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LM3915 LED VU meter problem - pictures included

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Oh... I thought that 'location' was some electronics term...

Finland.

OK,
You can connect two leds in series in order to reduce the dissipation in the LM3915 or add a low value resistor in the commoned anodes.

For a single LED, a common resistor of about 10R or 12R at 0.5W should be OK.
 
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Ok, good to know.

I guess the next step is to find the actual audio signal levels coming the MP3 player.

Once these audio voltage levels are known we can redesign the rectifier and perhaps a low gain preamp.
 
I'll get a 3.5mm PCB audio jack for proper signal input tomorrow with the 5K pot. I could also get rest of the possible parts needed if I'd know what to get. Could you give me some info what might be needed, I could buy some spare parts just in case.

I will also document this project for other beginners to use, I think these kind of topics are quite familiar on electronics forums...
 
I'll get a 3.5mm PCB audio jack for proper signal input tomorrow with the 5K pot. I could also get rest of the possible parts needed if I'd know what to get. Could you give me some info what might be needed, I could buy some spare parts just in case.

I will also document this project for other beginners to use, I think these kind of topics are quite familiar on electronics forums...

hi,
This image shows what the datasheet says about the accuracy of the simple rectifier circuit.
You have to decide on the degree of accuracy you want for a mP3 player display.

AAesp03.gif
 
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Well, I'm not looking for much accuracy here, more in the lines of visualization of the music audio. I probably will be happy if the LEDs are unlit during silence and fire up according to the music. I'm not sure how much accuracy this needs.
 
Well, I'm not looking for much accuracy here, more in the lines of visualization of the music audio. I probably will be happy if the LEDs are unlit during silence and fire up according to the music. I'm not sure how much accuracy this needs.
OK,
I would connect the MP3 player to the input of the rectifier as drawn on the original circuit that you posted.

See what sort of results you get on the LED's.

Lets know what you see.:)
 
But isn't there still the problem of the LEDs already lit up? Lit even without any signal?

hi,
According to the datasheet the offset should be in the order of 100mV.

I have done some simulations with LTspice and I also have all the components to construct the project for myself.
I will build the LM3914 and rectifier circuit tomorrow and I see will if I get the same problems as you and if possible suggest a solution.

I know its a pain, but can you recheck and post the voltages you measure on pins 1 thru 8 only.
 
Ok, but I have no idea what 'offset in the order of 100mV' means... :eek:

And sure, they are:

#1 = 8.4
#2 = 0
#3 = 12.25
#4 = 0
#5 = 1.15
#6 = 1.25
#7 = 1.25
#8 = 0
 
Ok, but I have no idea what 'offset in the order of 100mV' means... :eek:

And sure, they are:

#1 = 8.4
#2 = 0
#3 = 12.25
#4 = 0
#5 = 1.15
#6 = 1.25
#7 = 1.25
#8 = 0

hi,
All those except pin5 are what I would expect.
I'll contact you tomorrow.

I see that Finland has also closed down it air flights because of the Iceland volcano.:)
Its been chaos in the UK for about 2 days with flights being cancelled.
 
hi,
All those except pin5 are what I would expect.
I'll contact you tomorrow.

I see that Finland has also closed down it air flights because of the Iceland volcano.:)
Its been chaos in the UK for about 2 days with flights being cancelled.

Ok, great.

It's actually very ironic, the airport people here went to a strike as of today morning, and then they closed the airports down anyway... :)
 
The pin 5 input of the LM3915 has a PNP transistor with a max positive input bias current of 0.1uA which causes an offset voltage of +0.1V max in R5.
The transistor has a typical base-emitter voltage of 0.65V and the diode has a very low current so its forward voltage is only 0.2V. Then the input of the LM3915 gets 0.65V - 0.2V= 0.45V. The 7th and LEDs above it will light when there is no signal.

If the reference voltage at pin 7 is set to 10V then the 1st LED turns on with an input of +0.45V so it might or might not turn on when there is no signal.
 
Ok, great.

It's actually very ironic, the airport people here went to a strike as of today morning, and then they closed the airports down anyway... :)

hi,
I have the LM3915 built on a project board.
Using the same components as the original circuit except for the PNP transistor I have a BC214.

The offset at pin #5 is only +0.29V, so only the first 2 LED's are lit with no audio signal input.
By using a 5K pot on pin #4, connected between pin #7 and 0V, with the wiper of the pot to pin #4, I can back off the +0.29V on pin #5, so that no LED's are lit.

When I connect a signal generator to the input of the 100nF I see that the 1st LED lights when the input signal is 0.5Vppk and the 10th LED lights when the input is 8Vppk.

As I said previously we need to add a pre amp for the MP3 input.

You will have to find a way to measure the output signal voltage of the MP3 so that I can post a preamp circuit, use a scope if possible.

Regarding the 1V signal you have on pin #5, its possible the transistor is damaged when you had it reverse connected, try another pnp.

I'll wait for the MP3 tests...:)
 
Hello,

let's start with the 5K pot. You said previously to connect its wiper to pin #6, how come now you put it in pin #4?

Then the mp3 player output voltage. I've no idea how to do this at all, I could use more information.

I also tried to change the transistor, but nothing changed, I still have that 1.15V in the pin #5
 
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Hello,

let's start with the 5K pot. You said previously to connect its wiper to pin #6, how come now you put it in pin #4?

The the mp3 player output voltage. I've no idea how to do this at all, I could use more information.

I also tried to change the transistor, but nothing changed, I still have that 1.15V in the pin #5

hi,
Because when we used it on pin #6 it was for a test.
Now we need to OFFSET the +0.3V so that the 2 lower LED's go off.

I have tried a number of transistors and I get +0.29 or 0.3V on pin #5.????

I measure on the emitter of the pnp transistor +0.964v, thats on the diodes anode and +0.3v on pin #5 thats the cathode of the diode.
On the base of the transistor I have +0.323v

Measure the emitter and base voltages of the pnp.
 
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Well, iPods usually have an average peak of 2.5V..that must be same with almost range of MP3's. I think the present circuit should work if everything is ok being the iPod volume turned all the way up.

Gaichute, why can't you go for an entire re-assemble from opposite part of the board, so that any board related problems can be overcome?
 
Well, iPods usually have an average peak of 2.5V..that must be same with almost range of MP3's. I think the present circuit should work if everything is ok being the iPod volume turned all the way up.

Gaichute, why can't you go for an entire re-assemble from opposite part of the board, so that any board related problems can be overcome?

hi BC,
I just did a web search and the best I can find for an MP3 player is about 300mVrms into about 100R to 200R load.
 
To reduce the offset voltage of the transistor peak detector then you need a very high current gain like from a BC558C transistor and use a fairly low voltage like 3V for the supply for the emitter resistor. The BC558C has its pins reversed from a 2N3906.

With a 3V supply then the collector/emitter current is 0.22mA. Its current gain is typically 600 so its base current is 0.36uA.
The 0.36uA base current in the 101k input to ground resistors cause the base voltage to be only +36mV.

With your 12V supply and ordinary 2N3906 transistor with a current gain of only 200 then the input offset voltage is about 0.58V on the base of the transistor with no signal.
 
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