Circuit Schematic Diagram of LED Audio Level (VU) Indicator and Alarm
The following schematic is a LED Audio Level (VU) Indicator and Alarm whose Dot Turns to bar when the signal reach the maximum level. This LED consists of 10 LEDs to indicate the level of an input signal. Only LED#1 will light if the signal is small, and when the signal is getting stronger, the light dot will move through LED#2-LED#10. When the signal reach LED#9, the next increasing will also turn on all LEDs changing the dot to a full-scale bar. This will help you to give a better alarm, telling you that the maximum level has been reached.
Chip at pin#5 will detect the input signal. It should range between 0-1.2V. Suitable for standard audio line level. LM3914 is used because it has linear scaling for the LED, the indication move linearly with the input signal : when the input is maximum, LED#10 will light. When the input is half maximum, it will light the LED #5/#6. Led #1 will light if the signal is 1/10th of the maximum.
For the input signal conditioning, connecting this circuit directly to audio signal would give fast blinking display with vague dot position, because the audio signal oscillates between an equilibrium point. To fulfill this purpose, the single transistor peak detector will be suitable.
For the logarithmic scale purpose, the National Semiconductor provides LM 3915, an IC series with pin-to-pin compatible with LM3914. Simply change the LM3914 to LM3915 and then the display will be in logarithmic scale. However, take note that you need more precise peak detector for logarithmic scale. This is because the display will have more precise resolution at low level indication. For this purpose, the precision half-wave peak detector would be suitable. Better dynamic indication in almost all level of volume control is certain if you use the logarithmic scale indicator. [National Semiconductor Application Notes]