The MARK and SPACE are the HIGH and LOW values of a waveform. When a waveform is HIGH, it is called the MARK.
The diagram above shows MARK and SPACE durations of different lengths. Marks and spaces can be any length and can change during the production of a waveform. If the length of the mark is equal to the space, the waveform is said to have a 50:50 Mark:Space ratio, as shown below:
A waveform with a 50:50 Mark:Space ratio is produced by a 555 when the top resistor (called the DISCHARGE resistor) is very small compared to the TIMING resistor. This is shown in the diagram below:
To increase the MARK, the Discharge resistor must be LARGE compared to the Timing resistor.
To increase the SPACE, a diode is needed as shown in the diagram below:
The MARK:SPACE ratio can be adjusted without altering the frequency by connecting two diodes as shown in the diagram below:
