NE555P Connected in Monstable Mode didn't turn off after T = 1.1RC

Thread Starter

andychen

Joined Feb 4, 2020
4
Hi, I connected the 555 timer as shown in breadboard below. It didn't turn off after 8 secs as calculated. I tested the 555 IC in astable mode and the green LED blinked on and off. I have also rechecked all the pins, orientation of 555 IC and connections but can't find the problem. Could you spot any issues with my design or wiring? Any suggestions to troubleshoot the circuit? Thanks for your help.
 

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WBahn

Joined Mar 31, 2012
26,143
Schematics are the language of electronics. Don't expect others to guess which holes some of the pins go into (though your annotations help a lot) or what way polarized components are turned or what the underlying connect matrix of your breadboard is (though this one looks pretty straightforward). Sketch a schematic and post it.

Do you have a bypass capacitor across the supply pins.

How are you triggering the circuit? It looks like you have the trigger input hard tied to ground. The 555 is a retriggerable timer. Look into what that means and the implications for your circuit.
 

Thread Starter

andychen

Joined Feb 4, 2020
4
Schematics are the language of electronics. Don't expect others to guess which holes some of the pins go into (though your annotations help a lot) or what way polarized components are turned or what the underlying connect matrix of your breadboard is (though this one looks pretty straightforward). Sketch a schematic and post it.

Do you have a bypass capacitor across the supply pins.

How are you triggering the circuit? It looks like you have the trigger input hard tied to ground. The 555 is a retriggerable timer. Look into what that means and the implications for your circuit.
Hi, I have attached the schematic.

I don't have a bypass capacitor.
 

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WBahn

Joined Mar 31, 2012
26,143
Hi, I have attached the schematic.

I don't have a bypass capacitor.
But how are you triggering it? Are you connecting that white wire to GND and then leaving it there? If so, then that is why you need to go read up on the implications of the 555 being re-triggerable.
 

Dodgydave

Joined Jun 22, 2012
9,431
You need to pulse pin 2 like this...


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This is Homework HELP. Please give the TS the opportunity to take the hints and suggestions they've been given and struggle with it and figure out on their own how to get over the finish line.
 
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Thread Starter

andychen

Joined Feb 4, 2020
4
Hi All,
Thank you for your help. I looked into the retriggerable feature and included a switch between ground and pin 2 of the NE555. I also checked the pull-up resistor to Vcc. It's working fine now. Appreciate all your inputs. :)
 
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