Monostable or one shot or pulse extender

http://www.555-timer-circuits.com/monostable-555.html When the circuit is turned on, the output is LOW and a brief negative pulse on pin 2 will make the output go HIGH for a period of time determined by the value of R and C. If pin 2 is low for longer than this period, the output will remain HIGH while pin 2 is LOW and immediately go LOW when pin 2 goes HIGH.  CIRCUIT OPERATION  When the circuit is turned on, the capacitor is uncharged. Pin 6 sees a LOW and pin 2 sees a HIGH.  Remember: Pin 2 must be LOW to make the output HIGH.  Pin 6 must be HIGH to make the output LOW.  Neither pin is "controlling the chip" at start-up and the chip is designed to output a LOW with these start-up conditions.  In other words, the chip starts in RESET mode. Pin 7 is LOW and the capacitor does not charge.  When pin 2 see a LOW pulse, the chip goes to SET mode and the output goes HIGH. Pin 7 goes OPEN and capacitor C charges via R. When pin 6 sees 2/3 rail voltage, the chip goes to RESET mode with pin 3 and 7 LOW. The capacitor instantly discharges via pin 7 and the circuit waits for a negative pulse on pin 2. Seems by playing around R = 25k and C = 200u makes the time about 5.5 seconds Time formula: time = 1.1 x R x C

published 6 years ago