555 timer is a commonly used integrated circuit, which can be used to form pulse generation and shaping circuits. The following are three commonly used pulse generating and shaping circuits composed of 555 timers:

1. monostable trigger: monostable trigger is a common pulse generating circuit, which consists of a 555 timer and a diode. When the input voltage is lower than the threshold, the output terminal will continuously output a high level; When the input voltage is higher than the threshold, the output will flip to low level.

2. Schmidt trigger: Schmidt trigger is also a common pulse generating circuit, which consists of a 555 timer and a capacitor. When the input voltage is higher than the threshold, the output terminal will flip to low level; When the input voltage is lower than the threshold, the output terminal will continuously output a high level.

3. multivibrator: multivibrator can generate various waveform signals, including square wave and sine wave. It consists of several 555 timers and a capacitor or resistor. Each 555 timer has a clock input pin and a control input pin, and the frequency and duty ratio of the output waveform can be changed by changing the level of the control input pin.