View ne555n datasheet:

NE555N (1/2) IL08D PRECISION TIMER —TOP VIEW— 7 DIS 3 1 GND VCC 8 Q 6 THR 2 CLOCK IN 2 7 DISCHARGE 5 CONT R0 Q OUT 3 6 THRESHOLD 4 RESET IN 4 5 CONTROL IN MONOSTABLE MULTIVIBRATOR VCC R 7 3 INPUTS OUT DIS Q RD CLK Q 6 0 : LOW LEVEL THR 0 X 0 2 1 : HIGH LEVEL CLK 1 5 X : DON’T CARE CONT C R0 0.01 CLK 4 VCC Q 1.1CR RETRIGGERABLE MONO. MULTIVIBRATOR (MISSING PULSE DETECTOR) VCC R 7 3 INPUTS OUT DIS Q RD CLK Q 6 0 : LOW LEVEL THR 0 X 0 2 1 : HIGH LEVEL CLK 1 5 X : DON’T CARE CONT R0 C 0.01 CLK 4 VCC Q 1.1CR 1.1CR NE555N (2/2) ASTABLE MULTIVIBRATOR VCC RA 7 3 RD Q DIS Q RB 0 0 6 0 : LOW LEVEL THR 1 2 1 : HIGH LEVEL 5 C CONT Q R0 0.01 4 TH TL VCC TH = 0.693C (RA+RB) TL = 0