This simple peak reading VU meter circuit uses six LEDs to indicate six signal levels at – 14, – 8, – 3, 0, +3, and +6dB, or any other levels having the same spacing (e.g. – 17, – 11, – 6, – 3, 0, and +3dB, if preferred). About 24mV peak to peak is needed in order to activate the highest LED indicator, so the circuit is sufficiently sensitive to be used with any normal item of audio equipment.
Circuit in lines: (click to enlarge)
The circuit is based on an LM3914N bar graph display driver device (IC1), which can be used to drive up to ten LEDs. This is connected so that with OV12 at the input only the first LED indicator switches on.
With the input raised to OV24 the second LED switches on as well; OV36 causes three LEDs to switch on and so on up to an input of 1V2 or more whereupon all ten LEDs are activated. In this circuit only LEDs 1, 2, 3, 5, 7 and 10 are included in the display, and these are D1 to D6 respectively.
The input signal is taken to a variable attenuator which enables the sensitivity of the circuit to be set at the correct level. The signal is then passed to a low gain common emitter amplifier based on Q1 which gives a tenfold boost in the sensitivity of the circuit. C2 couples the output from Q1 to the input of IC1.
R5 is the input bias resistor for IC1, and D7 protects IC1 against an excess negative input voltage. R6 sets the current to each LED at about 12mA, but as IC1 responds only to positive half cycles the LEDs can switch on for a maximum of 50% of time. This gives an effective LED current a 6mA. The quiescent current consumption of the unit is about 8mA rising to an absolute maximum or 44mA with all six LEDs activated.
To calibrate the unit, a 0dB test signal should be fed into the equipment and R1 adjusted for the lowest sensitivity that does not cause the 0dB LED to extinguish. The input impedance of the unit is about 80k and it will only lightly load the monitored equipment.
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