LED Blinking to music. Simulation OK but on Breadboard it doesnt work.

[Nathaniel]

Hi guys,
I'm trying to make a LED blink to the sound of the music captured by a microphone.

I used PSpice to simulate the circuit:
**broken link removed**
(The 1k Resistor at VIn is the mic impedance)

That's the Bias Point simulation.

And this is the transient simulation for a 500Hz frequency:
**broken link removed**


(Yellow is V in the collector, Purple/Blue is Voltage at the LED, Green is Voltage at the base; In the upper plot we see the current in the LED)

After that, I tried to build the circuit and expected the led to turn off when I talked to the mic.
But... Nothing
Here's the results I've got:
Voltage at the base:
**broken link removed**



Max amplitude at 580mV, similar to the simulation.

Voltage at the collector:
**broken link removed**





Constant
LED always on =/

Photo of the circuit:
**broken link removed**




What did I do wrong?

Thanks in advance

 

[audioguru]

It looks like your microphone is an electret type.
It is usually fed 500uA (not just 11uA like you have). Its 1k resistor is usually fed from +5v to +9v but you are feeding it only +0.6V or less from the base of the transistor. There must be a coupling capacitor between the mic and the base of the transistor. R10 must be fed from +5V.

Why are you using a low gain power transistor? Its current is very low.
Why do you have R2 in series with the LED? Its max current from R4 is only 1.6mA which is almost nothing.

 

[Nathaniel]

Thanks for the fast reply.
I have never used a microphone before in a circuit so I had no ideia how to use it.
The transistor I'm using is a TIP31C. Why am I using it? I have no idea, to be honest =/

So, I updated the circuit to this:
**broken link removed**

And it's working =D
Thanks a lot, I really appreciate it.

 

[Nathaniel]

So, to prove that it works, I've uploaded a small video with me blowing to the microphone =P
YouTube - Produce

Tomorrow, I'll try to adjust the sensibility so I don't have to wake up the whole house just to see a LED blinking =P
Once again, thanks

 

[audioguru]

Your circuit has so many things wrong that its voltage gain will be very low and you must scream at the mic for the LED to flicker.

The electret mic should not have R10 in series with it.
R29 should be 4.7k ohms.
C8 should be a 1uf non-polarized film capacitor.
The transistor should be a high gain small signal transistor like a 2N3904.
The transistor should have an emitter resistor for DC stability and a bypass capacitor acroiss it.
The base bias resistors R23 and R24 should be calculated to provide enough base current for the transistor.
R4 should be about 180 ohms for the LED to be bright.
A second transistor is probably required.

 

[Nathaniel]

Your circuit has so many things wrong that its voltage gain will be very low and you must scream at the mic for the LED to flicker.

A The electret mic should not have R10 in series with it.
B R29 should be 4.7k ohms.
C C8 should be a 1uf non-polarized film capacitor.
D The transistor should be a high gain small signal transistor like a 2N3904.
E The transistor should have an emitter resistor for DC stability and a bypass capacitor acroiss it.
F The base bias resistors R23 and R24 should be calculated to provide enough base current for the transistor.
G R4 should be about 180 ohms for the LED to be bright.
H A second transistor is probably required.

Thanks for the reply.

Concerning A, that resistance is to simulate the impedance of the mic. The characteristics can be found in this page (they are in Portuguese): **broken link removed**

B, you said that the mic should be fed with 500uA but I only had 2k resistors at home =/
C, Only had 47u, 100n and 10n.
D, how can I know what kind of transistor should I use?
E, I actually had that in the previous schematic but since it wasn't working I started removing stuff and leaving only the basic. I forgot to re-add it.
F, how much current is "enough current"?
G, with the circuit designed as it is now, by lowering the R4 value the LED stays always on. A tried with 470Ω and the LED stopped blinking. I also remember from class that the gain is related to the Resistance connected to the Collector, in this case, R4
H, I have another TIP31C right here =P

Thanks for the help