Help on Sound-to-Light Kit Analysis

[appleaaaa]

Can someone please tell me how this circuit amplify the electric signal which is collected by the electret microphone?

I am award that T1,T2,and T3 are used to amplify the signal, but how?
I only known that T2 is a universal voltage divider bias. How does it amplify the electric signal?

Also, what T4 does in this circuit?


Any answer would be helpful!
Thank you very much!

 

[Mr RB]

How much do you know about transistor amps?

You could google or wiki for "single transistor amplifier types" and read up there, knowing the basic types of single tranny amps is useful.

I'm assuming this is a proper homework question so I won't answer re T3 and T4, but some googling on "transistors and LEDs" might help there.

 

[audioguru]

When you learn about transistors then you will see how this extremely simple circuit works.

 

[appleaaaa]

I am aware that T1, T2, and T3 are act like common emitter amplifiers. Input is fed to the base and the collector current is β times base current. Therefore there is a amplification of approx β times. Also, RV1 can change the detect frequency of this circuit by changing its resistance. T4 acts like a buffer for matching of the output stage. The transistor amplifies this small current to allow a larger current to flow through from its collector to its emitter. This collector current is large enough to make LED works.

I still have a question about how to calculate the detective frequency of the circuit (for example, when RV1 is at its largest value, 220K, how to calculate the high pass frequency). Also, how the negative feedback of T1 and T2 works on this circuit.

 

[Mr RB]

"I am aware that T1, T2, and T3 are act like common emitter amplifiers." - Check again. You are correct that the amp type is common emitter.

" Input is fed to the base and the collector current is ? times base current. Therefore there is a amplification of approx ? times." - Check again. It's best to think of the "gain" of a common emitter amp as voltage gain, a quick google will show how the gain can be calculated based on the parts values.

"Also, RV1 can change the detect frequency of this circuit by changing its resistance." - Check again. RV1 is not for changing detect frequency.

"T4 acts like a buffer for matching of the output stage. The transistor amplifies this small current to allow a larger current to flow through from its collector to its emitter. This collector current is large enough to make LED works." - Seems ok. Needs more information, ie re how T3 affects T4.

 

[Ramussons]

The value of R12 has foxed me! (I know the value, but the purpose ...)

Ramesh

 

[Mr RB]

It looks like they tried to conserve the battery by using very high value resistors in the current paths. R12 has a high resistance but should still do its job.

 

[WTP Pepper]

T3 is biased off and will momentarily conduct when audio of a sufficient level in present. This level is set by RV1. Now look at how T4 will react as T3 conducts.

 

[audioguru]

The value of R12 has foxed me! (I know the value, but the purpose ...)

R12 shorts any leakage current from T3 or T4 so T4 does not turn on with no signal.