Average vu-meters operate within limited dynamic range boundaries and there are not suitable for professional audio or broadcasting. Here comes the 60db Led vu-meter which can be used to cover the wide dynamic range needs of any demanding project.
The 60-db Led vu-meter has been inspired from LM3915’s datasheet. LM3915 is a popular IC which is able to sense analog voltage levels and it is able to drive 10 LEDs, providing a logarithmic 3 dB/step - 30 dB range- analog display. 30db? That’s not sounds too much for a 60db vu-meter! Indeed, however, multiple LM3915 devices can be cascaded to provide a display with a range of 60 or 90 dB.
In our 60db vu-meter, we use a pair (IC1 and IC2) of cascaded LM3915s, to achieve a total dynamic range of 60 dB. R5 is used to adjust the current in each LED on IC2 while network R5-R6-P1-R7 sets the reference voltage that determines the full-scale input signal level for IC2. The full scale for IC2 is set to 5V.
There are also additional components used to shift the reference voltage of IC2 at a level which corresponds to 30db below that of IC2. The most critical component for this precise shift is the P1 potentiometer. The current of LEDs 1 to 10 (are driven from IC1) is set from R3. R3 serves for the same purpose as R5 does too, and sets LEDs 1 to 10 current to be exactly the same current as the one used to drive LEDs 11 to 20. However, the value of R3 is not the same with that of R5. Actually, it is smaller than R5 to compensate for IC2’s internal voltage divider which is connected in parallel with the reference voltage source in IC2. The chosen value for R3 ensures that that there are no differences in LEDs brightness from IC1 to IC2.
A double-pole switch, S1, allows the readout to be switched to ‘dot’ mode instead of ‘bar graph’. IC3 is used to reduce the voltage applied on the LEDs in order to keep the power dissipation of IC1 and IC2 within safe limits.
Pins 5 of IC1 and IC2 are connected together, and both of them accept simultaneously the audio signal from the input. Actually, the input signal is not applied directly on those pins, but first is filtered and attenuated from C1-R1-R2-C4. C1 is used to block any DC component at the input. R1 and R2 form a voltage divider and C4 is added to provide a small time delay to suppress high frequency responding of the display. With R1 at 18 K as shown in the schematic, full-scale indication is reached at about 14V, which is equivalent to 50W on a 4Ω load. Depending on the output power of your amplifier, suitable values for R1 may be selected from the table below:
C4 is actually used to adjust the rise and fall time for the display by acting as an integrator – in conjunction with R2. The VU-meter intentionally "slows" measurement, averaging out peaks and troughs of short duration. The “speed” of measurement is preset according to author’s preferences but can be easily set to another value. By replacing C4, or even R2 and R1, and after some trial and error you will be able to find the ideal values according to your preferences.
The VU meter requires only one, simple adjustment of P1. The P1 must be set to provide 158 mV (5.0 V / 31.62), on pin 6 of IC1, that is, –30 dB relative to the reference voltage on pins 7 and 8 of IC2. This adjustment corresponds to an initial calibration which can be easily achieved with the use of an accurate digital voltmeter.
The circuit may be powered from a 8 to 12V power supply unit. A mains adaptor with an output voltage of about 8 VDC is an inexpensive and safe way to power the circuit.