caution
New Member
I'm looking for a little bit of help with a circuit I'm building to drive some LEDs for a VU meter project going into a JVC M70 radio. Actually I'm using the ones already there for a different function, and moving the old signals to new LEDs behind the originals so they can still shine through.
I finished that part and began to install some LM3915 kit boards when I realized they didn't work at all, so I had to wire up my own circuit. After getting some LM3916s with the semi-log audio stepping and getting it running with all 10 LEDs on a test setup, I determined it has to have peak detection to look decent, so I tried the half-wave peak detector in the datasheet notes.
From what I can tell, the forward bias of the diode across the 2N3906 ruins the first 0.7V of my line-level (1.2V?) audio signal, so the first 4-5 LEDs are always on. I thought about using the headphone output to get more oomph, but I don't want the meters to vary with actual volume, as some models out there do.
I almost decided to live with the first few LEDs being unusable on the half-wave peak detector because I only have 6 LEDs per channel (actually there's 5, but I'm OR-ing an 11th, center LED with the first LED from each channel), but that doesn't really look right because although you have five LEDs in a row at the top end (+3/+2/+1/0/-1 dB) it doesn't even respond to half of the audio you hear, so I need to have at least the first LED (-20 dB) and probably the third (-7 dB) to look right.
So, I breadboarded the precision full-wave peak detector. I tried using the TL082, since it was available in the stores and is what seems to be a good-enough version of the TL072, which I've seen referred to as an equivalent to an LF353, which is what the example uses in the LM3916 datasheet. This circuit, after building it a few times, turns on the LEDs solid. My knowledge of op amps has me against a wall. I'm using precision resistors, so that's not an issue.
From what I can find, these op amps (LF353/TL072/TL082) cannot function unless there is at least 3.3, preferably 5 volts on the negative rail. If this is true, what can I do? My lowest level in the radio is ground. Can I do something with another TL082 to shift the signal down some?
I finished that part and began to install some LM3915 kit boards when I realized they didn't work at all, so I had to wire up my own circuit. After getting some LM3916s with the semi-log audio stepping and getting it running with all 10 LEDs on a test setup, I determined it has to have peak detection to look decent, so I tried the half-wave peak detector in the datasheet notes.
From what I can tell, the forward bias of the diode across the 2N3906 ruins the first 0.7V of my line-level (1.2V?) audio signal, so the first 4-5 LEDs are always on. I thought about using the headphone output to get more oomph, but I don't want the meters to vary with actual volume, as some models out there do.
I almost decided to live with the first few LEDs being unusable on the half-wave peak detector because I only have 6 LEDs per channel (actually there's 5, but I'm OR-ing an 11th, center LED with the first LED from each channel), but that doesn't really look right because although you have five LEDs in a row at the top end (+3/+2/+1/0/-1 dB) it doesn't even respond to half of the audio you hear, so I need to have at least the first LED (-20 dB) and probably the third (-7 dB) to look right.
So, I breadboarded the precision full-wave peak detector. I tried using the TL082, since it was available in the stores and is what seems to be a good-enough version of the TL072, which I've seen referred to as an equivalent to an LF353, which is what the example uses in the LM3916 datasheet. This circuit, after building it a few times, turns on the LEDs solid. My knowledge of op amps has me against a wall. I'm using precision resistors, so that's not an issue.
From what I can find, these op amps (LF353/TL072/TL082) cannot function unless there is at least 3.3, preferably 5 volts on the negative rail. If this is true, what can I do? My lowest level in the radio is ground. Can I do something with another TL082 to shift the signal down some?