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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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3 Answers 3

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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\$
    – user173271
    Jan 12, 2021 at 12:21
  • \$\begingroup\$ Yes, thank you James. I have added D-flipflop to save the last state. \$\endgroup\$
    – Mohammad
    Jan 12, 2021 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\$
    – user173271
    Jan 12, 2021 at 19:13
  • \$\begingroup\$ Yes he could improve it by adding debouncing circuit at the switch. Thanks James \$\endgroup\$
    – Mohammad
    Jan 12, 2021 at 21:46
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Here is an idea that manipulates the switching threshold point, by the D-type flip-flop, to terminate the timing cycle early. D1 is used as a low reverse-leakage diode with 2.2 V forward drop.

Circuit simulated in Proteus.

555_cir

Normal triggered output duration.

normal

Triggered output terminated by next trigger pulse.

normal_off

Output can be turned on or off with alternating trigger inputs. While the output is in the on state, it will time out after the normal timing period set, if no more triggers gets applied.

normal_off_on

Slower sweep. The wide blue pulses are from the set mono-stable timing RC values.

of_on_tog

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