Rodents and insects (including mosquitoes) are allergic to high frequencies above 15kHz. My repeller is using this fact. It is generating ultrasonic tone with a frequency of around 20 kHz, which is inaudible to humans and harmless, but reliably repel uninvited guests such as mosquitoes, mice, rats, martens or spiders. The heart of the device is integrated circuit 555, a source of ultrasound is piezoelectric speaker. You may also be interested in Powerful ultrasonic repeller of insects, rodents and martens with periodicly changing frequency.
