Check my Lm3915 vumeter circuit

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JAY X

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Hi!

I draw this vumeter buffer and circuit from the D self book. I want to know if the paralell diode to give the peak output is correct. Also i would like to know if the resistor values 1k2/7k5 are ok for the vumeter ref level. I expect to adjust it respect to 0.775v. 0dbu.

Jay x
 

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JAY X

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Hi!

¿Is there a way to make the leds draw less current in bar mode? I saw some schematics with a resistor in the +V rail to the leds.

When calibrating for reading input level (output of a preamp), in theory i can decide the maximum input level in dbu/ volts RMS, and then decide that the top led represents that level. I send a sine wave with that level and adjust the trimmer till the top led lights up.

Jay x
 

JAY X

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Hi!

After reading more the app notes of the LM3915, i found a schematic (the left diagram powered at +24v), to reduce the current draw in bar mode. Despite, the power supply rail is +24v. I want to make it work at +/- 16v. (32v), So I redraw the schematic for dual rail supply (the right diagram). I added a 400R resistor in the +v rail to drop the total voltage to 24v. (each led has a voltage drop of aprox 2,2-2.4v). I think it should work, but I want to hear some opinions. There is a That design note that does a simmilar thing, but with a small current source.

Thank you for your help!
Jay x
 

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JAY X

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I forgot to mention that i don't need the leds to shine too much, to reduce current draw.
Jay x
 

JohnRoberts

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Its been a long time since I messed with LM391x chips but I seem to recall that sink current was adjustable . The data sheet should explain.

JR
 

JAY X

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Hi!

I try the circuit at the right, and it does not work!. Only two leds are lit up. I did not put the electrolytic on the negative rail. Only on the positive rail. My voltage rails are: -15.5 & +15,2v so a total of 30,7v. 30.7- 22v/0.02amp= 435R limiting resistor on v+ rail. Lm3915 works only on +v rail....may missed something..?

Current adjust is not well explained, or at least i did not understand well...:(

Jay x
 

ruffrecords

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I am surprised the right hand circuit works at all. Basically you have the string of LEDs right across the supply. You cannot connect the right most LED to a supply.

What is the maximum supply voltage for a 3915?

Cheers

Ian
 

JAY X

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Hi Ian!

The max supply for lm3915 is 25v. I have 30.7v, and 435R drop resistor at v+ rail to drop the voltage to 22v.
 

ruffrecords

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Hi Ian!

The max supply for lm3915 is 25v. I have 30.7v, and 435R drop resistor at v+ rail to drop the voltage to 22v.
It would if the V- of the chip was connected to -16V but it isn't. To make this work you need to change all the 0V references for the 3915 to the -16V rail and AC couple in the audio signal

If you do that then you would have about 8V across 435R which is 18mA. So the chip is drawing 18mA and has 22V across it which means it will dissipate about 400mW. I think that is probably a little on the high side.

Cheers

Ian
 

JAY X

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Hi!

Ok!. but right now i can't do those changes in my pcb. I will stick with single supply for the LM3915. I found another schematic that could make it work with the single supply i already have. It divides the led chain in two chains. With some minor changes i could try it!

Jay x
 

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ruffrecords

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Now that is a very neat circuit and just what I need for my own LM3916 design to run off 12V heaters. As this works with the chip in dot mode, only a single output is on at a time. So it works as normal up to when pin 15 goes constant current. From pin 14 onwards Q1 turns on which in turn turns on Q2 as a constant current source to replace the one that was on pin 15. Very neat.

Cheers

Ian
 

JAY X

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Hi!

Glad to read this!!. It also can work in bar mode with pin 9 tied to v+. Maybe drawing a bit more current, but less than with leds in paralell.
 

ruffrecords

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Hi!

Glad to read this!!. It also can work in bar mode with pin 9 tied to v+. Maybe drawing a bit more current, but less than with leds in paralell.
It actually works as a bar type display even though it is in dot mode. I know this may sound crazy but I think it is true. It is the only way to make the 'string' of LEDs work. If you set the chip to bar mode I think this particular circuit will no longer work at all.

By the way, where did you find that particular schematic with the two transistors?

Cheers

Ian
 

JAY X

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Hi Sonic!

Interesting schematic!. But a bit too late! I already finished breadboarding the mods to my vumeter board. I will test the vumeter today, and see what happens!! :)

Jay x
 

JAY X

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Hi!

Good and bad news.
It works, but in a strange way. I have built this circuit around bc547/557 transistors, instead of 2n3904/3906.

There are two strings of 5 leds each. The first one lights up to a level, and when this level is surpassed, the next string of leds light up. It does so, but alternatively, maybe because the transistors are not switching as fast. So there are moments where the lower led string is off when upper led string is on.
 
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