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Generating 40K Hz Audio with a 555 timer Answered

I'm looking for ways to generate a 40K Hz signal with a 555 Timer. I've been through the 555 timer calc and I'm not liking the values its wanting me to use for R1 and C2.

So i found the schematic shown below. Will i be able to replace the IR LEDs with a small amp and speaker and get a good 40kHz tone out of it?

Are there any better options i should be looking at for creating a 40K Hz audio signal?



Best Answer 5 years ago

  • First you cannot expect an ordinary speaker to vibrate at 40 Kc
  • Try a tweeter
  • A piezoelectric disc can easily vibrate at 40 KHz and higher.
  • Some electrostatic speakers can do 40 Kc.
  • Flame ( Plasma ) speakers can do 40 KHz too.
But you have to loose R1 and push the supply voltage to 12-15VDC.

Try this link


Yes i understand R1 is only there as a current limiter for the LEDs. running it at a higher voltage isn't ideal so i'll have to keep looking at other options.

But by using dual complimentary 555s in a single 556 IC
that 5 volt supply will deliver the 10 volt oscillator output..

Be aware that there are Pe
piezzo discs that have 3 wires and a transistor to produce
the 40 KC you desire without a single 555///


5V- complimentry556 piezzo.PNG

i've been working on this as well. there are a few commercial products out there that are supposed to do what you're attempting, but are far too low power to affect the dogs i'm aiming at. those products, however, have a microphone that senses a few seconds of sustained barking before the ultrasonic noise is generated.

These plans seem to be exactly what you're trying to make:
http://www.williamson-labs.com/sdi.htm (scroll to near the bottom)

i've seen that ciruit completed with an arduino here:
though the author doesn't explain how he does it.

there are others using piezo tweeters with cones on the end to direct the sound. from reading posts, it seems like piezo speakers may burn out eventually, but they're a good starting point.

Here's another circuit that doesn't seem to have a microphone setup...

These plans have the microphone and high output speakers:


I believe you are driving 2 led's so you don't want to "hear" the tone anyway. All you need to do is try different values of capacitor C1. Use more capacitance to reduce the frequency... or use less capacitance to increase frequency. Also, you may want to place an ADJUSTABLE resistor in place of R2 and R3. I suggest about 10K ohms or 20k. You will notice the squarewave will not remain symetrical as you adjust the two pots. If the squarewave becomes too un-symetrical (50%duty cycle)... then add or subtract some capacitance on C1. From my experiments, the most i ever got out of a 555 timer was 1mHz.

I don't want to drive the LEDs i want to get a 40K Hz tone out of a speaker to use as a training aid for other people's dogs.

IMHO, I'd just get one of these: http://www.goldmine-elec-products.com/prodinfo.asp?number=G19090

As has been noted, speaker response is limited, horns and tweeters even are limited to around 20kHz. A certain mfg. guitar speaker, for example, might roll off very sharply at 2500Hz.

An ultrasonic speaker will produce the 40kHz, exactly what it's made to do.

That is, assuming, you aren't making some kind of oddball D-class amp where you are relying on the dampening factor of the RLC network to restore a smooth amplitude, perhaps?

Great speaker option if it can handle how powerful i'll want it. It needs to be able to reach across an average yard to effect a neighbor's dog(s).

The circuit might work but you'll never know whether or not it does. Most people,even the young, can only hear to 20K.

Up until I was about 25, I could hear ultrasonic burglar alarms.  But I think what I was really hearing was a lower harmonic of it not the real frequency.

I'm sure my dog can let me know if its working or not. Thats the target audience for this anyway. I've found a few other variations of a 555 that will produce 40K Hz. So i'll have to breadboard it up when i get home and see what the dog does. I think the trick will be getting it amplified where i want it.

This is going to be a training tool for people to use against there neighbor's dogs who bark too much. Should be a helpful training tool.

Increasing C1 ten times for testing purposes should bring down the frequency by a factor of ten, so you will be able to hear it. (talking from the top of my head, check with the data sheet)

Are you sure, the dog will bark _less_ if it is tortured with this sound?

40K Hz is in the range of a dog whistle. Friends of mine have been having problems with barking dogs. This is what has prompted this idea. They found an MP3 of a dog Whistle and play it through there PC speakers out the window and the dogs stop barking. I'm sure at a lower volume the sound will cause them to bark but if you can get it loud enough they'll stop. But of course further testing will be needed.

I know everybody likes the 555.  However, another way to make a R*C timed, relaxation oscillator is this circuit in the attached picture
which uses a pair of CMOS inverters.

How this trick actually works is explained here:

Note that these CMOS inverters usually come in the form of an IC that has 6 of them on it, e.g. the CD4069 from the 4000 series, and this is handy because you can use two inverters (plus one resistor and one capacitor) to build the oscillator. Then follow the output with an inverter.  Then follow the output from that inverter with another inverter. 

So you've got your oscillator signal, and you've got the compliment (or inverted whatchacalit) of that signal, and that can be handy if you want to do some kind of push-pull thing with whatever the final thing that you're driving is.

I guess all I'm saying is:  Give CMOS inverters a chance!

2008525124819_Cmos oscillator.PNG

Good One !

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