This electronic mosquito repellent circuit is easy to build but is kind of hard to adjust on the exact sound frequency which will reppel the mosquitoes. With frequencies between 2 KHz and 2.5 KHz which mimic male mosquitoes buzz, it seems you can get the best effect.
But sometimes you may need to readjust the potentiometer so the freq. be as high as 10 KHz. Anyway, the fact is that you may need to make a lot of experiments until get the best result. You can read this article about insects and how do they react to ultrasounds – Ultrasound and Insects.
You may add an audio amplifier but don’t forget to connect the small filter (damper) between the electronic mosquito repellent and the amplifier.