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Keys finder circuit


er3na

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Guest Alun

Yes, I agree with audioguru, a piezo buzzer has a built in oscilator that will interfere with things.

A transducer whether it be the 2 lead or three lead type is what you need and I'd go for the 2 lead one if you have the choice.

Sorry about that post, it was badly worded and therefore could be confusing.

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:o  thx for all the information.

here it say that N4, C3 and R5 will ensures that the circuit will not trigger from common noise, what does it mean? is this consider as a filter? can i change the value so that i would need a high pitch voice to activated it? how is the calculation?
after that it say that the voltage of N4 will increases to hight logcal level what does it means?

sry to trouble you all again due to i lack of knowledge about circuit analysis

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Hi Lapsap,
D1 charges C3 very quickly when there is any sound, including common noise. Then the oscillator is started and the piezo transducer makes its sound. The only things in the circuit that I see to reduce sensitivity to low frequency noise is the high frequency resonance of the piezo transducer and the low values of C1 and C2.

I don't think this project will respond very well to high pitched voices, except maybe it will respond to a baby's very high pitched crying. The piezo resonates at 3kHz to 4kHz which is far above an ordinary high pitched voice. The harmonics of the loud voice of an opera singer will probably activate the circuit very well with certain musical notes.
The piezo does have some output at the frequency of a high pitched voice so try it and see if its sensitivity is OK. Increase the values of C1 and C2 and it will be a little bit better.

The circuit description of the operation of N4 and C5 is backwards and poor.
1) Without an input sound, N4 self-biases itself as a linear amplifier with its DC output voltage at about half the supply voltage. The output of N4 is fed through R6 to the base of T1 which turns on with its output voltage low.
2) The low voltage at the input of N5 causes a high voltage at its output which causes a low voltage at the output of N6, stopping the oscillator and turning off T2. With T2 turned off, the piezo is a microphone.
3) N1 to N3 amplify the sound signal from the microphone and D1 passes positive half-cycles to the input of N4 causing it to go high. Therefore the output of N4 goes low and it turns-off T1.
4) With T1 turned-off then N5 and N6 oscillate and drive T2 which drives the piezo for it to make a whistle sound.

The project describes pretty well how D4, C7 and D3 turn-off the N1-N3 amplifier, and how C3 slowly discharges into R5 to stop the piezo whistle after a few seconds.

Did you figure out what is a fizzle?
I think it is the jingling sound made by keys shaking together. ;D ;D
 

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i have tested manually by turning on the T1, n let T2 on, in this case, you say that it will be triggered.but the sound is so soft can i increase the volume.

but when i connect acorrding the circuit diagram given, the sound is also be triggered without any wistle. but the only different compare with T1 off, is that,
when T1 is off, when is battery is connected the piezo start buzzing.
when connect acorrding circuit, the sound is seem like charging up n start buzzing.

another question is that normally the piezo will be connect at the collect and not the emitter. why?

is the N5 and N6 part is the osillator? the connection is also differen from what i have found.

thx you for your help...

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Hi Lapsap,
Without any sound, T1 is on and T2 is off. You can test its sound by shorting the base of T1 to ground, which N4 does when the microphone hears a sound.
You can make the sound louder by increasing the supply voltage, by housing the piezo in a resonant cavity adjusted to the same frequency as the piezo's resonance and by tuning the oscillator's frequency to the piezo's resonance by adjusting the value of R9.

The piezo is connected to T2's emitter instead of its collector so that the base current of T2 doesn't load-down the output voltage of N6. If the high output voltage of N6 is not high enough then it can't shut off the N1-N3 preamp.

N5 and N6 are a "Classic Cmos Oscillator" like millions that I have seen and used. ;D

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sorry to trouble you again, i have just reconnect the circuit few times, but now when i gound the base of T1, it also do not have any sound. do you mind to explain everything? can i test the oscillator it self? with only the N5 N6 R8 R9 C5 how should i connect it to do piezo? connect the terminal of piezo to 12 and C5?

