I'm looking to start a VU Meter project with an LM3915, and I was wondering about the audio input to the circuit. If I'm using a commercial stereo amplifier's headphone jack as the input, do I have to regulate voltage and current coming from that source? It seems to me that anything coming to the headphones would be relatively low power.
My second question would be, since that signal would be pre-amplified (I assume), would I need to use a chip like the LM386?
The levels should be just about right if you want 0dB (full scale) to somewhere around full volume. Headphone levels are usually around 1V (at high volumes) depending on the device and the volume level.
But for best results, you'll need to "condition" the AC signal and convert it to varying DC with a peak detector. There are examples in the LM3915 datasheet.
If you want to calibrate 0dB at a lower voltage/volume, you may need an amplifier. Any op-amp will do. It's probably easier to build an amplifier with the "right" gain (which you might have to find experimentally) using a regular an op-amp (just an op-amp and 2 resistors) than with the LM386.
But for best results, you'll need to "condition" the AC signal and convert it to varying DC with a peak detector.No Way, dvddoug.
How about ac-coupling the signal and clamping it (if that's necessary)?
But...
The LM3915 is for a log display.
The LM3916 is for a VU display.
(The LM3914 is for a linear display.)
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