Jump to content
Electronics-Lab.com Community

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


Recommended Posts

i just hope that the wrong value of R1 is the reason why this project is not working... i'll try it with a 4.7k then.. i'll let you know about the result as soon as i have them..

by the way. when you modified the circuit for a speaker instead of a buzzer, i think you forgot to indicate the polarity of the speaker connected with pins 14&15.. anyway, i'll try all the possible connection...

thanks.. and also, we have replaced the wires with shorter and smaller ones..

later...  :)

Link to comment
Share on other sites


  • Replies 161
  • Created
  • Last Reply

Top Posters In This Topic

pressure is on! i really need to get this thing beeping soon. i am running out of time..

i have made all the necessary changes. i've changed the wires. i've modified the circuit for a piezo speaker. i've replaced the R1 from 22k to 4.7K. i also measured the mic's voltage. this time, the rating was above the expected value of 1.5-2.5. it has 2.8V.. almost the same as the source..

i am concerned with the parts i have. one or two of them might be defective, just like the old transistor i had. i want to check if the speaker and mic are functioning. could i do that?  how?

our thesis adviser approached us this afternoon inquiring about our progress. audioguru, remember the first time i mailed you. i asked you to explain to me a certain circuit. it's also a key finder circuit. according to you, that circuit needed a much higher whistle  frequency than this circuit i am working on. i decided to go with this circuit because it's much simpler, with less components and therefore smaller. our adviser gave us two days to have this circuit working. after that, we would start working on his circuit. even if i protested against the idea, i can't make any more contentions, because i dont want him to blame me if ever i can't finish this on time.. my concern is that, if we work on that circuit, we would be back at square one.. i tried to explain that i have gone through a lot with this circuit, and maybe, just maybe, i'm very close in making this circuit work.. i just hope i could get this thing beeping as soon as possible..

please tell me what to do.. thanks a lot!

Link to comment
Share on other sites

1) To test the beep oscillator and the piezo speaker, temporarily connect the IC's pin 5 to ground (the negative wire of the supply).
2) Measure R1's 4.7k resistance with one end disconnected from the circuit, maybe it is bad.
3) Replace the electret microphone. Before (with 22k) its voltage was much too low, now (4.7K) it is much too high.

Link to comment
Share on other sites


1) To test the beep oscillator and the piezo speaker, temporarily connect the IC's pin 5 to ground (the negative wire of the supply).


if i do that, what should happen? i already brought my stuff to the internet cafe with me. i connected pin5 to the ground. nothing happened. it is as quiet as ever.


2) Measure R1's 4.7k resistance with one end disconnected from the circuit, maybe it is bad.

the color code definitely tells that it is a 4.7K resistor. (yellow, violet, red) but the meter reading is 5K. i'll see if i can find a replacement with a much closer value.


3) Replace the electret microphone. Before (with 22k) its voltage was much too low, now (4.7K) it is much too high.


can't do that right now, maybe tomorrow..


Link to comment
Share on other sites

Hi Logan,
When pin 5 of the IC is grounded, its pin 4 should go to 3V, its pin 2 should go to ground and its pin 10 should go to 3V. Then D2 becomes reverse-biased which allows the oscillator to drive the piezo speaker. I copied the values of the oscillator's R10 and C5 from the Keys Finder project but maybe they are wrong.
I don't have a CD4049 powerful hex inverter IC but have a CD4069 ordinary hex inverter IC that I will try with my piezo speaker and the same parts for R10 and C5.

I tried my electret mic again with 4.7K (and a 3.0V supply) and its voltage is 1.9V. With 10k its voltage is too low at 0.8V. I guess only a 3V supply is too low for our mics so the resistor's value must be selected for them to work properly. Yours might work with 5.6k, 6.8k, 7.5k, 8.2k, 10k or 12k etc.

Link to comment
Share on other sites

Hi Logan,
I tried the oscillator section driving my piezo speaker. My opamp is a CD4069 ordinary one, not a high power CD4049 so it didn't have enough power to drive the piezo speaker unless the supply voltage was at least 4V. It wasn't very loud so I connected the piezo from pin 15 (your IC's pin numbers) to ground and it was much louder. The frequency kept changing if the piezo was bumped or moved, so my IC isn't powerful enough for the job.
I added a transistor to drive the piezo like in the Keys Finder project and the volume was very low, so I reconnected the piezo from pin 15 to ground. I added a resistor in series with pin 11 to make it a Classic Cmos Oscillator and it worked best. Your IC has a different input protection arrangement so it might not need the added resistor.

Link to comment
Share on other sites

???  ???  ???

sorry but i really didn't quite understand what those  info means...

anyway, do you think there's still a chance i could pull this out! should i go to plan B? you have already seen the alternative circuit i was telling u about, would that be easier to build? assuming i can get all the parts it needed, would it be better..

after i grounded pin5, yes, it gave the expected DC voltages.. but still no  beep!  :(

i'll replace the mic later...

Link to comment
Share on other sites


can we still figure out this circuit?

if ever we really cant make this thing work, would the keys finder circuit (http://www.electronics-lab.com/projects/misc/002/index.html) be as much of a problem as this whistle responder? what do you think? should we try building the keys finder circuit? but i still say that we ought to continue with the whistle responder... it's just that i cant come up with any more guesses with what ius wrong, and especially how to fix it!!!

i need your expert advice, audioguru, thankz...
Link to comment
Share on other sites

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.

post-1706-14279142628804_thumb.png

Link to comment
Share on other sites

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:

post-1706-14279142629142_thumb.png

Link to comment
Share on other sites

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. 

post-1706-14279142631643_thumb.png

Link to comment
Share on other sites


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?!)
Link to comment
Share on other sites


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?

Link to comment
Share on other sites

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.

Link to comment
Share on other sites


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..
Link to comment
Share on other sites

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   

Link to comment
Share on other sites


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..
Link to comment
Share on other sites


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..
Link to comment
Share on other sites

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!!!

Link to comment
Share on other sites


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
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
  • Create New...