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 it.

Copilot AI gives the basics only.

Also a similar set of 3 x CD4047 but giving a 20 sec pulse of 25khz square wave.

Any ideas or suggestions

 

BTW, it will be less confusing to others if you refer to the 20 kHz part of the output signal as a burst rather than a pulse.

Do you need digital clock accuracy, or are simple R-C circuits adequate?

Also, details: Circuit operating voltage? What does the output drive?

Your are correct, two 555's will do this. So will one CD4093 configured as two oscillators. In either case, the first half-cycle of each 20 kHz output burst will be slightly longer than the remaining half-cycles. There are ways to mitigate this if this is an issue.

Look at the datasheet for the CMOS 555, such as the LMC555. It has an astable circuit that makes a nice 50/50 output squarewave with only one timing resistor instead of two. That is the basic approach. Have the 60 second period oscillator (0.01667 Hz) drive the enable or Reset of the 20 kHz oscillator.

Schematics on request.

ak

 

Clock accuracy - not very important, but not way silly
Voltage operating - 12v DC
Output driving a 50mm dia piezoelectric speaker (maybe an amplifier if needed)

And yes agree with burst being a better word.

For some reason I have avoided using logic gates as oscillators, and 555 not to accurate.

 

Logic gates can be used for either crystal or R-C oscillators. The R-C versions are not as accurate as a 555 circuit because logic gate transition voltages are not as tightly controlled as in a 555.

ak

 

Any ideas or suggestions

One suggestion using the 555 as a 20 second timer.