Hello Mr. Markus Drexelius,

The following web address has two references to frequency counters using 8051 type IC.

http://www.8052.com/forum/read.phtml?id=26293

It should be easy to build a 8051 type IC to detect the frequency's you indicated (1.0 kHz thru 2.5 kHz)

Good luck,

Charles Bannister