LM3915 Based Spectrum Analyzer
Posted by aonomus on July 8, 2008
While preliminary, I have schematics mostly drawn up for a LM3915 based spectrum analyzer using Fliege bandpass filters to give me bands at 60Hz, 150Hz, 400Hz, 1kHz, 2.5kHz, 6kHz, and 15kHz. At this point, digikey seems to have run out of LM3915’s, so in about a week when they recieve some in stock I’ll put my order through (I can’t stand their extra $8 handling fee ontop of a second shipping charge for 7 backordered ICs).
I’ll also have schematics finished soon, however last night I threw together a quick opamp + LM3915 circuit just as proof of concept, nothing all too important, but its a start. The song is “Oh What A Night” by Frankie Valli & The 4 Seasons played off of a youtube clip, fed into the circuit.
Read the full post if you want the nitty-gritty electronics details.
While I do have a quad opamp, I only used it since it was the only opamp I had in my parts drawers, I could easily use 2 for a Fliege filter and the last opamp to complete a proper audio amp (ie: push-pull config) since the single opamp can only amplify positive voltage, and the oscilloscope clearly shows that the lower half of the waveform is lost. I suppose I could place the rectification stage prior to the opamp and avoid the use of an extra opamp, however this was just a fast mini-project. On the far right is the LM3915 chip with a 10 LED bar.
The cause for all the silly wires on the far right side of the breadboard is simply due to the need to limit the current of *all* the LEDs. The LM3915 does have its own internal voltage/current dividing resistor network so that the LED’s all draw the same amount of current, however in doing so the chip gets hot (the circuit is run at almost 20V!). All the LED anodes are tied together and connected to a 5W 270ohm resistor in order to limit the current to the bunch. I really should be using one resistor per LED (which I likely will in the final product) however I was simply out of similar resistors in the 1/4w range.
Anyway, my schematics are almost done and to my knowledge it might be one of the few public audio spectrum analyzer schematics out there once its done (unless my google-fu is weak and I couldn’t find any other similar project). Stay tuned and once I have more news I’ll make another post.