0 volts, I need zero volts output at no signal of the peak detector circuit

Thread Starter

Arjune

Joined Jan 6, 2018
182
I need zero volts output at no signal of the peak detector circuit so I can drive a audio display level circuit that uses the lm3915. What voltage does potentiometer r3 has to be set at and what would be the value of r4 to achieve this? My output without r3 or r4 is 47 millivolts and I need it to go lower to zero volts if possible. I plan to show the level of an audio line amplifier with LED display using the lm3915.
 

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Audioguru again

Joined Oct 21, 2019
4,688
You are using a cheap old LM324 and your output amplifier is amplifying its input bias current in your R2.
I use my peak detector fed AC from the preamp and I use a better dual opamp but it is not available anymore.
 

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DickCappels

Joined Aug 21, 2008
8,710
Using your existing offset adjust circuit, choose a value of R4 that will allow up to 10 microamps to flow from the wiper of your offset adjust pot to the inverting input when the inverting input is at ground (LM324 pin 4). I would calculate it for you from V+/5 microamps but I don't what your V+ is.

Also, the output of the LM324 cannot be driven all the way to ground, so help out the output stage with a 1k resistor from each output to ground.


Alternate solutions:
Using information your provided and following Audioguru Again's advice, there are two ways to get very close to zero millivolts on the output when there is no audio on the input:

Alternate 1) Remove the offset adjustment circuit; Reduce all resistors by a factor of 20; Multiply all capacitors by a factor of 20, or

Alternate 2) Get an op amp designed in the last 20 years with very low input bias current. and with rail-to-rail input and output.
 

Thread Starter

Arjune

Joined Jan 6, 2018
182
Using your existing offset adjust circuit, choose a value of R4 that will allow up to 10 microamps to flow from the wiper of your offset adjust pot to the inverting input when the inverting input is at ground (LM324 pin 4). I would calculate it for you from V+/5 microamps but I don't what your V+ is.

Also, the output of the LM324 cannot be driven all the way to ground, so help out the output stage with a 1k resistor from each output to ground.


Alternate solutions:
Using information your provided and following Audioguru Again's advice, there are two ways to get very close to zero millivolts on the output when there is no audio on the input:

Alternate 1) Remove the offset adjustment circuit; Reduce all resistors by a factor of 20; Multiply all capacitors by a factor of 20, or

Alternate 2) Get an op amp designed in the last 20 years with very low input bias current. and with rail-to-rail input and output.
V+ is 12v dick
 

Audioguru again

Joined Oct 21, 2019
4,688
57 years ago I bought some NSM3900 National Semiconductor modules that have a surface mounted LM3915 and a strip of 10 LEDs.
I cascaded two modules as shown in the datasheet of the LM3915 to made a 60dB display that has 20 LEDs.
A couple of years ago some modules were on ebay for $50.00US each!
 

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Thread Starter

Arjune

Joined Jan 6, 2018
182
I set the potentiometer to 2.9 volts and used a one Meg r4 allowing 2.9 microamp and the output voltage of the lm324 was reduced to 13 millivolts. That was okay for me with the lm3915 input. I'm sorry but the 10K resistor should be R5.
 

Thread Starter

Arjune

Joined Jan 6, 2018
182
57 years ago I bought some NSM3900 National Semiconductor modules that have a surface mounted LM3915 and a strip of 10 LEDs.
I cascaded two modules as shown in the datasheet of the LM3915 to made a 60dB display that has 20 LEDs.
A couple of years ago some modules were on ebay for $50.00US each!
I bought the lm3915 kit on eBay for less than $2 each.
 

MrSalts

Joined Apr 2, 2020
1,762
57 years ago I bought some NSM3900 National Semiconductor modules that have a surface mounted LM3915 and a strip of 10 LEDs.
I cascaded two modules as shown in the datasheet of the LM3915 to made a 60dB display that has 20 LEDs.
A couple of years ago some modules were on ebay for $50.00US each!
Are those your LM39xx modules or are those available somewhere?
 

Audioguru again

Joined Oct 21, 2019
4,688
Since the LM3915 is not made anymore and since National Semiconductor was bought by Texas Instruments over 9 years ago, then the LM3915 ICs are old ones and maybe are defects that were found in the garbage. Maybe they are fakes.
The AliExpress modules have the IC mounted in an IC holder maybe for some to be tested to find one that sort-of works.
I never buy cheap junk from ebay or AliExpress.

I have three of the 57 years old NSM3900 modules that some people are selling on ebay for $50.00US and mine still work. But the old red LEDs are dim.

I increased the contrast of the module's image:
 

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Ya’akov

Joined Jan 27, 2019
5,663
The LM3914 which appears to be substantially the same is still available from TI only in a surface mount package. National Semiconductor also makes it, but though Mouse only has it SMD, MPJA has 18-pin DIP versions, here.

I don't have the impression that MPJA sells counterfeit parts. I have purchased many things from them and they've all been above board, so my surmise is this is old new stock they got from somewhere. It is also considerably cheaper than the new, SMD parts.
 

Audioguru again

Joined Oct 21, 2019
4,688
The LM3914 is a linear voltage meter that has a range of only 20dB. Each step has a different number of dBs.
The LM3915 was a logarithmic audio level meter that has a range of 30dB, Each step is 3dB.
 
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