On / Off / Flashing LED circuit help

cmartinez

Joined Jan 17, 2007
8,218
I just remembered I modeled your circuit in post #29, and it clearly shows that the LED never completely fades out... let me see what I can do and I'll get back to you
 

cmartinez

Joined Jan 17, 2007
8,218
I just placed a 1N914 diode between CV (pin 5) and the output (pin 3) and the LED seems to completely fade out, although it flashes at a far lower frequency, about every 8 seconds
 

cmartinez

Joined Jan 17, 2007
8,218
It would be really, really nice to know if you made the changes I suggested... if you placed the diode and changed C1's value...
did it work?
 

Thread Starter

aac52

Joined Dec 16, 2014
28
It would be really, really nice to know if you made the changes I suggested... if you placed the diode and changed C1's value...
did it work?
Sorry for the lack of response, I have been a bit occupied with other things and not had a chance to check back yet.

Unfortunately I do not have 9v available in my application, which is why I was using the circuit suggested by #12 since it should work on the 5v I do have, but please do not think that I haven't appreciated your efforts because that is not the case at all.

I will build the one you simulated, including the additional changes and compare the functionality.
 

cmartinez

Joined Jan 17, 2007
8,218
Sorry for the lack of response, I have been a bit occupied with other things and not had a chance to check back yet.

Unfortunately I do not have 9v available in my application, which is why I was using the circuit suggested by #12 since it should work on the 5v I do have, but please do not think that I haven't appreciated your efforts because that is not the case at all.

I will build the one you simulated, including the additional changes and compare the functionality.
Hey... no worries... I was just curious (and impatient) that's all ;)
Anyway, I just checked and it seems that 5V ought to make things work with modern 555s, so just adding the diode I told you about should be able to fix things up, you don't need to change your circuit or build another one... let me know how it went
 

Thread Starter

aac52

Joined Dec 16, 2014
28
Sorry for the delay again, I have been waiting on a few things. Plus there has not been much time to work on this little project unfortunately.

My transistors have finally arrived after two errors with stock where I received incorrect items. My capacitors have also arrived.

Since my previous messy breadboard attempt I also ordered some jumper leads.

Tomorrow or the day after I will be testing the circuit, including the extra diode you mentioned cmartinez :). Thank you again for the suggestion.

I will post back with the results.
 

Thread Starter

aac52

Joined Dec 16, 2014
28
Hey... no worries... I was just curious (and impatient) that's all ;)
Anyway, I just checked and it seems that 5V ought to make things work with modern 555s, so just adding the diode I told you about should be able to fix things up, you don't need to change your circuit or build another one... let me know how it went
So I have finally got round to testing this, and it works perfectly, thank you!

Adding the extra diode does indeed allow the LED to turn fully off. Looking at the graph I was a little worried that it would be off for too long, however it appears to be fine.

I've also taken a short video to show the real-world effect.


Next stage is to make the PCB and integrate it into the rest of my project. I will again post back with updates as I make progress.

I'd like to thank everyone who has helped me so far in this thread, it has been really appreciated.
 

cmartinez

Joined Jan 17, 2007
8,218
So I have finally got round to testing this, and it works perfectly, thank you!
It's nice to see things working in the real world, other than a simulator. I suppose you also had to change the value of C1 as well to get the frequency you wanted, right?
 

Thread Starter

aac52

Joined Dec 16, 2014
28
It's nice to see things working in the real world, other than a simulator. I suppose you also had to change the value of C1 as well to get the frequency you wanted, right?
Yes, like with the previous issue I had, the real-world circuit showed the true importance of having a capacitor between Vcc and GND of the 555 timer.

For the capacitor I chose it in the simulator, and since the resistor could not be very large due to the problems mentioned before, I had to go with a fairly large capacitor instead.

In the simulator I just used a ~2k resistor, but in the circuit I built I used a 5k pot, and plan to keep that so I can adjust it in future if I need to. The two component values give a nice range between fast flashing (not what I want though), and very slow fading.
 
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