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Patrick

Mon May 13 2013, 04:04PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i have taken apart two radio shack piezo speakers. these will be used to warn the human pilot that the battery is first at 50% and then at "20% land now" warnings. one is 102dB the othe 105dB, there are tiny circuit boards on both.

the problem: they are to big and heavy as manufactured.

the solution: take out the piezo component and run my own AC tone through it, via external SMT circuits.

My question: isn't it just an HF AC wave that the piezos' need? it seems I can choose several frequencies through my own wave generation circuits, right? is pulsing DC less effective that AC ?

pics to follow:

Conundrum

Mon May 13 2013, 06:06PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Best to drive at acoustic resonance for max. output.

I've noticed that this can vary with temperature, so a self resonant single transistor circuit would be best here.

2Spoons

Mon May 13 2013, 10:04PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

If you want high sound pressure (~100dB) out of a piezo disc it needs to be mounted in a Helmholtz resonator. I've been down this path before - without the resonator you will get little more than a squeak out of your piezo.

Patrick

Tue May 14 2013, 02:46AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

2Spoons wrote ...

If you want high sound pressure (~100dB) out of a piezo disc it needs to be mounted in a Helmholtz resonator. I've been down this path before - without the resonator you will get little more than a squeak out of your piezo.

ok, fair enuff, let me look up "helmholtz resonator" , but please elaborate more...

Electroholic

Tue May 14 2013, 03:09AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Both of the about, and I think you will find square waves louder than sine waves. Easiest tho is to probe the original circuit and try month mimic the waveform/frequench.

Patrick

Tue May 14 2013, 03:15AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

it looks like they both have the plastic "resonators" built in...

[it also looks like they both use a three terminal inductor, possibly an auto-transformer, for voltage/impedance reasons..]

in the below oscillogrram I put in a 9v, measured 97 V across the piezo wires (V-pk-pk) so im certain now its a 1:10 Auto Tmfr... also the period is 320uS, which means 3,125 Hz, so right down the middle of what the specs claim...

heres one:

pics on piezo warbler :

Patrick

Wed May 15 2013, 04:13PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

if the resonator was meant for 3kHz, can I use 7kHz and still have it be reasonably loud (60-80% of the 3kHz loudness?) id like to have several frequencies, and warble patterns for emergency purposes. so maybe 3kHz, 5kHz and 7kHz, im thinking. or is that Helmholtz resonator a tuning device as well?

id also like to use a 555 timer and a H-bridge to drive the autotransformer/piezo. Im hoping to sink one pair, then source the other pair out of pin 3 to provide the alternating waveform.

heres two vids :

2Spoons

Wed May 15 2013, 10:04PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Usually the resonator will be matched to the mechanical resonance of the piezo disc, as thats where the highest sound pressure can be obtained. You are unlikely to get anywhere as loud output off-resonance, but I'd suggest doing a sweep as there are probably other frequency peaks.

Patrick

Thu May 16 2013, 12:11AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

2Spoons wrote ...

Usually the resonator will be matched to the mechanical resonance of the piezo disc, as thats where the highest sound pressure can be obtained. You are unlikely to get anywhere as loud output off-resonance, but I'd suggest doing a sweep as there are probably other frequency peaks.

yes that was what I was worried about, I was hoping it would be a broad-band pass instead of a narrow pass type, ,but I guess ill sweep it to find out. ill try to put a schematic together for the 555 timer source sink driver.

Erlend^SE

Thu May 16 2013, 05:03PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Note: the transformer can also be part of the resonance-circuit.
The piezo disc works as a cap, the the transformer as inductor.

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