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now i only connect N5, N6 R8, R9, C5. n connect the pie at output of N6 to the ground. there is a buzzing sound coming out. is this suppose to be the output sound?

in this circuit T1 n T2 is suppose to be acts as switch? when the T2 turn on, the piezo is going to triggered, why and how? where is the signal come from to trigger the piezo?

can i test the mic function by oscilloscope? how? which point should i connect?

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Hi Lapsap,
The output of N6 isn't very strong and won't directly drive the piezo very well. The circuit uses T2 as a emitter-follower current-booster so that the piezo produces an amplified  strong 4kHz whistle, not just a little buzz. The piezo is used as a speaker and T2 is its ampliier/driver to make the sound of the 4kHz N5-N6 oscillator.

If your T1 and T2 transistors don't work then maybe you have their pins mixed-up. Look at the picture of the BC547B on its datasheet to see which pin is what.

You can connect an oscilloscope to pin 6 of N3 to see the amplified output when the piezo is used as a microphone. The DC voltage should be about half of the supply voltage and the AC output will be the highest when the sound is near the piezo's resonant frequency of about 4kHz. ;D

post-1706-1427914228488_thumb.png

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after testing the circuit part by part, the circuit oscillation part finally working (i think so).

but when i connect it to the battry,  the piezo start buzzing, after few second it stop. and then start buzzing again. when i short the base of T1, the sound will non stop buzzing.

R5 C3 N4 is the filter circuit? whats the name of this part?

Quote:
The integral circuit

sound.rar

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mp3 is just a audio file, any media player will be able to play it.
just in case here is the zip format, because this forum does not allow to attach mp3 files, i ll need to zip it up.

now the problem i m facing is that, when i connect the circuit to the battery, the piezo will start buzzing for non stop, even there is no any sound in the environment except the piezo buzzing.

when i remove the C1, the circuit will stop buzzing after a short buzzing. does that work as when there is no sound from the mic?
but when the C1 remain there, i just disconnect the connection between C1 and R10, it will be same as the previous case, buzzing, stop, buzzing, stop...

why do we need a C1 over there?

sound.zip

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mp3 is just a audio file, any media player will be able to play it.
just in case here is the zip format, because this forum does not allow to attach mp3 files, i ll need to zip it up.

Hi Lapsap,
Thanks for the zipped intermittent beep.

now the problem i m facing is that, when i connect the circuit to the battery, the piezo will start buzzing for non stop, even there is no any sound in the environment except the piezo buzzing.

It should stop buzzing a couple seconds after connecting the battery, if the room is quiet.

when i remove the C1, the circuit will stop buzzing after a short buzzing. does that work as when there is no sound from the mic?

Yes.

but when the C1 remain there, i just disconnect the connection between C1 and R10, it will be same as the previous case, buzzing, stop, buzzing, stop...

Then that is the key to your problem. D4 is shorted or backwards, or C7 is open.
D4 is supposed to rectify the oscillator's output, charge C7 and feed the positive voltage through D3 and R1 to the input of N1. Then N1 is disabled and shouldn't pass the oscillator's signal through N2 and N3. Without an output from N3, then D1 won't pass positive peaks to the input of N4 so its output remains at half of the supply voltage and causes T1 to turn on which stops the oscillator.

why do we need a C1 over there?

C1 passes the piezo's signal as a microphone to the input of N1. It also blocks the DC ground voltage from R10.

With the project buzzing when it shouldn't, measure the DC voltage at the output pin 8 of N4. Also measure the DC voltage at the output pin 6 of N3.
What is your supply voltage? ;D
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i m using a 12V battery to test it, but when i using a 12v power supply it works fine. the only thing is that the mic cant pick up signal.

can i put a mic parallel with the piezo, so that it can receive the whistle or some other signal.

btw way, can i increase the gain of the amplifier? (N1...N3) what is this call? do you have any document for this amplifier so that i can refer?

