The homepage of a relatively simple microcontroller-based frequency counter. This design is common to just about all of the kits that you'll find on eBay or Amazon (you know the ones - they don't have any instructions). Usually you'll be looking up construction variant 2 (five digit display) with a 20MHz crystal.

Designed by Wolfgang "Wolf" Büscher, DL4YHF.

Microcontroller used: PIC 16F628

If you're at a security conference, and you've got a ham radio license (of any class), grab your radio and set it to 146.580 simplex (no offset).

A website that takes a parrticular frequency (in KHz) and a duty cycle (at least 50%, or on half the time, off half the time) and calculates the values of the two resistors and the capacitor required to generate that frequency using a 555 IC.

A simple webapp that calculates the resistor and capacitor values to make a 555 generate a signal of a particular frequency.