Explanation of 555 monostable mode operation sequence.

555 has been widely used for more than 40 years as a timer IC. There are bipolar type which is original, and CMOS type which is lower power consumption than original bipolar type. 555 can be used to make 10us to 100s timer which is configured by externally added R and C. Simplified block diagram is shown in Fig. 1.

Fig. 1 Simplified block diagram and example usage Fig. 1 Simplified block diagram and example usage

Operation sequence

  1. As an initial state, Trigger input is more than 1⁄3 Vcc, and RS-FF is reset. Transistor TR is ON, therefore Vc is 0 [V]. Output is LOW.

  2. If Trigger input gets lower than 1⁄3 Vcc, transistor TR becomes OFF. C starts being charged through R and voltage potential of Threshold input starts increasing. Output is High.

  3. When the voltage potential of Threshold input reaches 2⁄3 Vcc, RS-FF is reset. Transistor TR is ON, and C starts discharging. Output is LOW.

  4. The output pulse width t [s] is given by

$$ t \approx 1.1 \times RC $$

References/External links

TI NE555 datasheet