How can I turn an old Smoke Alarm into an Ultrasonic Dog Silencer?

Thread Starter

Sam111

Joined Sep 26, 2018
5
I am pretty new to the area.

So far I have experimented with 555 at 10-25 kHz, but the dB is too low. Even at 10 kHz the output power is pretty low (measured by my Samsung app, Oscope at 2-4 cm distance it's just a few dB). I am using a piezo transducer from an old sound system, no spec, just a piezo disk 2.5cm diam with two leads black and red).

My question : Is it possible to turn an old smoke alarm into an Ultrasonic Dog Silencer?
 

AlbertHall

Joined Jun 4, 2014
10,563
If you mean using the alarm sounder from the smoke alarm then it probably isn't possible. The ones I have seen use a piezo disk to make the noise and these only work well near their resonant frequency so the one from a smoke alarm, intended to work at medium audio frequencies, isn't going to do much at 25kHz.
 

Thread Starter

Sam111

Joined Sep 26, 2018
5
If you mean using the alarm sounder from the smoke alarm then it probably isn't possible. The ones I have seen use a piezo disk to make the noise and these only work well near their resonant frequency so the one from a smoke alarm, intended to work at medium audio frequencies, isn't going to do much at 25kHz.

I mean:
1) direct replacing the existing smoke alarm sounder with a piezo transducer for 25-30 kHz. and
2) lifting the output frequency to 25-30 kHz. The end ampl wont struggle with the capacitive load of the piezo.

Is this possible?
 

bertus

Joined Apr 5, 2008
20,881
Hello,

As @AlbertHall said, the piezo will not do much outside the resonance frequency.
When you look at the attached PDF, you will see that the resonance frequency of a 27 mm piezo is about 4.6 kHz.
(page 7 has a table).

Bertus
 

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AlbertHall

Joined Jun 4, 2014
10,563
I mean:
1) direct replacing the existing smoke alarm sounder with a piezo transducer for 25-30 kHz. and
2) lifting the output frequency to 25-30 kHz. The end ampl wont struggle with the capacitive load of the piezo.

Is this possible?
But then what are you actually using from the smoke alarm - unless it is intended for dogs that smoke?
 

Thread Starter

Sam111

Joined Sep 26, 2018
5
Hello,

As @AlbertHall said, the piezo will not do much outside the resonance frequency.
When you look at the attached PDF, you will see that the resonance frequency of a 27 mm piezo is about 4.6 kHz.
(page 7 has a table).

Bertus
My understanding is that the piezos respond well to high "audio" frequencies - above 15 kHz. Although their curve is not flat like for "the magnetic" ones for mid range frequencies, they still can work well (outside their resonance freq) in a wide range, in the ultrasonic area to us. Or am I wrong here?

But then what are you actually using from the smoke alarm - unless it is intended for dogs that smoke?
Good point. Well, the whole setup with a 2 small replacements might be able to do the job (I hope :)), Instead of building the whole thing from scratch
 
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AlbertHall

Joined Jun 4, 2014
10,563
We had a cat that smoked.
Once when she stood very close to a radiant electric fire. You could see the char lines on her fur matching the fire elements.
Second, when we had a power cut and were using candles. She whisked her tail through the flame. It brought a new meaning to 'put the cat out'.
 

AlbertHall

Joined Jun 4, 2014
10,563
If you search the forums for 'dog barking' you will find threads where this has been discussed before, though I don't remeber any them using a smoke alarm.
 

dendad

Joined Feb 20, 2016
3,618
Often the case of the Piezo is a resonant cavity. You may find formulas to calculate that and make a new one or modify the original case to suit the new frequency. That will increase the output dramatically.
 

ian field

Joined Oct 27, 2012
6,539
I am pretty new to the area.

So far I have experimented with 555 at 10-25 kHz, but the dB is too low. Even at 10 kHz the output power is pretty low (measured by my Samsung app, Oscope at 2-4 cm distance it's just a few dB). I am using a piezo transducer from an old sound system, no spec, just a piezo disk 2.5cm diam with two leads black and red).

My question : Is it possible to turn an old smoke alarm into an Ultrasonic Dog Silencer?
Sounds like too much hard work - they usually use a piezo sounder, resonant cavity housing and auto-transformer to step up drive. You might get an above human audio sounder assembly, but you probably won't be able to adjust the oscillator.

Magazine projects turn up every now and then.
 

Dodgydave

Joined Jun 22, 2012
9,682
Dogs + cats hearing range peaks at 4Khz to 18Khz, between these frequencies they are the most sensitive, so you can use the lower output -35dB, but also humans can hear it too .
I made one for cats and dogs using a sweeping frequency from 20-45Khz using an ultrasonic speaker from an old car alarm, these are ideal i used a 556 timer, one half as the sweeping osc,the other centred at 30Khz, and a push-pull output with Tip35/36 transistors.

The best way to get higher voltages across the transducer is a Transformer output.

ultrasonic-pest-repellent.jpg
 
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Dodgydave

Joined Jun 22, 2012
9,682
Any 40Khz transducer or tweeter speaker rated for 10Khz above will do, use one from an old car alarm or from ebay.

The circuit shown just makes a single tone, i would modify it for pulsing or sweeping.

Here is a sweeping frequency one which can be modified for the desired centre frequency, and you can use the two spare gates as a Complimentary Output for the speaker driver, intead of a Transformer.


Sweeping-Tone-Alarm-Beeper-4069-TIP29C-NECD882.jpg
 
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Thread Starter

Sam111

Joined Sep 26, 2018
5
@Dodgydave
Would you mind posting the actual circuit that you have successfully built? (As I mentioned above I am new to the area, so please be patient with me :)))

Jeee, this looks like an Intel Pentium processor :)))
 
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Dodgydave

Joined Jun 22, 2012
9,682
@Dodgydave
Would you mind posting the actual circuit that you have successfully built? (As I mentioned above I am new to the area, so please be patient with me :)))

Jeee, this looks like an Intel Pentium processor :)))
Here is one that i used years ago, a sweeping frequency output, with the Tip41/42 or TIP35/36 transistors for higher power.

You can use a piezo on the chip outputs instead for lower output . Circuit works on 5V to 18V.DSC_0171.JPG
 
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