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Key finder too sensitive

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KevinW

Member
I purchased a key finder that uses a piezo for a sound sensor but it's way too sensitive.
Is there a way to reduce the sensitivity without altering the alarm tone or can a different piezo be purchased that is less sensitive?
It only consists of a switch,led, battery and piezo.
 

Ramussons

Active Member
A key finder with the only components you mentioned uses the human brain to find a key and press the button so as to sound the piezo and flash the LED.
Maybe feeding a bit of Ethyl Alcohol to the brain may make it less sensitive :D:D:D:D:D
 

unclejed613

Well-Known Member
Most Helpful Member
It only consists of a switch,led, battery and piezo.
how does this locate keys? if it's something that works like ramussons described, you wasted your money.
 

dr pepper

Well-Known Member
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Presumably theres also a blob on the board that does all the control work, not much you can do without affecting the beep volume.
Stuffing it with cotton wool will reduce sensitvity at the cost of some volume.
 

audioguru

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Many years ago a keys finder project was posted at another website, but the project did not work properly then it was deleted.
Yes, it was too sensitive and beeped when there was almost any sound.
It used the piezo transducer as a microphone and as a beeper but many sounds produce the whistle sound you make to activate it.

Here is the same project but on another website. They say "fizzle" instead of whistle, maybe a poor translation. If you reduce the value of R2 then it will be less sensitive but then you must whistle very loudly or be very close to it.
Here: http://www.diy-electronic-projects.com/p104-Keys-Finder
 

unclejed613

Well-Known Member
Most Helpful Member
there is more to the device than was described... most of the circuit is inside that blob of glue. you could put a capacitor across the piezo to reduce the sensitivity by shunting some of the high frequency signal.
 

KevinW

Member

The key finder on the left has the capacitor, I used a 1944 U.S.N whistle to excite it.
The only issue now is a vibration fault as it will start when moved.
 

audioguru

Well-Known Member
Most Helpful Member
When the piezo is a microphone the capacitor reduces its high frequencies (the whistle) sensitivity but the low frequency (movements) sensitivity remains too high.
Also the capacitor should be a film type, not ceramic because a ceramic capacitor also behaves like a microphone.
 

KevinW

Member
Thanks to everyone for helping with this.
I'll tweak it a little more but it has definitely made a difference by adding the capacitor.
 

KevinW

Member
The Electrolytic capacitor is working much better than the film capacitor as It knocks the high frequency down.
The film capacitor didn't have any affect on it.
 

audioguru

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The Electrolytic capacitor is working much better than the film capacitor as It knocks the high frequency down.
The film capacitor didn't have any affect on it.
A higher number of micro-farads cuts high frequencies. You said you used 0.1uF. An Oriental 0.1uF film capacitor is marked 104 and a European 0.1uF film capacitor is marked 100n. What marking was on your film capacitor?
 

KevinW

Member
It was a 102k that measures 1029pF.Maybe I need 100000pF?
Do these piezo sensors have to be encased to work?
When I removed it from the plastic enclosure it stopped working and when I reinstalled it back into the plastic housing it began to work again.
 
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audioguru

Well-Known Member
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102k is 1000pF +/-10%. 0.1uF is 100,000pF.
I have never seen or used a piezo transducer.
 

KevinW

Member
They don't work uncased.
I notched the plastic in order to pry it out of the case with a screwdriver.
It wouldn't sense or sound out of the case and when I reinstalled it the piezo would trigger with sound but the buzz was next to nil.
I filled the notch with my glue gun and it's back to normal.
It requires a rigid mount around the complete edge of it to work properly.
 
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