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i m using a 12V battery to test it, but when i using a 12v power supply it works fine, the only thing is that the mic cant pick up signal.

If it works fine with a power supply then its mic picks up a signal, right? Then what is wrong with your battery? Maybe it can't supply enough current.

can i put a mic parallel with the piezo, so that it can receive the whistle or some other signal?

Not without re-designing the circuit for a separate mic.

can i increase the gain of the amplifier? (N1...N3) what is this call? do you have any document for this amplifier so that i can refer?

It is a high gain inverting amplifier, its gain is determined by the ratio of R2/R1 and the value of C8.
It should work fine as shown. Maybe your piezo is incorrectly mounted which would damp its sensitivity, or maybe it isn't mounted in a suitable resonant chamber. Maybe it is too small.
Did you replace IC1?
Did you measure the DC voltage at the output of its preamp on pin 6?
Did you see the output of its preamp with an oscilloscope?
How did you fix it so that it doesn't continuously beep?


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after i change the 12V with a 9V battery, it works fine, it also can receive signal. but it seem like only whistle will not be able to trigger the circuit, i will need sound that louder.

when i test wif power supply again, reduce the current, it seem like continuous beeping again, why?


i did not change the IC1 to stop the continuous beeping sound.


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after i change the 12V with a 9V battery, it works fine, it also can receive signal. but it seem like only whistle will not be able to trigger the circuit, i will need sound that louder.

Hi Lapsap,
Good, it works. ;D
I tested a 30mm diameter piezo transducer just now from a cheap alarm clock. The piezo has a small resonant chamber in front of it with a 3mm hole for the sound. As a speaker, it is poor until about 4kHz when it whistles very loudly. As a microphone, it is also poor until about 4kHz, when it is very sensitive. I can't whistle above about 2.8kHz so it is not sensitive to my whistle.
The circuit is designed to respond to a fizzle, isn't it?

when i test wif power supply again, reduce the current, it seem like continuous beeping again, why?

Maybe your power supply is not regulated or has poor filtering and produces mains ripple. Then the AC ripple voltage passes through R3, D2 and D1 causing the input of N4 to be a little high, and therefore its output to be low. Then T1 is turned off and the oscillator is allowed to run.
You didn't post measured voltages so I am just guessing. ;D
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but why when using a 9V battery will work instead of 12V battery? 12V battery low current?

If your 12V battery cannot supply enough current then its voltage would drop far below 12V.
Maybe N4 doesn't self-bias itself at half the supply voltage. If its self-bias voltage is 1.2V lower than half-supply, then it will have its input voltage too high from D1. Then its output voltage would be low, turning off T1 and allowing the oscillator to run. Reduce the value of R4 or increase the value of R3 to fix it.

oh ya, may i have your name? i m doing this as a project, can i put your name in my report?

He, he. I don't have a name. Just call me "Audioguru". ;D

Did you try putting your piezo in a resonant chamber so that its whistle output is very loud and it is very sensitive as a 4kHz microphone?
Did you find something to make a 4kHz fizzle sound to activate this project?
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Try using a bigger piezo transducer as it will have a lower resonant frequency.

About the name thing, fair enough  audioguru for not wanting to reveal your real name over the Internet you're quitte shy aren't you? ;D There again you just don't know we might have an axe murderer here who will kill you if the the highly unlikely event you mistakenly give them some bad advice on a circuit. ;D I'm not scared though so I use my real name if someone wanted to find me I'm sure they could track my IP address and find out where I live and if they wanted to kill me I don't think they'll care what my name is. ;D

lapsap, (if that is your real name  ;D)
Why don't you mention this site and perhapps the name of the autor, hang on a second, does anyone know the author of this project, did audioguru design it? You could still mention audioguru anyway as he did help you and most importantly the name of this site.

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