555

The LM555 timer can achieve a 50 percent duty cycle as shown in the next diagram. The duty cycle adjustment range of the give components values is from 42 to 55 percent.
Resistors R1 and R2 were selected first and then resistor R3 was selected to give the best control range based on [...]

This circuit gives out an alarm when its sensor is wetted by water. A 555 astable multivibrator is used here which gives a tone of about 1kHz upon detecting water. The sensor when wetted by water completes the circuit and makes the 555 oscillate at about 1kHz.
The sensor is also shown in the circuit diagram. [...]

The basic circuit operates at a frequency determined by R1, R2 and C1 and has a pulse width range of 0 to 100 percent.
The following diagram shows a basic circuit with an open collector output that would require a pull up resistor at its output. The parts values are the nominal values of [...]