Question about project in Make Magazine

[markbernard]

I was going to build the lamp project I found at http://makezine.com/projects/make-32/little-big-lamp/. I built it up on the breadboard and the brightness did not vary that much. So I thought I would try something different.I have attached the schematic of what I came up with. I breadboarded it with 2 sets of LEDs and it work like a charm. Each set of LEDs was only drawing around 21mA max current at 12v and the 5K pot was able to create the dimming effect and bring the current down to 0.5mA. With 6 of these the max power should be around 1.5W.

Is there anything I am not seeing here? Should my circuit work for this application? Is there an advantage to using the 555 with the darlington instead?

Thanks.

[fcb]

So you are running 126mA through the pot at maximum brightness...

What size is your pot? Can it take the power for sustained periods? Does it become scratchy quickly? I'd be surprised if it can take it.

[markbernard]

Ok thanks. Now I see the problem. I think the one I have is only 1/2 watt. So if I get a 2 watt pot then it should be ok?

[fcb]

Maths and data sheets are your friends here.

Personally - I'd stick with a low cost pot for the control and create a constant current generator with a transistor - far cheaper than a 2W pot.

[Supper_slash]

I would go with the Pulse With Modulation circuit. Even a strong potentiometer would breakdown after a short amount of time.If you need help with the PWM circuits, you can ask me for what it's worth.
There is also PWM chips instead of the 555 timer, or a microcontroller. Although that may be over kill unless u was to put a "sleep timer" in it or something like that so it would cut off after so long without activity.

Otherwise, I'd just use the 555 timer.

[markbernard]

Thanks a lot guys. I will give the 555 circuit another go and see if I can get it working. When I first tried the test on a single set of LEDs I did not have the darlington in the circuit. I'll try with it in this time to see what happens.

[markbernard]

Thanks again for everyone's help. I figured out the problem. The 555 I was using was blown. I replaced it with another one and it started working. The only issue now is that the 100k pot speced in the schematic only changes brightness over half the range and the other half is just dim. So I am going to try a 50k and a 10k.

[tooki]

FYI, I don't like the PWM dimmers that just use a 555, because the PWM frequency changes quite a bit when dimming. The ones that use a 555 and a comparator (usually an LM358 op-amp) work better and are only trivially more complex to build (or buy: they're the kind you get on eBay for $2).

Here's a circuit: https://www.circuitlab.com/circuit/c8m48y/pwm-led-dimmer-12v-8a-96w/

And here are some dimmers on eBay that use that circuit:
http://www.ebay.com/itm/PWM-Dimmer-Switch-In-line-Manual-Knob-Style-for-1-Single-LED-Strip-Light-12V-24V-/391775561775
http://www.ebay.com/itm/DC-12V-8A-LED-Light-Protect-Strip-Dimmer-Adjustable-Brightness-Controller-MC-/162023123417

[markbernard]

I am just starting out so I'm going to keep it simple and follow the project. But I will definitely keep that in mind for a future build.

Thanks. 

[Zero999]

What 555 timer circuit are you using? Is it like the one below?


Here's the one with the comparator, which tooki was talking about. I don't know why people use the LM358. The LM393 should be used instead, because it's a proper comparator. The LM358 is designed as an op-amp and performs very poorly when used as a comparator. A MOSFET driver may be required, if the turn on time is too slow.

[markbernard]

It's the one in the article linked above.

I will see if I can embed the image here.