i need help extending the keys finder project. can someone help me out????

[audioguru2]

Hi Logan,
I also had a difficult time trying to drive my piezo speaker with a low power Cmos IC and a low supply of only 3V to 5V.
I looked for Piezo Beeper Circuit in Google and found this frequency response graph of one that shows how it has extreme resonance at only a couple of frequencies. It has a capacitance of 41nF which would be very difficult for a Cmos IC to drive. If your oscillator operates at a frequency between the peaks and if the piezo speaker is not mounted in a resonant chamber which is tuned to the peak then it won't be heard.

I think the Keys Finder project would also be difficult because I tried its oscillator and output transistor to drive my piezo speaker and the output was a very low level.

View attachment 38367

 

[audioguru2]

I'm not finished with improving this project:
1) I tried driving my piezo speaker with two ordinary Cmos inverters paralleled, to get more drive current and it worked well but wasn't very loud with only 3V for a supply.

2) I tried two sets of paralleled inverters connected as a bridge to double the voltage across the piezo speaker. It was great with a 9V supply but with only 3V the current was too high for the low power Cmos inverters so it wasn't very loud.

3) Since the piezo speaker needs a fairly high current to charge and discharge its high capacitance at about 4kHz, I will try using two sets of complimentary push-pull emitter-followers connected in a bridge. With a 3V supply they will have a voltage loss of 2.6V but the piezo speaker will still have 3.4Vp-p across it which is 1.2V RMS.

I found the spec's sheets for my favourite piezo speaker. It is pretty big at 34.5mm diameter and comes in its own resonant housing. I think it was used in a phone:

View attachment 38369

 

[logan_dslasher]

Time's UP!  :'(

i have reached the deadline.. we are told to do the other circuit for our thesis. i'm going to need your help again... should i start another topic for this in the forumor should i just use this topic?

am going to purchase the parts this afternoon..

thanks...

 

[audioguru2]

Hi Logan,
It is too bad that your time is up before you found out why your circuit didn't work. It probably would have worked if you had a piezo buzzer with a built-in oscillator as spec'd in  its parts list.
You should continue talking about the other project in this thread because it has its name.

I have a modified circuit that beeps my piezo speaker perfectly. Its frequency must be tuned to exactly the resonance of the piezo and the piezo must be in a tuned cavity for the loudest beep tone. It starts working when the supply is 2.8V and its frequency increases as the supply voltage is increased to about 4V. At a supply voltage above 4V then its frequency doesn't increase anymore. Therefore it needs a trimpot to tune it and a regulated 3V supply. If the supply is higher than 3V then its battery won't last very long. 

View attachment 38376

 

[logan_dslasher]

anyway, i'll still continue working on this circuit... i want to make it beep    not just now.. maybe after i have the new one working and i have already made my final thesis defence..

i'm sorry for causing too much trouble...  :-[

waaaa!! i have already started a new topic... i'll just delete that the next time i log in..  i have to go for now.. i have a class, and i'm late again (what's new?!)

 

[logan_dslasher]

i really didn't quite understand your last two posts ;D, audioguru.. but get this. i have already found a piezo buzzer!!!! it's almost the same as what you have shown.. it beeps pretty loud with 3V.. i went back to the old circuit, the original one hoping that with the right buzzer, it would work, but.... as usual, nothing.. i tried making all sorts of sound/noise using different sources.. still, nothing./ now that i have the right buzzer, what could be wrong. maybe i don't need to work on a new circuit since i have the buzzer. i just have to make it beep when it is connected to the circuit...

remember the DC voltage values i should get from different test points? they matched!! even the mic voltage (1.5-2.5V), were correct! i got a volatge of 2.3V across the mic!!

what could still be wrong?

 

[audioguru2]

We removed the transistor and modified the circuit to make a 4kHz oscillator for a piezo speaker.
If you install a new transistor (the one you had is damaged) like in the original project or slightly re-wired like in my 1st modification, and use the original values for R10 and C5 then a piezo buzzer will work (if you don't short the transistor again with an LED).
Pin 10 of the IC should go to 3V when you whistle into the mic then the oscillator should turn-on, turn-off, turn-on and turn-off slowly the transistor and the buzzer for a couple of seconds.
If the buzzer is connected backwards then the transistor might be destroyed.

I hope you get it to work, if not, try 4.0V or 4.5V for its supply voltage. Don't go as high as 6V.

 

[logan_dslasher]

i tried working on the circuit again. guess what? the moment i turn on the power, the buzzer kept beeping.. i tried to measure the circuit's voltage.. and found out that the transistor's base has a 0.5V which should be 0V.. i think i have to replace my transistor again. i have asked a friend to buy one for me..

i am using 1M5 for R10 and 47nF for C5.. there's is also a 4.7K at the transistors base connected to pin15.. the negative terminal pf the buzzer is in the transistor's collector and its positive terminal with +V source..

i already tried it with 4.5V, it didn't work. dont worry i won't go higher than 6V. the last time i did that, my transistor got damaged..

