The LED meter circuit is simpler and smaller than its analogue counterpart, and is very common in audio equipment. This circuit is based on LM3915 IC and uses the logarithmic version. This circuit is using a single IC and a few discrete components. The extra diode (D3) is included to ensure that the DC to the LEDs is almost unfiltered. C1 is included to make sure the IC does not oscillate, and is not a filter capacitor. This allows a higher LED current with lower dissipation than would be the case if the DC were fully smoothed, and full smoothing would also require a much larger capacitor. This is the figure of the LED audio VU meter circuit;
How is this circuit work? We will explain with simple ways. L1 to L8 will normally be green (normal operating range) and L9 and L10 should be red (indicating overload). This gives a 6dB overload margin when the unit is calibrated as described below. As shown, full scale sensitivity (with VR1 at maximum) is 12 Volts peak (approximately 8.5 volts RMS). This is designed for direct connection to the speaker output of an amplifier, but is still suitable for use with preamps if the sensitivity is changed. Power comes from a 15-0-15 transformer (connected to AC1-Com-AC2). You can generally use the smallest one available, as average power is quite low. The peak current is about 120mA DC, so a 5VA transformer will be sufficient to power two meter circuits. One 15V winding goes to the terminal AC1, the other goes to AC2 and the centre tap is connected to Com (Common).
How is this circuit work? We will explain with simple ways. L1 to L8 will normally be green (normal operating range) and L9 and L10 should be red (indicating overload). This gives a 6dB overload margin when the unit is calibrated as described below. As shown, full scale sensitivity (with VR1 at maximum) is 12 Volts peak (approximately 8.5 volts RMS). This is designed for direct connection to the speaker output of an amplifier, but is still suitable for use with preamps if the sensitivity is changed. Power comes from a 15-0-15 transformer (connected to AC1-Com-AC2). You can generally use the smallest one available, as average power is quite low. The peak current is about 120mA DC, so a 5VA transformer will be sufficient to power two meter circuits. One 15V winding goes to the terminal AC1, the other goes to AC2 and the centre tap is connected to Com (Common).
