Combining a 555 PWM with an LED Flasher Circuit Advice

[denno]

Hi all,

I am making myself a nightlight with some LED's and components that I have on the way, and I would just like to run this schematic past people with more experience than myself to see if it's workable.

The reason I am using the flasher circuit is because I'm pretty sure that if I set the flash rate fast enough that it isn't noticeable, then I will be able to double the number of LED's that my power supply can run. I bought an AC to DC power module from eBay, which outputs 12V but with a maximum current of 300mA. I am using 100mA LED's (as I have these laying around and have nothing better to use them for).

The blue dashed square is the 555 PWM signal generator circuit, and the original came from this website:
**broken link removed**
The red dashed square is the LED Flasher Circuit, which came from this site:
https://wild-bohemian.com/electronics/flasher.html

Does my plan make sense and better yet, will it work?
I tried to label as many things as I could to make it very clear exactly what I plan to do .

Thanks in advance
Denno

 

[alec_t]

Rapidly switching two sets of LEDs on alternately will have the same visual effect as continuously powering one set of LEDs.The multivibrator has a 1:1 mark/space ratio, so the average output from either set is half what the continuous output would be. Two times a half = 1.
Your power supply can only provide 300mA, period.

Be aware that even if you could double the light output it wouldn't seem much brighter, because of the non-linear response of the human eye to light intensity.

 

[denno]

Thank you for the reply.. However I don't quite understand what you're getting at.. Are you saying that what I've suggested above won't work, or that it will be ok?

 

[alec_t]

The flasher will certainly switch the two sets of LEDs, but they won't appear brighter than one un-switched set. So in that sense the circuit won't work as you hoped.

 

[denno]

Oh ok.. I guess I didn't mention anything about that did I.
I don't want the LED's to appear brighter. I want them to still be the same brightness of whatever I turn the POT to in the PWM circuit.. if that makes sense.. I wasn't trying to be tricky and somehow get brightness without using the current...

 

[alec_t]

The reason I am using the flasher circuit is because I'm pretty sure that if I set the flash rate fast enough that it isn't noticeable, then I will be able to double the number of LED's that my power supply can run.

So, if you're not trying to boost the output then I'm curious: why are you doubling the number of LEDs?

 

[denno]

You will notice in the quote you added to your reply that I said 'double the number of LED's', not 'double the brightness output'. The reason for wanting as many LED's as possible, is because these 10mm LED's that I have don't have a very wide spread of light, so the more I use, then the more even I can spread that light.

Anyway, my question initially was would I burn up the power supply if I made this circuit, not would I double the light output.

So I would still like it if someone could tell whether connecting the circuit above will blow my power supply or not.

Thanks,
Denno

 

[carbonzit]

I am making myself a nightlight with some LED's and components that I have on the way, and I would just like to run this schematic past people with more experience than myself to see if it's workable.

I bought an AC to DC power module from eBay, which outputs 12V but with a maximum current of 300mA. I am using 100mA LED's (as I have these laying around and have nothing better to use them for).

Looking at your schematic, it appears you have a problem.

You say you're using a 300mA power supply. Your schematic shows 6 strings of 100mA LEDs on the two sides of your multivibrator.

This means the LEDs alone will be drawing ~300mA at any given time. Plus whatever the 555 and transistors draw (not much, but something). So your power supply would be pretty much maxed out. Would it work? Probably, but who's to say how long it would last? Did the manufacturer build in any wiggle room when they specced it?

Kind of curious why you'd want to build a night light with such high-brightness LEDs. Even the 20mA ones would seem to be overly bright for such an application.

 

[denno]

Thanks for that reply, I didn't even think about the transistors! The reason I'm using these LED is because I have nothing else to use them for.. And I'm pretty sure these are the only white ones I have at the moment, which is the colour I'm after for the light.

If I up the resistor on the LED strings, would that give me enough left over for the transistors? I'll up the resistor's so that the LED's are running at 90mA instead of 100... Does this sound good?

Thanks,
Denno

 

[carbonzit]

Thanks for that reply, I didn't even think about the transistors! The reason I'm using these LED is because I have nothing else to use them for.. And I'm pretty sure these are the only white ones I have at the moment, which is the colour I'm after for the light.

Have you looked at those LEDs yet? You might find they're waaaaaay too bright for your night light. (Or maybe not.) 100mA is a lot for a white LED. I've been fooling around with the little (T1-3/4) 20mA jobbies, and they're plenty bright.

If I up the resistor on the LED strings, would that give me enough left over for the transistors? I'll up the resistor's so that the LED's are running at 90mA instead of 100... Does this sound good?

Could work. You can basically "put your LEDs on a diet" (current-wise), starve them a little and they'll still put out plenty of light.

You could build it, then measure the total current from the power supply with a multimeter to see what you're actually drawing.

 

[denno]

You could build it, then measure the total current from the power supply with a multimeter to see what you're actually drawing.

Probably the most sensible thing to do isn't it . I shall give that a go.

I plan on putting the night light over the kettle area in the kitchen so at night we don't have to keep turning the kitchen light on to make a cuppa, so it wouldn't be too bad if it was bright.

Thanks again,
Denno

 

[alec_t]

my question initially was would I burn up the power supply if I made this circuit

That's not quite how the original post was worded

The reason for wanting as many LED's as possible, is because these 10mm LED's that I have don't have a very wide spread of light, so the more I use, then the more even I can spread that light.

Thank you for satisfying my curiosity.

I think it's sensible to reduce the LED current as suggested above. Always take manufacturers' specs for power supplies etc with a pinch of salt and allow a good operating margin.
Like CZ, I think you'll find the light much too bright (when your eyes are dark-adapted). By way of comparison, I've built night-lights for a child's bedroom and for a stairwell. Each is quite bright enough using a single LED at 7mA !! I guess your use could benefit from something a bit more than that though.

 

[denno]

You're right alec_t, I was thinking that I had wrote something similar to that when in actual fact I didn't lol.

I guess I'll wait until the parts arrive and see how we go.

Thanks for the help guys, really appreciate it.

 

[carbonzit]

Maybe this is a good time to throw out a note concerning due caution in building circuits like this.

I think it's OK for experimenters and hobbyists (I include myself squarely in the middle of this category) to have somewhat less than UL-certified (UL being the name of my (American) Underwriter's Laboratories: insert name of your national regulatory agency here) standards when approaching a situation like this (i.e., will this wall wart blow up or not?). My approach, since the rated output of the power supply is almost exactly what the circuit should draw, would be to try it, seeing as all the parts involved are not that expensive and there's a good chance of it not blowing up. While testing it, I would pay particular attention to how warm or hot the wall wart was getting: if it became uncomfortably hot, then I'd know that it was operating right at the edge of its capabilities, and should therefore be replaced with a higher-output supply.

In other words, we can rely on good old-fashioned common sense here, provided that we know enough about what we are doing and take all reasonable precautions against fire and electrical shock.

In other words, we don't necessarily need someone going all nanny-state on us and insisting that we only play in some kind of Macdonald's sandbox until we get our EE degree, or some such.

Now, if the person undertaking the experiment is an idiot, then all bets are off. I'm proposing guidelines for reasonably intelligent people.