hi there,

[chinsoon]

need some help to understand this.

i cant understand the circuit that i found on internet. so please help me.

1. N1, N2, and N3 are inverters that form amplifier? is that true? that is what my friend told me but i am not sure about that.

2. What are N5 and N6 doing there? whats their function?

3. whats the effect of inverters having feedback using a)resistors b)capacitors.

huh, this circuit is driving me crazy.

thanks in advance

 

[Hero999]

I'm a bit stumped too.

Knowing what this circuit is supposed to do would help; I don't suppose you know?

I do know that the ijnverters are all configured to behave as linear amplifiers but I don't know why the designer didn't just use op-amps like any normal person would.

N1, N2 and N3 form an amplifier with nearly open-loop gain, C8 provides a roll-off to prevent high frequency oscillation.

D1 and D2 are biased by the R3 and R4 to overcome the 0.7V forward voltage forming a prescion hlaf wave rectifier.

N4 acts as another amplifier with C3 acting as a low pass filter.

R6, R7 and C4 from an attenuator and low-pass filter, with T1 forming a peak detector.

N5 and N6 look like an oscillator.

Is BZ1 a piezo transducer rather than a buzzer with an internal drive circuit?

I think I've got it, it's a keys finder circuit!

When you whistle at it, it beeps back!

The piezo transducer acts as a microphone amplified by N1 - N3.

D1 and D2 (as I said before) form a precision rectifer.

When sound is detected the output of N4 is pulled low, discharging C4 and turning T1 on, which allows N5 and N6 to act as an oscillator which drives the piezo transducer.

The low pass filter action of C3 prevents the output from the oscillator from feeding back through the circuit and triggering it.

 

[audioguru]

Why not read about the circuit in its project?
https://www.electronics-lab.com/projects/misc/002/index.html

My daughter bought a Chinese keys finder and asked if I could fix it. It beeped whenever she talked and whenever the TV was on.
Inside it had just a black blob for an IC and a piezo transducer.
I stopped its continuous beeping with tape over the piezo but then you could hardly hear it.

 

[Hero999]

What does fizzle mean?

Chinsoon,
You could have posted the link to the origional project so I wouldn't have had to waste my time figuring out what it does.

 

[audioguru]

A fizzle is when a pyrotechniques rocket fails.
I think the author means to say "whistle".

Most piezo transducers are most sensitive to about 3kHz to 4kHz but I don't think anybody can whisle that high.

 

[chinsoon]

What does fizzle mean?

Chinsoon,
You could have posted the link to the origional project so I wouldn't have had to waste my time figuring out what it does.

oh actually the circuit came from my friend. and the purpose i am asking is because i just don understand how those inverters can work as amplifiers. isnt there any explanation on how they work?

by the way, as audioguru said, it keeps beeping... no matter what type of noice it receives. so i wonder could it be the values of capacitor and resistor have gone wrong? as in they have a large bandwidth. so if u narrow down the bandwidth that it will work just fine. am i correct? (referring to the integrator)

anyway, my main intention is just to find the explanation of how inverters with feedback can serve as amplifiers.

thanks

 

[Nigel Goodwin]

anyway, my main intention is just to find the explanation of how inverters with feedback can serve as amplifiers.

Simply apply negative feedback around the inverter - this only works on specific devices though, depending on their construction.

 

[audioguru]

The Keys Finder that my daughter bought kept beeping probably because its gain was too high or because the piezo transducer was too sensitive as a microphone. It has nothing to do with bandwidth.

A Cmos inverter is just a single N-channel Mosfet and a P-channel Mosfet joined together as a complimentary push-pull amplifier. It has 180 degrees phase shift so a negative-feedback resistor from its input to its output self-biases it so that its output voltage is close to half the supply voltage.
It is very distorted when the signal swings near the positive supply and near ground.

Here is a graph of gain and bandwidth vs power supply voltage: