RGB Strobe Light

[Ramzy]

Hi everyone.

I am about to get into my first solo build and the plan is to make a strobe light by way of a 555 timer and some RGB LED's(3-5mm). I plan on using a 12V supply and a bunch of potentiometers.



1. How do I achieve a 50% duty-cycle?
My calculations tell me that R1 must be 0, so that R2 can be a pot which I vary to change the speed of the strobe.

2.a) How am I to best wire my LED's?
My idea was to put them in parallel with about 6 LED's per row in 4 columns.

2. b) How am I to control color best?
I was planning on having all the separate color probes together for each color then vary the current in those by way of potentiometer and manually make colors from those.

3. Am I using the right type of LED?
I know I could be using an addressable LED strip to make my life better but I do not own any microprocessors just yet. Apart from those, would high power RGB LED's be better for such a build? My fear is all about the heat dissipation as I would not like to have anything get super hot. (The plan is to keep it as cheap as possible till convinced otherwise so I am avoiding heat sinks)

4. Is using a transistor better that leaving my output as the LED panel?
I plan on using a transistor because my 555 output voltage is not too constant but by using an inverter logic layout, I can have my (perfect) 12V supply to the LED panel to help with the consistency of the brightness and action.

[sokoloff]

If you want fast switching, high power, and low losses in the switch, you're going to want to look into using a MOSFET rather than a BJT as the output transistor. By all means build with a BJT to learn how to use the 555 and get it working, but the BJT will drop enough voltage (and therefore power and therefore heat up) where a MOSFET can have a much higher current flow with lower power dissipation due to the lower voltage drop.

You might also want to just order a cheap Arduino Uno clone (sub $5 on Aliexpress), build the 555 while you wait on the micro, get the experience from the 555, but you're likely to find that nearly everything is easier in code than in discrete ICs. (I'm glad I went through the discrete ICs/BJT phase in my teens, because the fundamental understanding sticks, but damn, it's so much easier to whack things together with a $2 microcontroller...)

[endevor100]

As far as how to wire your LEDs you'll have more efficient use of power if you wire them if series configurations. If the LED's have a 2.1V forward voltage and your source voltage is 12V you can put 5 of them in series (10.5V) and then use a 100 ohm resistor to limit current to ~15mA which is plenty for most jelly bean LEDs. Once you have the series array designed you can put as many of them as needed in parallel.

I suggest this Falstad Circuit Simulator for you to mess around with your duty cycle needs. They have an example PWM circuit that is a good example of you you might get the full range of duty cycles. http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+4.818269829109882+56+5+50%0A165+240+128+256+128+0+0%0AR+304+96+304+80+0+0+40+5+0+0+0.5%0AR+304+288+304+336+0+3+20+2+3+0+0.5%0AO+368+192+416+192+0%0Aw+240+160+208+160+0%0Ar+208+160+208+96+0+300%0Aw+208+96+304+96+0%0Aw+208+256+240+256+0%0Ac+208+256+208+304+0+0.000003+1.1194804884586058%0Ag+208+304+208+320+0%0Aw+208+224+208+256+0%0Aw+208+224+240+224+0%0Ar+208+160+208+224+0+300%0Ao+2+32+0+4102+5+0.000390625+0+2+2+3+ctl%0Ao+3+32+0+4102+5+0.00009765625+0+1+output%0A

[sokoloff]

Also, if you're making a strobe light, you probably want less than a 50% duty cycle and will want to look into overdriving the LEDs with a short pulse of high-current. The current that an LED can take for a short burst is higher than what it can take for long steady-state operation, so look into that. This makes it more important to drive with a low resistance and fast switching output, either a dedicated LED driver or a discrete MOSFET with suitably low Rds.

Also, to your question of how to handle the individual channels, well, once you have switched over to a micro and are driving each channel individually, you will likely find that varying the duty cycle for each color will allow you easy control over the color.