The pot value will affect the frequency of the output but does not necessarily need to be similar to Ra or Rb. It's essential that Ra is of a reasonable size to prevent too much current flowing from the positive rail through the Discharge transistor to ground.

Hi, thanks for Subscribing, First of all, the 555 can be powered from 12V supply and will give you a 12V output (or very near to 12v). You will probably need to hang a transistor off of the 555 in order to support the current of your fan. With regard to turning off in relation to your PWM signal, I don't quite understand the arrangement/what's required. Is the PWM driving the fan, if not what is it driving. Are you looking to detect when the PWM signal stops, and then stop the fan 20-30 seconds after that, or looking to detect the PWM has stopped for at least 20seconds before turning the fan off?

@paulpkae thanks for your help. This is a cooling fan on a small laser diode. 12v to the fan is always on. The pwm signal fires the laser beam. So trying to stop the fan running as it's very loud after the pwm signal has stopped for 20 to 30 seconds to allow the laser to cool down. Meaning the laser had finished it's lasers job, the module as cooled down, so now turn the fan off. When pwn signal comes back on the next laser job, the fan starts again. The 12v, ground and pwm is all in the same 3 core cable

@@FlixmyswitchLASER OK. I think you can arrange a simple circuit to do this with an Op-amp, diode, transistor, capacitor and a couple of resistors. If you configure an op-amp as a voltage follower with a diode within the loop back (see my "All About Op-Amps" video, "Perfect Diode" section 11:07), then connect + terminal of a capacitor to the output & - to GND, the capacitor will act as a peak detector to whatever signal is fed into the non-inverting input (which will be your PWM signal). By using an op-amp there will be no additional load pulling down your PWM. You will need to add a blead resistor in parallel with the cap so that it has a discharge route when the PWM signal stops (something large 100K+). Now connect another resistor from the +terminal of the cap and into the Base of an NPN transistor. 12V+ to one side of your fan, then the negative side of the fan onto the Collector of the transistor and the Emitter to GND. While the PWM is running the capacitor charge is regularly being topped up and will experience minimal discharge between cycles. The charge keeps the transistor switched ON and allowing current to pass through the fan to GND. When the PWM stops, the cap will discharge through the blead resistor and Base/Emitter junction and eventually the transistor will turn OFF. You will need to play around with the resistor and capacitor values a little. I will do a little sketch later and give you a link to download, should make it easier to follow. Hope it helps.....

@paulpkae thanks time to do some study

@paulpkae thanks for your detailed reply. If you are able to produce a sketch that would be very helpful

Thanks for your comment, much appreciated.

Thanks very much, glad you enjoyed it.

Thank you, glad you enjoyed.

"My friends, as I have discovered myself, there are no disasters, only opportunities. And, indeed, opportunities for fresh disasters."

Quite possibly :)

Many thanks for your comment. I never really learnt much from school. It was only after being exposed to real world situations that I became interested in certain topics and was then highly driven to learn.

Many thanks, much appreciated.

Many thanks. There are countless iterations of the 555 circuits out there and some have been stretched well beyond what people thought possible. Like all electronic components and ICs, they are just the building blocks for us to play around with and create all sorts of configurations.

That's great to hear. Thanks for the comment and thanks for Subscribing too.

Thanks for your comment. My heat gun is still going strong, used regularly and I can honestly say that model is great quality for the price.

Hi, thanks for the comment. That video felt like a 100 mile an hour run through so much stuff it was not really feasible to delve to much into each aspect and circuit. I do intend doing a follow up video whilst putting together an audio mixer project for my office. A lot more detail will be given in that. Watch this space..... 🙂

Many thanks.

Thank you. More videos coming......

Hope you enjoyed this one then :) . Thanks for watching and thanks for the comment.

Glad it helped.

That's great to hear. More videos coming......

Hi, thanks for the comment. I have just uploaded the spreadsheet and you can now download it from here:- 1drv.ms/f/c/10f323cd9840b5df/EknQIwciMO1NnKcfaTDeZZQBiHeleHQV0ED1RoaRtsEXzw?e=HF4PbV

Thank you so much. I'm really grateful.

It took quite some time, but comments like this make it all worthwhile thanks. And many thanks for Subscribing.

Many thanks for the comment. Glad you enjoyed it.

Many thanks.

Glad you found it useful. Thanks for the comment.

Many thanks for Subscribing, it makes a big difference.

What a great bit of history, many thanks.

Thanks very much, glad it made you laugh :)

Hi, not sure which comment you are referring to, but there are two follow up videos given in the description (click on "...more" above).

Haha, indeed it would. Lets campaign for Boris ratings on all Op-Amps.

Great to hear.

Indeed they do. That is Electron current flow, I usually present using conventional current flow. Either way, the rules are the same.

Current is indeed a flow of negatively charged electrons.. which of course are attracted to the positive source. Unfortunately.. way way back in time before this was known... it was postulated that current flowed positive to negative... and it stuck ! Everyone knows its back to front... and its called conventional current flow. So what you will see on almost all electronic circuits is current flow shown in the direction +ve to -ve . That is why the diode is marked that way - it assumes conventional current flow. Its also why an npn transistor "emitter" terminal is shown with an arrow pointing outward.. showing conventional current flow from the collector which is biased positive And a pnp transistor with the emitter arrow pointing inward.. showing conventional current flow towards the negatively biased collector Just one of the things you have to get used to!!

I'm not going to get used to it, so just assume everyone else is WRONG. I believe that electrons flow from negative to positive. Full stop. I keep this in mind at all times. When I see nomenclature like Vss or Vdd (source or drain), it makes more sense than Vcc (collector).

@@davidgari3240 Yeah they do and we all get it smarty pants and you do you but as said " Rules of the game are the same" it works just fine for us and has done for years.

@@andymouse Right. If I'm consistent and get it backward _every_single_time_, everything works fine.

Brilliant! Thanks for watching and thanks for the great comment.

Many thanks. Yes, more Op-Amp videos to follow. I will probably do the Audio Mixer project mentioned in this one.

That's great, so glad you are finding these videos of use. Best of luck with your studies.

Thankyou, glad you enjoyed.

Great comment, thank you. Glad you liked and special thanks for Subscribing too.

Wow, that's saying something! I do highly recommend that viewers also take a look at Dave Jones' Op-Amp videos (if not already done so). A mix of alternative explanation techniques is always helpful (partly why I started this channel). Anyway, many thanks.

Glad you liked it. Thanks for the comment.

Thank you. And yes, I intend to press on.

Cheers Les.

Thanks for watching.

Many thanks for your comment, much appreciated. I particularly like the Boris tug of war too 🙂

Less dodgy to connect hot

Hi, many thanks for your comment. The answer to your question is Yes. You can achieve the exact behaviour you queried, by configuring the 555 in a monostable arrangement. I will be doing a video in this in the near future, but in the meantime, you can find this circuit on the 555 datasheet.

@@paulpkae Thanks a lot! Now I remember that I have somewhere a datasheet book where 555 is also discussed, I just need to find it :-)

@@MrJPI if you Google "555 timer" and click on the "ti.com" link, you can view the datasheet online or even download it.

Thank you. Not a great video really, but it certainly demonstrated some great effects that can be achieved with addressable LEDs. This is permanently setup in my office now. If you want to know more about the WLED project, check out the link below:- kno.wled.ge/

Thanks for watching, and for your comment.