VOX is not working - any help?

[Con1]

As I can understand, there is not enough input sensitivity from the mic in order to trigger the circuit. When I tried to connect, the input pins A & B (mic input) with a piece of wire then it was working good.
What is wrong?

View attachment 36598

 

[audioguru2]

Hi Con,
On your picture, it looks like an electret microphone, but the circuit doesn't have any way to power it. The circuit is probably designed for a dynamic microphone that has a coil and magnet to generate a tiny amount of electricity when it hears sound.
When you used a piece of wire, it picked-up electrical hum and made the circuit work.

 

[Con1]

Oh audioguru, I guess you are right. Tank you.
I am going to test it and then post it back here for the public.
By the way, do you know any similar circuit using electret microphone to sagest?

 

[audioguru2]

Hi Con,
Your circuit is easily modified to use your electret microphone. Post its schematic and we will show you how.

 

[Con1]

Well here it is, unfortunately, there was no schematic diagram with the KIT so I tried to make it of my own and I hope to be right.

I tried to use a dynamic microphone as you told me to do and also a small speaker (0,4w 8ohm) but the circuit it seems to be

View attachment 36618

 

[audioguru2]

Hi Con,
The LM358 should be cold in this circuit. Is it installed backwards? It should have a notch on top on the end between its pins 1 and 8, or a dent on top beside its pin 1. Is the battery installed backwards or was it ever? If the IC or battery was ever installed backwards then the IC is destroyed and will need replacement. Are you using a 9V battery?

 

[Con1]

Hi again Audioguru,
Yes i am using a 9v battery and the LM358 is installed at the right way, If an electret mic connected Instead of dynamic mic the circuit is not responding and also there is no high temperature at the LM358.
Anyway, can you show me how to modify the circuit in order to use an electret mic?

 

[audioguru2]

Hi Con,
I don't know why your IC was at a high temperature before. Maybe your electret mic has long wires and needs a shielded cable.

Modifications for electret mic:
1) Relace C1 with a metalized-plastic 0.47uf/63V cap.
2) Connect terminal B to terminal C.
3) Add a 10K resistor from terminal A to terminal D.

If the mic is too sensitive, reduce the value of R1. Try 330K, 100K, 33K or 10K.

View attachment 36619

 

[Con1]

Hi audioguru,
I did as you said and I tried every possible way.
With that configuration, the circuit is not responding at all.
That was it; we have spend too much time for nothing;
I will try to find other circuit unless if you or someone else have to sagest something else.
Anyway , Thank you for your time;

 

[audioguru2]

Hi Con,
I have used a circuit like my modified one many times with success.
Maybe your IC got damaged when it was hot.