Am I scammed by the electronic ultrasonic insect repeller?

Insect and pest repelling using the electronic way isn't something new and has been around for many years. However, until today, there is still debates and doubts whether it works or not. There are lab tests showing that it totally not working, and other lab tests showing that it does work but not very effective.

In fact, there are 3 common kinds of electronic insect and pest repeller in the market:

• Electromagnetic - the device changes the wavelengths of the frequency signals that already exist in the wiring of homes, thus claimed to be able to agitate rodents and insects with these signals.
• Ultrasonic - the device emits high frequency sound above 22 khz which is unheardable by human being (thus called ultrasound) but is detectable by certain insects and rats. The effect is like putting a loud siren beside their ears and chase them away.
• Ionic - it is said that certain insects and rodents are sensitive to negative ions, so they won't go near an ionizer.

Nowadays they are even 3-in-1 products which combine all the features above, but selling at high price of above RM100.

I have 2 electronic ultrasonic insect repellers in my home, which I bought for about RM20 each in Giant hypermarket. Picture as below.

In fact, you can easily find in the Internet for circuit diagrams about electronic ultrasonic insect repellers, and can DIY make your own with some electronic knowledge and devices. Some are very simple, and here is a more complicated one from Electronic Circuits website.

Beside that, there also exist computer software that make use of the speakers to generate pest repelling ultrasonic waves, such as SEA Anti-Mosquitoes XP, KP-Anti Mosquitoes, Ultrafobos, etc. Similar applications are available for mobile devices including iPhone, Palm, etc. For normal handphones, there are also ultrasonic ringtones that you can find and download from Internet.

Is the small electronic ultrasonic device that I've bought effective to me?

I put one in my kitchen, quite near to the dining table.

Well, before using it, I used to disturbed by one or two mosquitoes at night when I used my laptop on the dining table. After using it, so far no more such mosquito disturbance.

And before using it, sometimes I saw cockroach in the kitchen when I turn on the light there during midnight. Now, so far no such engagement after using it.

It seems like it works fine for me so far, and I don't feel like scammed, although I've found quite a number of reports in the Internet saying electronic insect repellers are useless.