NE555 Adjustable PWM Circuit


So I think I’m just being stupid, but I thought I’d ask on here to confirm. I wanted to just play around with some simple circuits just have some fun. I thought that an adjustable PWM output would be nice for some LED lighting for… Something.

After some googling I found a circuit on EEVblog’s forums that I thought I would try to replicate. I put the parts on a breadboard and it doesn’t work as I thought. When I adjust the pot, there is a large band (maybe from 10-90% of the pot range) of little to no change in the brightness of the LED. The main issue though would be that either direction I go with the pot, I get this behavior. The brightness increases in both directions and when I measure the duty cycle (with a DMM) I never get lower than ~50%. So basically from 50% to 90% regardless of which direction I move the pot.

I don’t show the LED on the output, but one is there on the board. Although I get the same issue without a load on the 555. Everything simulates okay (but of course it would). I also attached the simulation file.

Thoughts? (669 Bytes)

Hardware Project Ideas

Can you try the circuit explained in this website:

Can you also provide the link to where you found this circuit? It should work, but it’s more complicated so I’d like to read why it was designed this way.


I will certainly try that circuit when I get home. Although I did find a very similar one on a different website. This one at the bottom of the page. It had the same issue with the pot though.

This is the forum post that I found it on. Sadly there isn’t much discussion on why it works (your link is much better for that).

I might try making nice short wires and cleaning up the breadboard and take a picture of it for everyone. Maybe I’m just hooking it up wrong. I was thinking maybe I have the pot in wrong, but I’m not sure. I have the two pins which always measure 100k connected together/connected to the diodes. From there I have the middle pin connected to the 1k resistor from my link.


It sounds like your pot is not wired correctly.

No two pins of the pot should be connected together, the two fixed pins each connect to a diode. The other side of the diodes connect together.


You know, you say that and it’s like. Ugh. UGHHHHHHHHHHHHHH



So I got home and tested it out with the pot in the right wiring. That was it. Doh! I also tried the circuit from your link @Dillon1337, that worked as well. Do you consider it simpler since it doesn’t have the cap on pin 5 and no resistor/wiring to pin 7? Just making sure I understand why you say that.

Now time to drop it onto some vero board to save it for later!

I think the next thing I’m going to work on will be a simple audio amp/VU meter (or peak detection) done with DTL logic… Maybe? Or a series of comparators. Haven’t decided how I want to handle it.


The capacitor on pin 5 is used to filter the internal control voltage, it is a good idea to include it.

The circuit posted by @Dillon1337 uses the output pin to control the charge/discharge instead of using the resistor and pin 7. Although this can work well it can also cause problems, in general it is not a good idea. See this post for more information.


Cool, yeah that makes sense. I do remember seeing the post from that build long now that you quote it here. It’s one of those moments where you read something, but didn’t know the application of it.

Thanks for the help everyone.

On a completely side note. Who knew that the LM386 was $1 per chip! Makes me want to rethink the idea of using it for an audio amp.


I got some time last weekend to solder up the circuit (even using a SOT23 FET on the veroboard). It works! Which is always good.

My soldering isn’t the best in the world, that’s for sure. but it’s getting better.

In the last photo, you can even see @jonthomasson amazing Solder Sniffer (with my horrible SMD soldering) :slight_smile: It works great.


That looks great! Glad you’re getting use from the Solder Sniffer. :slight_smile: I like that little Adafruit USB breakout board too. It seems like a handy way to get 5v on a breadboard.


Yeah it’s working great (both the fan and the breakout). I’ve used the fan a few times already. I soldering some small kits I got off of amazon with my niece.

The fan lasts a good long while really. So I can’t complain whatsoever.


Decided to respond here since it’s directly related. I’ve had another project come up which uses this circuit, so I thought I’d write something up for it.

I have a lightbox that is pretty bright and wanted to add dimming to it without modifying it too much. I decided that I could likely put a dimmer box in line with the power cord. There is a pigtail coming out of the lightbox with a plug on it. And the AC > DC converter has the other plug end on it. I figured I could get a pair of jacks and have the box in the middle. The specs aren’t too bad at all. 12V at 1A max based on the wall-wart. So that was my design goal.

