Why I have the sound at that freq!

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epilot

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Hello,

I have made an ultrasonic pest repeller based 555 IC(it has a volume for changing the freq).
As you know these devices work with ultrasonic sensors or tweeters,
but I connected the output of 555 via a capacitor into a common 4 or 8 ohm loudspeaker; just to see what will happen(I knew that the diaphragm is not suitable for ultrasonic frequencies),
I had an audible sound(like whistel) at ultrasonic frequencies too!
Can anybody explains the reason?
Ok, the diaphragm is not suitable, but why any sound at that freq???!
 
What frequency is it set to?

My guess is it's producing a sub-harmonic of the fundamental, so 22Khz would become 11kHz.
 
If your circuit has an adjustable frequency then what frequency was it? Maybe it was an audible frequency, not ultrasonic.

Why don't you attach the schematic of your circuit so we can calculate the frequency of the 555 oscillator, check to see if it has a supply bypass capacitor and to see if the 555 is overloaded by the high current in the speaker.
 
Hero999 said:
What frequency is it set to?

My guess is it's producing a sub-harmonic of the fundamental, so 22Khz would become 11kHz.
Click to expand...

The freq is adjustable between 35kHz to more than 60kHz.
 

Here is the schem.

I am suspect into sub harmonics like Hero999 too
 

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I'm suspecting them even more after reading **broken link removed**.

 
The poor little 555 is severely overloaded by the high current of the low impedance loudspeaker. It might be cutting in and out at an audible rate.

The circuit doesn't have a supply bypass capacitor so the supply voltage might be jumping at an audible rate.
 
You are probably right, on second thoughts sub-harmonics are unlikely to be the cause because the 1/2 harmonic of the frequency range is still ultrasonic.
 

Do you know any high quality square wave genarator with adjustable freq and 100 or 200mA output rather than 555?

I am thinking if I use a matching transformer then what will happen?

I forgot to say that the circuit has a 330uF capacitor between supply pins.

P.s: I'll put a power amplifier on the 555 output and then try to see if there is any audible sound or not.
 
You will need to attenuate the output of the 555 for it to feed a power amp.
 
epilot said:
Oh, you meant that the amplitude of the output voltage is high for an amplifier.
Click to expand...
Yes. The output voltage swing of your 555 is around 10.5Vp-p. An amplifier needs about a few hundred millivolts.
 
Or build a simple h-bridge driver with MOSFETs if you don't want to waste your money on an expensive amplifier.
 
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