Lighting my miniature theatre

[weedrummerkid]

Hello!

I am building a miniature theatre, which will contain 50 or so led's to light the stage. They run on 3-3.4v, max continuous forward current of 24mA, max peak forward current of 75mA. I want to have 20 or so circuits that I can dim independently, so some circuits will be running 2-5 led's in parallel. Now, I am aware that I need to have some kind of PWM in order to get smooth changes in brightness.

My question is: What is the cheapest, simplest way to do this?

A few options:
1. Have a circuit board printed, or use stripboard, and make circuits with 555 timers. This might be over my head.
2. Use a microcontroller of some kind, probably in combination with stripboard and a bunch of switches. As for microcontrollers and software, I use a mac, and I do not know much from programming–so simpler and mac-compatible is better.

I would really, really appreciate your help. Thanks!

 

[Sceadwian]

Given your level of current knowledge, buying a commercial unit that does what you want is the only solution that won't require you to learn something 'over your head'. 555's aren't that complicated on perfboard, and micro controllers are far more complicated.

 

[carbonzit]

Not sure I agree with that.

While it's difficult to know exactly what the O.P.'s skill level is, PWM controllers are pretty simple to cobble up on perfboard. F'rinstance, check out this simple PWM circuit. (Lots more like this out there on the web.) Just a single 555 and a few other small parts. One of these could control a string of lights. You could also use a much smaller output transistor.

(Note that if you do use this circuit, you won't need the snubber diode, as there's no inductive load like a motor.)

 

[Diver300]

Now, I am aware that I need to have some kind of PWM in order to get smooth changes in brightness.

I don't know where you got that idea. If you can change the current smoothly, the brightness will change smoothly. It does not have to be turning on and off. I have a desk light that I converted to LEDs. There is a capacitor in parallel with the LEDs, and when I turn the light off, the LED brightness fades and the lights are still visible after more than a minute. There is no point where they brightness changes suddenly.

You should not put LEDs in parallel. If you do, the current will not be evenly distributed among the LEDs. It is always best to put each group that you want to light at the same time in series, and to control the current in each group.

Also if you do use PWM, go for at least 1 kHz, or the lighting will appear to flicker in some conditions and to some people. I know that some people say that 100 Hz is fine, but I have been able to detect that an LED was flashing at 5 kHz. If the frequency is above 1 kHz it is generally not noticeable.

 

[carbonzit]

So instead of PWM, how about just using a potentiometer connected to the base of a transistor, with the LEDs driven by the transistor? That should be really simple.

 

[Diver300]

You should put a resistor in series with the emitter of the transistor. That will give you better control of the the current.

Without the resistor, it could be touchy and temperature dependent.

 

[weedrummerkid]

Thanks, everyone! I guess I will go with the 555's on perfboard. As for just a potentiometer setup, I have tried it, and there were tangible steps in the gradient. It might have just been a cheap part, admittedly, but I think I will try to learn some stuff.

Could anyone recommend a beginning-level book that would educate me enough to complete a project like this?

I greatly appreciate your help.

 

[KeepItSimpleStupid]

Bear in mid, I'm not advocating this as a solution, but this module http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_120539_-1 is available from Jameco. It's also overkill.

You might breadboard one of the suggested one and then use a free PCB layout program. Most are probably PC based, unfortunately. The steps that the manufacturer goes through is outlined here: http://www.pcbexpress.com/technical/tutorial.php Also see http://www.expresspcb.com/ which uses a propretary software so your locked in to their service.

 

[carbonzit]

Here, try this little circuit:

**broken link removed**

I just tested it, so I didn't just pull it out of my butt (or off the web somewhere). It works pretty well.

Not perfectly well, mind you: the problem is that the range within which you can adjust the brightness of an LED is fairly narrow, hence the top and bottom resistors. You could play with these values, with your particular setup, to arrive at a more pleasing adjustment action. But it is smooth, without any abrupt changes in brightness. The transistor should handle up to 8 or so "standard" (i.e., ~20mA) LEDs or so, or you can get a beefier transistor to control more.

Rled needs to be set to limit the current through your particular diodes. As others have stated, you should string them in series, not in parallel. (Or you can have series strings, each with its own resistor, in parallel.)

 

[KeepItSimpleStupid]

Nice. Gave me an idea. Why not the LM337L (100 mA) **broken link removed** 100 mA and small.

You can try the LM317T (1.5 Amp) which is usually available at Radio Shack. Setting the low end should be more predictable and not dependent on current gain. Once your satisfied, you can buy the cheaper part.