EEVblog #392 – 555 LED PWM Hack

[aluck]

Wouldn't it be simpler to replace one of the resistors on LM317 with a resistor and a pot connected in series? It will become a nice regulated current source with just a couple of components.

[T4P]

yeah but ... you need a fat pot as well as it having to be at the bottom

[aluck]

Put that pot on a long wire... nothing wrong here.

Yes, I know, Dave doesn't want to void the warranty...

[odokemono]

Nice hack Dave.  I gotta admit I was worried the 317 wouldn't like the pulsing but it behaved fine.

You've convinced me to get a good camera.  Mine has serious auto-brightness issues: https://sites.google.com/site/dannychouinard/Home/atmel-avr-stuff/cheap-led-lamp-micro-controller-upgrade

Video at bottom.

Sort of same hack, different solution.

Fun stuff!

[EEVblog]

You've convinced me to get a good camera.  Mine has serious auto-brightness issues: https://sites.google.com/site/dannychouinard/Home/atmel-avr-stuff/cheap-led-lamp-micro-controller-upgrade

I've never regretted spending $1600 on a decent video camera.

Dave.

[dentaku]

Very beginner question...
What's the cap between the 10k pot and ground?
I don't understand the writing and the particular way he wrote it seems like some sort of shorthand or code.

[c4757p]

That's the timing capacitor. It's the same capacitor as is almost always on pin 2/6 on any 555 implementation. Timing is accomplished by repeatedly charging and discharging it between two thresholds.

0µ1 means 0.1µF. The purpose of writing it that way is that decimal points are easy to lose if written sloppily. It's really common in EE.

Edit: Sorry, misunderstood the question as being two questions. Hope you're not offended that I thought you didn't know its purpose 

[dr_p]

FYI, the same rationality applies to resistors:
120R = 120 Ohms
0R1 = 0.1 Ohms
2K2 = 2.2Kilo Ohms
6M8 = 6.8Mega Ohms

[dentaku]

That's the timing capacitor. It's the same capacitor as is almost always on pin 2/6 on any 555 implementation. Timing is accomplished by repeatedly charging and discharging it between two thresholds.

0µ1 means 0.1µF. The purpose of writing it that way is that decimal points are easy to lose if written sloppily. It's really common in EE.

Edit: Sorry, misunderstood the question as being two questions. Hope you're not offended that I thought you didn't know its purpose 

If I wouldn't have already built another 555 circuit recently I probably wouldn't have known what it was for
Thanks for the answer.
I ended up using a tiny 104 capacitor which IS 0.1uF so I guess I figured it out.

[xenocide702]

Been meaning to post this for awhile, finally got around to making an account.






Thanks for the help! I got my scope and had this sucker within an hour or two. I happened to have an LED panel out of an old flashlight and some components lying around, so I hacked this thing up. A little hot snot onto the mount and we're good. It's not as much light as I really want, but I'll route a board for an LED ring later. You video really helped, I knew this configuration w/ the 555 existed, I just didn't know much about how it worked.

[scratchypots]

I think it would be nice to add a toggle switch to bypass the PWM and revert the mod.  This way you can have maximum lux, or a dimmer adjustable setting as desired.

Thank you Mr. Jones for the great blog!

[c4757p]

I think it would be nice to add a toggle switch to bypass the PWM and revert the mod.  This way you can have maximum lux, or a dimmer adjustable setting as desired.

It's been a long time since I saw this video, so I don't remember how it worked out for Dave, but I've used the exact same circuit before and IIRC, at the maximum end of the pot range, the duty cycle becomes too high for either the 555 or the switching transistor (I used a fairly large MOSFET with a high gate charge, though, and couldn't be bothered to figure out the "problem" - I called it a feature), and the load just turns fully on anyway.

Of course, if your pot is too scratchy, you might have trouble slamming it hard up against the end stop for full resistance. I recommend newer ones.

[SZA]

'm using a simiar circuit to generate a 50kHz PWM signal , can Any1 please confirm the formula used. And can we replace the 1k with a pot to achiving variable frequency.

[Ivanko1]

Why bipolar transistor, maybe mosfet will be better?

[bjcuizon]

Hi Dave,

How would you know the output voltage of the LM317 if it is configured to constant current mode?
I already know it has a reference voltage of 1.25V but I want to know its output voltage. Does it depend on the resistor or is it the same as the input voltage?

Thanks.

[Teza]

I want to use this circiut with a frequency of 3 kHz, at maximum light.
Having problems to find out how to calculate the needed values of the 1kohm resistor, 10 kohm pot and 0,1uF capasitor.

Anyone that kan assist with some calculation examples please?

I tryed the following, but I don't know if this is correct:

[vince76]

hi,
what can cause the brighness of the leds be at maximum either way of the pot? ( the pot itself seems ok )
In the middle is kinda working
thanks