Simple Sonic Cat Repeller

Cats, teenagers, kids: at least one of them likes to use my garden as a toilet, but fortunately they can all be repelled with some simple electronics. I’m going to use a 555 astable to produce a high frequency square(ish) wave signal that should range from annoying to painful for the “targets”, whilst remaining inaudible for most adult humans. I’ll try to make my instructable easy enough for relative beginners to understand.

Step 1: The Plan

This is how I’ll make the repeller.

The frequency of the output of the chip in this setup is given by the formula
Frequency = 1.44/((R1 + 2R2)C)

From experience I know that 12,000 Hz is a good frequency for the job, but feel free to try out others with different values of R1, R2 and C (if you have a breadboard I recommend trying a 100kR variable resistor as R1 and finding a suitable frequency). Do bear in mind that R1 should be at least 1kR.

12,000 = 1.44/((R1 + 2R2)C)
1.44/12,000 = (R1+2R2)C
1.44/(12,000 x 0.0000001) = R1 + 2R2 = 1200
I’ve used 100nF as my value for the capacitor, and I’ll use a 1kR resistor R1 and a 100R for R2.

As I’ll be using a 9V PP3 battery for +Vs, and my red LED has a voltage drop of 2V and draws 30mA of current, I ideally need R3 to be 233.3R. I have 220R resistors, but as they have 1% tolerance (i.e. they go up to 222.2R) they aren’t suitable to protect the LED from excessive current. The next highest value of resistor in my supply is 270R, which would limit the LED to drawing 25.9mA – perfectly acceptable. Check the ratings for your LED – don’t complain to me if its ratings are different to mine’s and it blows!

Ohm’s law: Current = Voltage/Resistance
Therefore Resistance = Voltage/Current
(9-2)V/(30 x 10-3)A = 233.3R

R4 is to increase the impedance (read resistance) of the output into the speaker. As with the LED’s resistor, less resistance means more current, and as power (in Watts) is given by current x voltage, less resistance means more power. “Nice!” I hear you remark in your best Jeremy Clarkson voice, but not so when you try to pass 2W into a speaker designed to take a maximum of 0.25W and you’re left with an expensive magnet. Read the specs for your speaker – do the maths and aim for the nominal power and don’t go over the maximum power. The nominal power of my speaker is 1W with a maximum power of 1.5W. A 47R resistor for R4 in series with my 8R speaker will give 1.02W of power (having measured the output of the 555 as 7.5V with a 9V supply).

Comments

[Beenay25]

Great Instructable. I'm building one of these to keep the local cats from using the gravel in our garden as a latrine.

Can you give more detail in calculating the value for the resistor on the speaker?

I've got a 6v power supply and a 3 watt speaker.
Can you give the actual formula you used to calculate your 47r resistor?

[Beenay25]

Actually, I think I've solved it.
Something like....

voltage / desired watts = current
5v / 2.5w = 0.5A
(voltage and watts are approximate - will check when at home)

Then R=V / I
5v / 0.5A = 10R

[Giveitago]

Hi Beenay, thanks. As Watts are Voltage x Amps, Watts / Voltage or Amps will give the other. (E.g. 2.5W/5V = 0.5A).
5V/0.5A = 10R, so you would need to make the load impedance 10R by adding resistors in series or in parallel with your speaker (after checking the impedance of the speaker).
However, a 555 timer can't supply more than 0.2A of current. To get a 2.5W output, I suggest adding an audio power integrated circuit between the 555 and the speaker. You may need to use a larger power supply Voltage (whilst remaining within the range of the 555's supply) and reduce the output Voltage of the 555 with a voltage divider.
Thanks for reading my Instructable.

[Dr.Bill]

Might even be useful for mosquitoes as well.

[javajunkie1976]

How effective is it against teenagers? Why don't adults get affected by this? Will it work against other annoying things, like politicians?

[trailleadr]

Politicians....lol. Love it!

[Giveitago]

The approximate human hearing range is 20-20,000Hz, but this is seriously affected by age and exposure to loud noises- from machinery to excessive use of headphones at high volumes. Basically (and depressingly) your hearing will be deteriorating for most of your life. Most teenagers will be able to hear 12,000Hz (quite an annoying frequency when heard for a while), but most adults won't. If you can- congratulations! As for politicians, given what I've said about age, I'd go for a wet sponge...

[lafnbear]

What if you don't have a sonic cat, just a regular one?

[jimmytvf]

throw'em some water

[Giveitago]

+1

[Giveitago]

Ok, admittedly its effect is limited on deaf cats... ;)

[sst1]

Does this frequency can disturb my dog? great device btw!

[Giveitago]

Thanks! Dogs and cats have much greater hearing ranges than humans (dogs about twice as wide; cats about thrice as wide), so if your dog's hearing is healthy then it should be able to hear the device at 12,000Hz. Whether it'll disturb a dog... Try it and let me know. :)

[Robot Lover]

This is definitely a great use of 555 timers. Keep up the great work!

[Giveitago]

Thanks for the encouragement! :)