Metering and Instrument Circuits

LED VU Meter

VU is an abbreviation for volume unit. Thus, a VU meter is a device for measuring the level of SOUND intensity with audio equipment, such as AMPLIFIERs and TAPE RECORDERs.

Although the measurements indicated on such a meter are in DECIBELs, the zero level should not be confused with 0 dB, the THRESHOLD OF HEARING. On such meters, zero indicates the maximum DISTORTION-free level that can be handled by the device with other values greater or less than the zero level indicated as positive and negative decibels relative to it respectively.

Although there are several ICs on the market with which it is quite simple to construct an LED VU meter, there is, no doubt, still interest in building such a meter from discrete components. The meter shown in the diagram is based on eight opamps, contained in two Type LM324 chips, which function as comparators. The inverting input of each of the opamps is provided with a reference voltage derived from potential divider R3-R10. The values of these resistors are chosen to give a 5 dB step between adjacent resistors. Resistors R1 and R12 ensure that the reference voltages are higher than half the supply The non-inverting input of each opamp is supplied with the rectified input signai (D1 and D2), which is also superimposed on to half the supply voltage.

VU meter

When the voltage level at the positive input of an opamp rises above that at the inverting input, the output of that opamp goes high and the associated LED lights. The higher the input signal, the more LEDs will light. If D3 and D4 are red, the circuit may be used as a simple peak indicator, showing when the input signal exceeds a certain value.

The supply may be between 10 V and 15 V. The current drawn from it depends largely on the number of LEDs that light: it is not more than 160 mA at 10V and 110 mA at 15V

The components List
Q1-Q20: 2SC458, 2SC828,C945,C1815, 45V 0.1A, NPN Transistor
D1-D40: 1N4148, 75V 0.15A Diodes
R1-R20: 15K, Resistors 0.25W +/- 5%
VR1: 10K, potentiometer POT
C1: 33uF 50V, Electrolytic
C2: 3.3uF 50V, Electrolytic
L1-L20: LED 2 x 5 mm.

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