I’ve already built of one these circuits just for fun/testing. But this came up and was the perfect time to break it out again. I changed out the MOSFET I used last time for one that can reliable drive more current. I think I went overkill, but I don’t have many options for heat sinking I don’t think.

I choose the project box kind of at random and had to get the size of the protoboard to work with. To do that I looked at the box’s datasheet to get the dimensions and then transferred them to paper to mock it up. From there I cut it out and traced it onto the protoboard that I have. That gave me the number of holes I had to work with.

From here I decided I needed some help laying out the circuit to see if it would even fit. I could have done it by hand, but I decided to look for some software help. I found some nice software (really easy to use with some nice features). It’s called DIY Layout Creator, located here.

It worked out great and I’m able to fit the circuit with protoboard in the project box. In hindsight, these steps should have been reversed. That being said, there isn’t much to this circuit so it isn’t so bad this time.

And that’s where I am now. I’ll be adding more when I get to it.


I bookmarked that Layout Creator software for later use. Will definitely come in handy. Thanks for the link. :slight_smile:


:slight_smile: That is part of the reason for starting up the log. I figured that people would like the software at the very least!


It looks like DIYLC has really moved on since lasst I used it - probably around V1 Beta. I had to program a couple of my own footprints in Java. Even so, it still made working with Veroboard much easier. It’s nice to see it is still around.


That sounds really intense, having the program your own! I can definitely see that it would be helpful though with a larger circuit. It’s nice that I didn’t have to go through that. I would have just used random shapes like I did already! hahaha


Thanks for the diylc tip!


Build focused log here. I built most of the circuit and just have to attach the input and output connectors and drill the box for the connectors and the knob. I have issues still with soldering in such tight confines/having to layer many wires on top of each other on the same node, but I’m sure I’ll get better in time.

I did have to cut down the protoboard more than I thought and somehow I managed to mess up the drill hole locations. shrug I did a lot of sanding as I don’t have any good cutting tools. I tried using a hacksaw blade that I had, but that wasn’t the best. I ending up using a razor blade and snapping the board with pliers.

Need to order a few more parts for it for testing, but I was wondering if I should get Kapton tape for future stuff instead of using electrical tape. Thoughts on that matter?

Pictures of the build are located here:


Decided to come back to this to see if I could figure out what was going on with it. I noticed when I tested it on the lightbox that I wanted to dim that no dimming was happening. To check to see what was going on, I hooked up a normal LED to the output and same thing!

I don’t have a scope, so it’s hard to see what is going on (need to get that AD2 bundle when I have the chance). But, I was able to figure out that the output of the 555 is okay and that I’m able to dim an LED hooked up directly to pin 3. So… it looks like the FET isn’t switching off like it should. Two things to note about the design. I don’t have a gate resistor from pin 3 to the gate and I don’t have a pull down from the gate to ground.

I did hold a 1k between gate and ground to see if that would solve the issue, but it didn’t seem to make any difference.

For reference, this transistor, is the one that I’m using for the output to handle ~1A of current though it at max.

Thoughts on what could be the issue?


I finally got the AD2 after putting it off for so long and it’s great. It’s also helped me test the dimmer and see what was going on with it.

What I see happening is that below ~5V, I can use the dimmer as expected and everything is hunky-dorry. But above a voltage threshold, if I go to 100% DC on the dimmer, then the gate drive gets forced high and stays there. The only way of recovering the gate output is to cycle the power to the dimmer and also reduce the voltage.

Here are a pair of pictures. They were taken at 8V, which is stable when there is no load, and when a load is plugged in, the gate locks high.

Gate Drive, No Load:

Gate Drive, 12R Load:

At 12V, the voltage I was hoping to run this at, I see this:

While I was writing this post. I started probing the input voltage to the dimmer since I noticed that most of the time the issue would occur around the point that the PSU would change ranges (typically around 8V). I have a cheap BPS-305. After being confused by the issues on the output, I finally decided to probe the input and saw that, at higher voltages, there were large voltage spikes! So I whacked an input cap on there and suddenly, no issues with the mosfet! WOO

I connected the dimmer to the lightbox and it works perfectly! So glad.