I found this circuit diagram on a website and thought that it would make an interesting project for a beginner, me. It came with the notes following.




Notes:

Repel those repugnant insects from your Garden this summer with this insect repellent circuit. Designed by Graham Maynard the circuitry consists of a phase locked loop (CMOS 4047) wired as a 22KHz oscillator. The output is amplified by a pair of complimentary output transistors and drives a Motorola 3.25 inch Piezo. Current drain is around 120mA so an external power supply is recommended.

The piezo used was a standard 85mm square Motorola Horn, Maplin part number WF09K or WF55K. These are rated +/-3dB to 28kHz.

My thoughts: If f = 1 / (2 x R x C)
where f = frequency, Hz R = resistance, W C = capacitance, F
then f = 1 / (2 x 4700 x 10-9 x 4.7) = 22635 Hz

However, the datasheet indicates that Tm = Ts = 1.1 x R x C
where Tm = mark time Ts = space time
then Tm = Ts = 1.1 x 4700 x 10-9 x 4.7 = 0.00000243 secs
and a frequency of 20577 Hz.
Dilemma!
Next if the pot was turned down so that resistance approaches zero, the frequency exceeds 1Mhz which is far above the range for the piezo horn.
Would it not be better to have a fixed resistor of 4k7?
I look forward to your views.

 

I wouldn't worry about the dilemma, since the components will introduce more error than that anyway, and will drift with temperature in use.

You could put a resistor is series with the pot (or a pot with a smaller range, for finer tuning of the frequency), to set the max frequency.

 

If all you want to do is generate a loud 22Khz sound there are simpler ways. They throw in a lot of buzz words (Phase Locked Loop jumps out) that are meaningless in this context.

You have one thing down, the speaker, not the electronics, is where it is at.

While the circuit may work, I am dubious.

 

If all you want to do is generate a loud 22Khz sound there are simpler ways.

+1
I don't know why you'd use that IC unless you just happened to have it in your junk box waiting for a home. And four transistors? No need.

 

Actually the four transistors are making a full H bridge, which will feed the piezo double the power supply voltage minus the 2X BE drops. I would not use that specific scheme, but it does have merit in drive voltages to the piezo. A audio transformer would work better IMO, provided it could handle the frequency.

Nu2lec, if you want to built this I would say go for it. I am a firm believer in learning by doing. I am not so sure it will perform it's intended function, but when I was starting out I build a lot of U/S (ultrasonic) sound generators.

When your cat climbs up your front while you are standing up, stares you straight in the eyes, and says (in cat) "Make it stop!" you know you got it right.

If you want some alternate designs (I am a fan of the 555, it was what I used) you have but to ask. U/S can make great spooky effects, since they are hard to hear (but can be sensed) and harder to trace.

 

The piezo tweeter has peaks at certain frequencies where is is very loud but at nearby frequencies it has nulls that are not loud. It is difficult to "hear" ther loudest peaks and the frequency will drift anyway.

Motorola has not made that old tweeter for many years. You are looking at a cheap copy that might not even produce ultrasonics.

If you look at Ultrasonic Pest Repellers in Google then you will see that they DO NOT WORK.

Here is a spec sheet for a cheap copy of the Motorola piezo tweeter. Its cost is so low that I think each one has a different frequency response.

 

Thanks for the input. I have learnt something from that which was the point of the exercise.

 

Are you interested in alternate ways?

Here is a simple one...

555 Hysteretic Oscillator