20khz

I am looking to have a period of say 60secs, and within that 60 secs a pulse of 20khz square wave for 20 secs. That pulse of 20 secs can be anywhere within that 60 secs. And then repeated. I am lead to believe 3 x CD4047 can do it ( a bit like using 2 x NE555) but I cannot find anybody doing...

As part of my ongoing project, I need to drive a Piezoelectric ultrasonic speaker. I have 1 of each that resonates at 20khz and 25khz. The max my sine wave oscillator can give out is 5v RMS, and the max the speakers can handle is 20v RMS. So to get the max out of the speaker I need a...