarithmic bar graph driver
- LM3916 – a monolithic integrated circuit that detects analog voltage levels and powers ten LEDs, LCDs or vacuum fluorescent displays, supplying an electronic version of the popular VU meter which can be calibrated by decibel values and used as a bar graph/LED driver chip
The VU meter specifies the non-periodic, level of complex, and non-concurrent signals like speech and music. The decibel meter and the VU meter must be treated as very different devices since the decibel meter should not be used for complex waves while the VU meter should not be used for steady-state waves. The ratio of voltage or power pertains to the unit known as decibel. The volume units are used to calibrate a volume indicator since the volume unit is a measure of a complex audio waveform. A value of -20 to +3 dB is the typical scale of a volume unit.
Both IC LM3915 and LM3916 have a change of the display from a bar graph to a moving dot display by one pin. A single supply as high as 25 V and as low as 3 V can operate the whole display system. To eliminate the need for current limiting resistors, the LED current driver is programmable and regulated. Both ICs contain an accurate ten-step voltage divider and an adjustable voltage reference. Ten individual comparators referenced to the precision divider is being driven by the high impedance input buffer which receives signals from the ground and up to within 1.5 V. although it does not require protection from +35V, the accuracy is normally better than 0.2 dB. Since both ICs are very easy to use, only a single resistor may be added to the ten LEDs for a 1.2 V full scale meter. To program the full scale anywhere from 1.2 V to 12 V in dependent of voltage supply, another resistor is required to be added. A single potentiometer can easily control the brightness of the LED.
Both ICs are very versatile that their outputs can power vacuum fluorescents, LCDs, incandescent bulbs, and LEDs of any color. In a resolution or an increased range of 60 dB or 90 dB, multiple devices can be cascaded for a bar mode display or a dot. The temperature rating of both ICs during operation is from 0ºC to +70ºC while they are enclosed in an 18-lead molded DIP package. They both have features such as directly powering CMOS or TTL, current regulated outputs for open collectors, able to withstand +35 V without false outputs and damage, programmable current output from 1 mA to 30 mA, down to ground input operation, 3 V to 5 V single supply operation, referenced internal voltage from 1.2 V to 12 V, expandable display of 70 dB for LM3916 and 90 dB for LM3915, externally selectable bar or dot display mode by user, and 3 dB/step at 30 dB range.
The IC1 utilizes the high speed JFET TL072 which has dual input operational amplifier. They are designed as low noise versions with low input bias, offset currents, low offset voltage coefficient, and fast slew rate. It is suited for high fidelity and audio preamplifier applications due to its low harmonic noise.
The bipolar output stages are coupled with each JFET inputs on a single monolithic chip.
In the circuit, the TL072 IC1 does the amplification and variation of signals the use of the trimmer TR1 to adjust the gain of the circuit. The half wave rectification of the signal is being done by IC2. The type of indication that will be shown from the LED can be controlled by the switch S1. The gain of IC1 at first stage is 1 since the input voltage is around 7.8 V due to the values of resistors R6 and R7 in the circuit. The difference between the LED 10 and 11 would result to a 3 dB volume unit. The positive supply has more current to provide but not overloading the current through the LEDs.
D1-19=LED 3 or 5mm any colour.
VU meters are typically utilized for recording live music rather than radio broadcasts and compressed television as they provide a manual level of control. They are useful in operating and designing audio systems since the magnitude of speech signal should be characterized and measured in some fashion. The trimmers are common in precision circuitry like A/V components with proper adjustment during the servicing of equipment.
The 3 d/step display of the LM3915 and LM3916 goes well with wide dynamic range of application such as audio light intensity, power, vibration, or level. The audio applications include indicators with peak or average level. The LED bar graph have replaced the conventional meters which result to a more rugged display with high visibility and a faster responding that retains the simplicity of interpretation of an analog display. The logarithmic LM3915 can be cascaded with LM3916 for an extended range VU meter and LM3914 for a linear/log display.