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I am new to working on 555 timer. I am running it in monostable mode. I am triggering it using a button on trigger pin. It works just fine. What I want is the LED connected to the out pin 3 to switch off when I press the trigger button again. If I don't press the button, LED will switch off after the specified time duration by the RC circuit. Is it possible? Thanks in advance. :)

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Terminating a monostable's output pulse

IC1 and its associated components are just a switch debouncer. C2 & R2 trigger IC2 on the falling (trailing) edge of the pulse coming from IC1. IC3a (a positive edge triggered D-Type flip flop) holds IC2 either in reset or enable dependent upon the circuit's output state at the time of the rising (leading) edge of the pulse coming from IC1. A transistor is used to drive the LED to ensure that IC3a's D input sees a sufficiently high logic 1 level voltage. In this edit I have added a power-on-reset circuit to the asynchronous set pin (S) of IC3a.

I have built and tested the circuit, it works well.

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Yes it's possible using logic gates: It will reset only if the LED is ON and you press the trigger button again.

enter image description here

Note: The extra inverters and the AND gate are for rising edge detection (When you release the trigger button). D flipflop to get the Previous state of the output.

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  • \$\begingroup\$ If the LED is off before the button is pressed it will immediately light when the button is pressed and then will switch off when the button is released, when it's required to stay on, won't it. \$\endgroup\$ – James Jan 12 at 12:21
  • \$\begingroup\$ Yes, thank you James. I have added D-flipflop to save the last state. \$\endgroup\$ – Mohammad Jan 12 at 17:16
  • \$\begingroup\$ You have the logic correct but beware the bouncing switch causing multiple sets and resets of the 555 monostable output leaving the monostable output perhaps set or perhaps reset after a switch press depending upon the number of switch bounces. \$\endgroup\$ – James Jan 12 at 19:13
  • \$\begingroup\$ Yes he could improve it by adding debouncing circuit at the switch. Thanks James \$\endgroup\$ – Mohammad Jan 12 at 21:46

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