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A NE555 timer with the output connected to an appropriate transistor should do the trick. You can tune the 555 times to just about any duty cycle you want.
So the 555 timer connected to a transistor will power that many LED's? What would the schematic look like? Not the 555 timing, but the power circuit. I can find a 555 timing circuit I am sure.
Also, I will be powering Red LED's with a voltage drop of 2.2V.
Let's say 30, with a voltage drop of 2.2V. That gives me some wiggle room with actual LED selection. If I design a power circuit for the LED's at given forward voltage, I could put a lower forward voltage LED array in it's place without any ill effects. Or am I mistaken on that?
You could put 4 red 2.2V LEDs in series which is 8.8V and in series with a current-limiting resistor that will have 3.2V across it. But the LEDs will not all be 2.2V. Some might be 1.8V and others might be 2.4V. You will need to measure them and sort them into strings. Each string might need a different current-limiting resistor which is calculated by Ohm's Law.
You can drive the LEDs directly from the output of the 555.
For a 12v supply, you will lose about 2v when sinking or sourcing 140mA,
Put 4 LEDs in series with a dropper resistor of 82R.
You will now have 7 strings, each with 4 LEDs and an 82R resistor. It does not matter if you put the 82R at the top of the run or the bottom, just remember to put the cathode leg of each LED towards the negative rail. You can either sink or source the array.
That is: connect the top of the 7 strings to pin 3 and the other ends to the 0v rail or connect the 7 82R resistors to the 12v rail, then the seven lots of 4 LEDs and the cathodes of the 7 bottom LEDs are connected to pin 3.
It seems to me that 30 LED's would be drawing way to much current to sink or source from the 555. I could be wrong(probably am). Could you explain how the 555 which can only handle 200ma can supply 30 LED's?
I don't mean to doubt you or anything, but I am fairly new to electronics, and rather than just have someone tell me to do it a certain way, I would like to know how that way works before I do it.
Also, are there any sites for setting up circuits to make pin 3 low or high at a certain frequency? Thanks again guys. You guys are great. It is all starting to make a lot more sense to me now.
Like sinking and sourcing. If I sink the current, the LED's would turn on when pin 3 of the 555 is low. If I source it, they will turn on when pin 3 is high. One other question, pin3 will supply the same voltage that my supply is, correct?
I already mentioned the 555 drops 2v (across the chip) when sinking or sourcing 140mA. In other words the output is 10v max or 2v min.
Red LEDs drop only about 1.7v to 1.9v Are you using HIGH-bright LEDs?
What frequency do you want and what duty cycle?
In this case, an external driver transistor is not needed as the chip is capable of delivering the current for the LED array.
But suppose we needed 500mA. The chip is not capable of delivering 500mA, so we need a buffer transistor, connected to the output of the 555 (via base resistor) and the load is connected to the collector of the driver (power) transistor.
The advantage of doing this is two-fold. We get our 500mA and also the power transistor will drop less than 1v when fully turned on.
I like cool electronics, meaning no hot running ICs, and even power supplies I designed for industrial applications don't run hotter than 40deg/C.
Here is my suggestion: Driving 30 LEDs from a 12V power source you might connect up to five LEDs in a string resulting in six strings for 30 LEDs.
Calculating the voltage drop of five LEDs per 2.2V the required forward voltage will be exactly 11V. Considering the worst case of 0.7V voltage drop in a transistor there is a leftover of 0.3V to "play" with. Using 20mA forward current for the LEDs the current limiting resistor for each string would then be 15Ω.
Without any doubt a timer IC NE555 can drive six transistors with a base current of 2mA each.
Each transistor (BC337) can handle up to 600mA collector current. Again this is very safe since the collector current won't be higher than 20mA.
Here is a schematic with a timer circuit adjustable from 1 to 2Hz.
There is a big mistake in only allowing a very small voltage drop across the dropper resistor. It does not allow for any variation in characteristic voltage drop across the LEDs. Use 4 LEDs in series and drop more across the dropper resistor.
And you certainly don't need more than one buffer transistor to drive the LEDs.
The high-bright LEDs I purchase are rated a 17mA with 20mA being their "tested value." I would use 20mA as a max.
The characteristic voltage drop across a LED depends on the colour of the LED and not its output rating. Our HIGH bright red LEDs are 1.7v to 1.9v
My Luxeon SuperFlux red LEDs have a typical forward voltage of 2.0V at 5mA, 2.2V at 21mA, 2.4V at 44mA and 2.6V at their max allowed continuous current of 70mA.
They are extremely bright at 53mA. I have not tried a higher current.
They have pins and use the printed copper on a pcb for their heatsink.
You will have to know which type of LED you have as the new SuperFlux LEDs have a higher characteristic voltage and current rating to the original 5mm LEDs.
This is the problem with all these posts, people want things designed without relaying full technical details.
Well there will be two sections to my circuit. The timing section has nothing to do with the power section. Also, if I design the power section to the recommendations that I laid out(2.2v and 30 mA) there should be no problem using lower value LED's if I need to. As a matter of fact, I could possibly add more LED's. I would rather overbuild the power section, than make it perfectly designed for it. It leaves me options later.
I would like more than just to be told what parts to use. I would like help in how you guys choose those parts as well. Like how do you know what size resistor to use for each strand of LED. How do you calculate how many amps each LED will draw in the strings of LED's? Etc. I really appreciate your help.
I really don't want this designed for me. I want help in how to design it.
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