Modular Circuit

[Atomic_Sheep]

Hi guys, I started my first project and I need a little help. For the time being, I'm just trying to get the design down pat and not worried about exact values of components... yet. What I'm building can be easily depicted by the diagram in the first image "Circuit 1".

Essentially... we have a power source, a dimmer and then the modules. The requirements of the whole thing are:

1.) Each module may contain a different number of lights (I haven't yet figured out the upper and lower bounds on eactly how many LED's I will be having in each module i.e. I don't know whether the module with the smallest quantity of lights will have just 2 for example and the largest 300, however the range I think will be something in the order of 20-50.

2.) Despite the difference in the numbers of lights in each module, the single dimmer must be able to control all the modules in a way whereby each module has the exact same brightness regardless of the number of lights present. (NOTE: the lighting difference doesn't have to PERFECTLY the same but it should be close enough for the naked eye to not be able to tell the difference or MUCH of a difference).

From some limited background reading and my basic understanding of electronics... I've decided that the 555 timer circuit is what I need... depicted in diagram 2 (Circuit 2) and taken from:

LED Dimmer Circuit - Lighting

In the third image, is how I see it working in the final state... i.e. each of the modules is a SERIES of LED lights with an appropriately large and thereby different resistor for each series.

Questions:

1.) If I were to connect say 10 of these modules, what would be the best way of plugging in 10 wires into the two terminals on the main control panel? If you wrap them around a screw or something of the sort it would be messy and wouldn't be easy to disconnect individual modules or would be fiddly and a bit annoying trying to separate the wires.

2.) Will what I proposed fulfill point 2.) of my requirements? i.e. resistors like any components can be made to varying performances, so I'm not sure whether my simple adaptation/modification of the standard circuit will do the trick.

Finally:

From what I've said, is what I'm proposing going to work?

 

[Reloadron]

You really need to sit down with some LED data sheets. The dimmer you propose using a 555 is a popular little circuit but it can't drive a small mountain of LEDs. That single transistor would be short lived. Thus you need to think about having the 555 drive maybe a MOSFET to get your current up there. Next do some homework on the lumens of LEDs. That will determine what you get per module. Getting the same lumesn for assorted numbers of LEDs will not be easy. You also need to consider viewing angles for your chosen LEDs. There is considerably more to doing what you want to do than meets the eye at a glance. You have a large collection of variables to contend with.

Ron

 

[Atomic_Sheep]

Thanks for the reply... I'm not aiming for the same lumens per module, all I need is for each individual LED on every module to glow equally brightly. I know what you mean though and it's really I think going to go down to trial and error to get the appropriate result in terms of the distribution of the lighting and trying to get similar brightness... I just want to get the same brightness per LED.

I did some quickish maths and it looks like I'll be controlling something like 500 LEDs (But I think if that's an estimate to go by then it's an overestimate... maybe by 100 but you get the idea) with that single dimming circuit. I think I'll be using SMD LEDs. I just had a look at it actually looks like they draw more power than normal LEDs???? 2.6VDC @28mA and the SMD LED which are tiny draw something like 1.25W? Obviously they differ but are SMD LEDs really more hungry than normal LEDs? So if that's the case... how much current will I need to power something like 500 LEDs? Maths says... 500 * 1.25W which is 625W!!!!??? That's 2.6A per LED so something is amiss here?

I'll more than likely use something like a PSU to power the circuit although exactly how that will be done is a completely different story as I'm not really familiar with Rails and converting AC etc. so I'll leave that question for later. I haven't really looked at this at all yet in fact... for all I know PSUs spit out DC but yer... just don't know at this stage.

The other aspect I wanted to cover was... would it be possible to simply put the transistors feeding the modules in parallel? So something like 1 transistor per module or something like that? (or more for the modules with more LEDs... looks like the module with the largest number of LEDs on a single module will be 50 but that's a very crude estimate at this stage).

Finally, should the LEDs on each module be connected simply in series or should I try to figure out how to connect them in parallel? If one LED goes out in a series circuit, just one module will go out and then it will be a matter of trial and error to find and replace the blown LED, right? Do you think it's worth making a parallel circuit? Probably too many resistors to worry about is what I'm thinking but I'm nowhere near an expert.

In terms of selecting the right transistors... well, let's just say I've got a lot of reading still ahead of me but we'll get there eventually I hope. I see what you're saying with the current problem though.

Any additional comments or hints are welcome.

 

[audioguru]

Your transistor has a 1k base resistor. The supply is 12V, the base emitter voltage is about 0.7V and the output high voltage of the 555 is +10.8V so the base current in the 1k resistor is only 10.1mA. Most transistors have a saturated output current of 10 times the base current so the collector current to the LEDs is only 110mA. The output current of a 555 by itself is nearely double at 200mA.

Your arithmatic is wrong.
I see 8 LEDs in series. If they are 2V red ones then their supply must be 18V, not 12V. if they are 3.5V blue ones then their supply must be 30V, not 12v..

 

[Atomic_Sheep]

The 3rd diagram is merely a representation of what I'm trying to do... I didn't actually do any maths for it. I took the original picture and fudged it a little to make what I want to make a little clearer. And I think I just realised I made a mistake in my other calculation because I just realised that my modules aren't connected in series (and I even drew them in parallel in the first post) so yer... disregard that calculation as well, I'll have to sit down and figure it out properly I think.

 

[Atomic_Sheep]

Your transistor has a 1k base resistor. The supply is 12V, the base emitter voltage is about 0.7V and the output high voltage of the 555 is +10.8V so the base current in the 1k resistor is only 10.1mA. Most transistors have a saturated output current of 10 times the base current so the collector current to the LEDs is only 110mA. The output current of a 555 by itself is nearely double at 200mA.

Your arithmatic is wrong.
I see 8 LEDs in series. If they are 2V red ones then their supply must be 18V, not 12V. if they are 3.5V blue ones then their supply must be 30V, not 12v..

I labeled the old diagram with what was written, just wanted to confirm that I understand what was said.

So effectively the transistor turns the 10.1mA which enters it into 110ma? Well this particular transistor that is? So basically I will have 110mA to work with in terms of driving the LEDs?