Look at it below, it should be easy to understand if you have some background in frequency, period, and duty cycle. Don't go away yet, it is really simple. Now look at the schematic down the page. It has on IC chip (the 555 timer) two resistors, and two capacitors. That simple! Now look at the five equations down the page describing t(high), t(low), duty cycle, period T, and frequency f. The graph just below this paragraph actually describes the frequency equations. You may use this graph to quickly design your square wave generator and fine tune the frequency and duty cycle later by adding variable resistor (pot) in place of R1, R2, or both. The graph below says: to get a square wave from the 555 timer look at the frequency you want to operate at from the frequency axis of the graph below, then project this frequency up against one of the inclined lines (1K or 10K or 100K or 1 Mega or 10 Mega ohms) then project it left against the capacitance C axis (in uF). The sum of (R1 + 2R2) in the 555 timer schematic should equal the value of the inclined line you selected and the capacitor C value in the 555 timer in the schematic below should equal the projected value C in uF. Now go ahead and do it for any imaginary value, just for a practice.

T = 1/f = t(high) + t(low)

Do you want to have fun with it? Build this circuit and proximate it to you AM radio, what do you hear? Add a piece of wire to the output (pin 3) and observe that the transmit range actually increased. If you can condition the output with filter circuit (will describe later) you can convert this circuit to wireless short range transmitter. Of course you can boost up the range by adding power amplifier to the output pin (pin 3). Careful not to interfere with other wireless close by equipment or violate FCC rules though. I bet if you drop the duty cycle to 50% and adjust the frequency to 1 Hz then hook an LED with series 220 ohms resistor to pin 3 they won't get you.

Did you know that some RFID systems operate over the rang of 125 to 148 KHz. This circuit can be modified to become part of RFID circuit or any other useful circuit.

Connect its output (pin 3) to a transistor driver and then to step up transformer and ---- oooops…hold on do not touch the secondary coil, you could hurt your self !!! well that depends on the transformer ratio. So play it safe.

Or may be you can adjust the frequency to 0.2 (T = 5 sec) and duty cycle to say 10% and connect a buzzer and use in you back yard to scare those docks that leave their !@#$ on the floor. I had hard time cleaning after them.

May be it is time to start using the Microcontroller… CU…

Firas Faham