 

[audioguru2]

Hi Logan,
It is normal for the base of the transistor to be about 0.55V to 0.7V when it is turned on. It should be off and measure 0V when pin 15 of the IC is also 0V. If the transistor has the 4.7k resistor in series with it like in my 1st modification of the original circuit, then you can test the transistor by shorting its base to ground to turn it off.
The IC's pin 15 measured 0V before so something has changed. Your mic is working properly now so maybe it is picking up the sound of the beeper which I think will keep the circuit turned on. Short the mic to ground for a minute to see if the beeper stops.

With R10 as 1.5M and C5 as 47nF as in the project's parts list then pin 10 of the IC should oscillate at about 8.5 beeps per second which maybe is too fast for your beeper to turn off beween pulses. Try 330nF or 470nF for C5 to slow it to about 1 beep per second.
Good! You almost have it working. ;D   

 

[logan_dslasher]

what is exactly the reason why the buzzer  kept beeping as soon as power is suppl ied?  how could i solve that? 

 

[audioguru2]

what is exactly the reason why the buzzer

 

[logan_dslasher]

1) "If the transistor has the 4.7k resistor in series with it like in my 1st modification of the original circuit, then you can test the transistor by shorting its base to ground to turn it off."

the buzzer continued beeping.

2) "Your mic is working properly now so maybe it is picking up the sound of the beeper which I think will keep the circuit turned on. Short the mic to ground for a minute to see if the beeper stops."

even for two minutes, it didn't stop.

3) "With R10 as 1.5M and C5 as 47nF as in the project's parts list then pin 10 of the IC should oscillate at about 8.5 beeps per second which maybe is too fast for your beeper to turn off beween pulses. Try 330nF or 470nF for C5 to slow it to about 1 beep per second."

sorry i havent done that..

 

[audioguru2]

It is obvious that when you short the transistor's base to ground to shut it off and it doesn't shut off, then it is bad or it is wired wrong.

 

[logan_dslasher]

i am working again on the whisatle responder circuit to be used in my thesis.  i have finally convinced my adviser that this is a better one compared to the key finder.

we have tested the circuit this afternoon and he concluded that my IC is damaged. i have replaced it already and guess what? it beeped. only two beeps. it beeped so fat that i think it didn't took a asecond for those two quick beeps. i don;t exactly know what made it beeped.. i tried making all sorts of sounds in hopes of making it beep again but to no avail..

i tested its test points (reply#28) my test results matched them except for the mic voltage which is 0.2V (nearly 0V).. when i tested pin5.. it made short beeps  until i removed the test lead on it. what does that mean? athat's the first time that happend.. the transistor's base has 0V..

i have the projct with me.. please tell me what to do with it.. i'll wait.. thanks..

 

[logan_dslasher]

auidoguru! audioguru!! it's beeping!!! i don't want to touch it cause it might not beep again... from time to time, it beeps!! it's so noisy here in the internet cafe so i dont' really know what is triggering it! gamiing sounds like explosions and special FX.. there's also a music being played... what should i do now?!! HEELLPPP!!!

 

[audioguru2]

we have tested the circuit this afternoon and he concluded that my IC is damaged. i have replaced it already and guess what? it beeped. only two beeps. it beeped so fast that i think it didn't took a asecond for those two quick beeps. i don;t exactly know what made it beeped.. i tried making all sorts of sounds in hopes of making it beep again but to no avail..

Good, the IC and transistor are working. It is supposed to make about 8 beeps per second.

i tested its test points (reply#28) my test results matched them except for the mic voltage which is 0.2V (nearly 0V).

Much too low. What resistor value do you have to connect it to 3V?

when i tested pin5.. it made short beeps

 

[audioguru2]

auidoguru! audioguru!! it's beeping!!! i don't want to touch it cause it might not beep again... from time to time, it beeps!! it's so noisy here in the internet cafe so i dont' really know what is triggering it! gamiing sounds like explosions and special FX.. there's also a music being played... what should i do now?!! HEELLPPP!!!

My daughter bought a Chinese-made keysfinder. It kept beeping whenever she spoke softly or if the TV was playing.
These kind of circuits are awfully too simple to do a proper job. I think it hears its own beep which makes it continue. Short the mic and it should stop beeping.

 

[logan_dslasher]

Much too low. What resistor value do you have to connect it to 3V?

R1 - 22K

i think it's very sensitive. is there a way to lessen its sensitivity.. i think i remembered something about changing the value of R4 to change its sensitivity? this is not a whistle responder, it's a noise responder!! ehehe.. but i'm really glad it's working..

by the way, i have it wired like when u first modified it -- with 4.7K to its pin15 and transistors base...

 

[audioguru2]

Hi Logan,
Many posts ago I said that R1 should be somewhere between 4.7k and about 6.8k with the circuit's 3V supply. With 22k then the mic's voltage is much too low.

The sensistivity is reduced by increasing the value of R2.

 

[logan_dslasher]

alright then, i'll change R1's value.. i'll let you know what will happen, and what would be the mics voltage..

hey! it suddenly became quiet!  ???