This astable uses two transistors which turn on and off constantly. When one turns on it turns the other one off. The speed of switching (or frequency) is controlled by the values of the base resistors and capacitors. The Crocodile Technology circuit shows 10k collector resistors whilst the PCB layout shows LEDs with 1k resistors.  There is one output from the block.

The 555 chip was built with timer circuits in mind and is easy to use as an oscillator or astable. The frequency is given by the value of the 1M variable resistor and the capacitor (the 1k resistor is so small it can be ignored). f=0.7/RC roughly. See Doctronic’s Beastie Zone for more details. The PCB layout shows two capacitors selected with a switch – they give quite a good range of frequencies. Use 100k with either 22nF or 100pF for higher frequencies. The 10nF capacitor on pin 5 is just to help stabilise the circuit. The other switch in the PCB layout is to allow the circuit to be “gated” – switched on or off – by a preceding block. When the output from the preceding block is high, the astable can run, otherwise it will not and its output will be low.

Here is a 555 astable with variable pulse width. The difference is the diode around the central resistor. This means that you can set the high time with one variable resistor and the low time with the other. The block can be controlled (gated) by the previous block just like the one described above.

A Servo motor as used in radio control, is controlled by the width of a pulse. It needs a pulse roughly every 20mS (50Hz) and with a mean width of 1.5mS. That width would turn the servo to the middle (90 degree) position. The width of the pulse can vary from about 0.5mS to about 2.5mS and these values will move the servo from fully anti-clockwise to fully clockwise.

Pulse Width Modulation again uses pulses of differing widths. The output of this circuit should vary from pulses that are almost as wide as the period of the waveform, right down to almost zero width. If you add a power driver to the right of this block, you can send pulses of power to a motor. Because the frequency of the system is sufficiently high, the effect is that the motor runs at full power all the time, even though its speed will change with the width of the pulses.