Sound Level Indicator

[jon1]

And I want to know, if I add one or two other LM3915, how would I connected them to let them work properly and extend the range? And should I not include the AGC-based dual range (and where is it?) if I decide to add more LM3915?

J.

 

[audioguru2]

Hi Jon,
The datasheet for the LM3915 shows how to cascade two of them for an extended range. Then the most sensitive one will have an input DC voltage of only a few milli-volts so an offset voltage adjustment should be made to cancel the input offset voltage of the most sensitive LM3915 and the full-wave peak detector opamp.

The automatic gain control in my project is provided by Q3, R14 and C9. When the audio level is higher than the amount that makes Q3 turn on and make the voltage at C9 (pin 6 of the LM3915) higher than the 0.5V by R15 then the range increases gradually by up to 20dB.

 

[jon1]

OK but the datasheet shows how to cascade two LM3915 in Dot Mode, does that mean the I can't change the mode to bar after? And if I cascade two LM3915, can I still use the 9V battery or should I change it with something with a higher value? I think I'll just keep it like it is for now...

J.

 

[audioguru2]

OK but the datasheet shows how to cascade two LM3915 in Dot Mode, does that mean the I can't change the mode to bar after?

They say how to connect the Mode pin 9 of the first LM3915 to pin 1 of the second LM3915 and a resistor across LED #9 for Dot mode. For Bar mode then leave both pins 9 disconnected.

And if I cascade two LM3915, can I still use the 9V battery or should I change it with something with a higher value? I think I'll just keep it like it is for now...

They show a reference voltage of 10V so the LM3915s should have a 12V supply. But then a power resistor would be needed to reduce the power dissipation in the LM3915s in the Bar mode.
A 9V battery cannot supply 500mA if 20 LEDs are lighted together with 25mA each.
A 9V battery drops to 6V during use.
The full-wave peak detector in the datasheet needs a positive and a negative supply, unless you make one like mine.

 

[jon1]

I did the circuit again, but now all the LEDs are working together and when I tap on the electret mic, the LEDs seem to react inversely, meaning that the LEDs turn off when I tap on the mic. (And it's really not as sensitive as it was when it was working well...) I verified the circuit three times and I found no problems, but I don't know why it isn't working.

J.

 

[audioguru2]

Again, without a sound, measure and post the DC voltages at pin 5 and at pin 6 of the LM3915.
Pin 5 should be 0V and pin 6 should be about 0.5V.

Pin 5 is the input. Pin 6 sets the input voltage that will light all LEDs:
With 0V input then no LED will light.
With 0.022V then the first LED will light.
With 0.088V then the 5th LED will light.
With 0.5V then the tenth LED will light.

 

[jon1]

Without the mic connected, voltage of pin 5 varies between 0 V and 5 V and there's no current on pin 6. But everything is connected well. I just don't get it...

J.

 

[jon1]

And I want to know, does it change something if I connected all the transistors' collectors together and then to +9V and all the three bases together to the 4.7 K connected to the ground and a wire going to pin 7 of the dual opamp?

J.

 

[audioguru2]

Didn't you use a special MC33172 dual opamp?
R7 grounds its input so the output at the emitter of Q2 which connects to pin 5 of the LM3915 should also be ground. It will oscillate if the 9V battery is low.
Didn't you use a special LM2931AZ-5.0 regulator?

Everything connected to +9V on the schematic are connected together. The collectors of the transistors are connected to +9V.

The bases of the transistors are connected to gether, are connected to R11 and are connected to pin 7 of the MC33172.

Pin 6 of the LM3915 is a 28k resistor to ground. It is fed current from R15 which makes its voltage 0.565V as here:

 

[petryna]

Hi Audioguru,

I am new to this site and I am in the middle of building this project but I can not seem to get it working properly. I know I have a serious problem as the the dc voltage of pin 5 of the 3915 is 5.2V and pin 6 is 1.25V. In dot mode the tenth set of led's is on and in bar mode, all ten led's are on. The signal is simply way to high. I have checked the circuit and everything looks the same. I used a 78L05AC 5V regulator. Could this be my problem.

Thanks, Adam

 

[audioguru2]

Hi Adam,
I used a low-dropout 5V regulator because sometimes my project is powered from a Ni-Cad "9V" battery that is actually only 7.2V and its voltage quickly drops to 6V that is too low for a 78L05 regulator.

When there is no sound, pin 5 of the LM3915 is fed 0V from the emitter of Q2. The bases of Q1 and Q2 are fed close to 0V from the output of IC1b. IC1b has its (+) input grounded by R7 and is a DC follower so the outputs of Q1 and Q2 are also at 0V.

IC1b is an MC33172 dual opamp which is similar but better than an LM358 dual opamp. Both of them have inputs that work at the negative supply which is 0V in this circuit and have outputs that go down to very close to ground. If an ordinary opamp is used then its inputs don't work if they are within a few volts from ground and their output goes only as low as about +1.2V.

With no sound, pin 6 of the LM3915 is about +0.56V which rises to about +6V when there are loud sounds.

 

[petryna]

I used the 33172 op amp so that shouldn't be the problem, however my signal to the 3915 is always high. Even if I take the mic out of the circuit the signal is like 5V or so which if my understanding is correct be at most 0.5V? Pin 6 is also high and the led's are always on. Do you have any idea what may be causing this. Everything in the circuit is correct. Could it be that my mic isn't working?

Thanks, Adam

 

[audioguru2]

With no sound, pin 5 of the LM3915 is supposed to be 0.00VDC, not 5.0VDC.
It becomes 5.0VDC only when the sound picked up by the micophone is loud.

Maybe the preamp is picking up mains hum. I used a shielded audio cable only 2cm long from the mic to avoid the cable picking up mains hum.
If you built the circuit on a breadboard then you will have plenty of mains hum all over the place which is amplified. I built mine on a compact stripboard which is like a pcb.

The output of IC1b feeds Q!, Q2 and Q3. These transistors all have the same DC input and output voltages so if the output of Q1 is +5V (to pin 5 of the LM3915) then the output of Q3 is also +5V (to pin 6 of the LM3915). 

 

[al_faughn]

Hello

I'm going to try this project. However, it seems that MC33172P is hard to find in my country.
Is there a good alternative to MC33172P?

Also, MC33171 and MC33174 are available. When I searched the datasheet, they have the same. Are they different? Can I use any of those 2 as an alternative? (Sorry, I'm new to these stuff)

 

[audioguru2]

... it seems that MC33172P is hard to find in my country.
Is there a good alternative to MC33172P?

Go to www.farnell.com and look to see if they have an office in your country and have the IC.
The MC33172 works well up to 35kHz. You can use an LM358 (its pin numbers are the same) but it has poor performance above 2kHz.
You can use an MC34072 (it also has the same pin numbers) but its supply current is higher.

Also, MC33171 and MC33174 are available. When I searched the datasheet, they have the same. Are they different? Can I use any of those 2 as an alternative? (Sorry, I'm new to these stuff)

The MC33172 has two of the opamps of an MC33171. An MC33174 has four of the opamps. The opamps are the same. The datasheet shows the different pin numbers.

 

[al_faughn]

Oh, I see. I'll try looking for MC34072.

MC33171 = 1 opamp and MC33174 = 4 opamps

Can I conclude that I can use two MC33171 instead of 1 MC33172 for this project?
MC33171 and 174 are available in our country. Thanks again!

 

[audioguru2]

Yes you can use two MC33171 instead of one MC33172. Both must be powered.