The LED Voltage Unit Meter is actually a voltmeter. The voltage is displayed on a 12-LED bar-graph. The circuit is based on operating amplifiers.
It is a classic analogue circuit based on voltage comparators. Each comparator is made up from an operational amplifier which operates on an open loop mode. There are 12 voltage comparators. Each one of them operates at a different threshold voltage and drives a LED, every time when the signal becomes greater than the threshold. The 12 different threshold voltages are provided by a resistors-based scale. The specific circuit topology has been extensively analyzed in a special article, in which you may refer to for more details.
Resistors R14 to R25 form a logarithmic scale network and provide 12 consecutive voltages, with step 3db. Thus the VU meter, becomes also a logarithmic one.
The D1 LED, by default, lights-on at 0db, D2 lights-on at -3db, D3 at -6db and so on. The D12 LED lights-on at -33db. Therefore, the dynamic range of the meter is 33db. The values of R14 to R25 can obviously set on a different manner to provide any different step or dynamic range. Any possible arrangement can be set, based on the design equations listed in the circuit analysis. In the case of all resistors will set to have the same value, the circuit will become a linear meter.
In addition to any possible arrangement of the resolution step and of the dynamic range, the topology can also be extended to have more than 12 LEDs, by using more comparators. In any case, however, the printed circuit we provide here is only for the 12 LEDs version.
The meter is actually a DC voltmeter. For AC readings (which is the case for audio), a rectifier and a peak detector are used. These units are based on C6, C5 capacitors, diodes D13, D14 and R29 resistor. The rectifier and the peak detector extract a DC component from the AC input signal. The response of the meter is mainly depended on the values of R29 and C6 and is set to be relatively slow. By altering the values, it may be adjusted to any desired level.
When the input signal is applied to P2 input and due to C5, the circuit operates exclusively as an AC signal meter. Alternatively, it may also operate as a DC voltmeter. This can be accomplished by applying the input signal to the DC input P3. By using P3, instead of P2 input, the C5 and the rectifier are bypassed and the signal is applied directly to the comparators.
Trimmers R27 and R28 can adjust the sensitivity of the circuit, in order to match it to any signal source. The R28 actually allows the input signal to be attenuated, while the R27 can set the minimum level at which the least significant LED (D12) is activated.
Assembling the circuit
The assembling procedure is relatively simple. The LED VU-meter and can be easily assembled on the printed circuit we provide below. All resistors are of the standard 1/4W type. You may use any resistors with tolerance of 5% or better. However, R14 to R25 must be of 1% or better accuracy.
You may use whatever type of LEDs you wish. You may also alter the values of R1 to R12 to adjust the brightness of the LEDs. The LEDs can also be soldered to any side of the board (top or bottom). Rectangular cross-section LEDs, or LED bar-graphs, usually result on the best aesthetic.
Τhe Printed Circuit Board Artwork for the 12 LED Voltage Unit Meter