YOU are OP - Original Poster. This is slang for the person who started the thread in a discussion on the internet.
R8 and C5 are on the NE567.
The NE567 is a PLL - Phase Locked Loop. It "locks phase" with an input frequency. This is the part that actually detects the 25khz frequency. If you want to detect 25khz, you need this part. The "decoded output" turns ON at 25khz. It is OFF at 26khz, or 24khz or any other frequency.
Unfortunately, ON and OFF are a transistor connected to ground, so you must provide a "pull up" resistor to read it, and when it is ON the output will be 0V and when it is OFF the output will be at Vcc, which may seem backwards, but is helpful if you are connecting a microprocessor that operates at a different voltage. Look at figure 16 and 17 on this page: **broken link removed**
The other parts of the circuit are an amplifier and a comparator, they will work at many frequencies. U1 is the amplifier. U2 is the comparator - this makes square waves, all the same amplitude (about 7.5V) out of the sine waves from the amplifier before sending them to the PLL.
In post #1 you said you wanted to "receive 25 kHz signal instead of 40 kHz". That frequency detection is done by the NE567. I did not understand your explanation in post #12. It would help if you drew a schematic (even a pencil sketch) of what you want to do.