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# Controlling Strobing LEDs Using A Potentiometer?

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#### sjaguar13

##### New Member
I have this good idea for once, and I can't figure out how to do it. I want a string of LEDs, about 10 LEDs, to blink. I want to be able to control how fast using a knob of some kind. I want to be able to turn the knob to 0 and they just stay lit. When it's at 10, they are going really fast. I really suck at this whole electronics thing. I can get the LEDs to come on with a battery, but that's pretty much it. What do I need to make them blink, and where does the knob come in at? I know I will need the LEDs (any certain kind?) and a knob, but what else I should get?

Thanks,
Justin

The best way to do this is to build a 555 timer circuit with one of the resistors as a potentiometer. I can't think of what some values you would use might be, but try this. First of all you want your 555 to operate in a mode called astable, where it switches on and off at the output. There are a ton of 555 timer calculators on the Internet, find one where you can supply the frequency and duty cycle (this should be around 63%) and calculate the resistor and capacitor values for the lowest speed you want, and the highest speed you want. Then buy a potentiometer that has a range about within those values. Then build the circuit to drive your lights. You will need to use an NPN transistor to pwoer the LEDs off the timer though, because of the current draw. Connect the timer's oscilating output to the base of an NPN transistor with a resistor (lets say 1K Ohm, but this will vary depending on your supply voltage), then wire your supply voltage to the positive of all your LEDs, and wire the collector of the transisor to the negative on all the LEDs. Finally wire the emitter of the transistor to ground. If your 555 oscilates in the frequency range you want, this should be all you need to do.

I won't go into detail about how the 555 works, as there are massive amounts of information about it available just by google searching 555 timer.

Thanks for the reply. I am really new at this and the 555 timer looks hard. I think I can do it. Once I get that done, that's pretty much the only hard part of the whole thing right?

I'm still looking for a schematic or something on how to build the 555 timer, but I don't really understand the potentiometer. Where is that coming in? Also, if I want the LEDs to stay on, what do I want the time to be? If I make it 999, it's still going to eventually turn off, right?

Thanks,
Justin

Oh, and I will be using a 9v battery as the power supply.

First of all, the oscilator will do just that, oscilate. If you want the lights to have a constant on mode, build a switch into the circuit that is configured as shown below, which will switch power from the oscilating circuit to the LEDs directly. You can even get a potentiometer (the knob) that has a SPDT switch built in, thus the "knob" will click at the end of its turn cycle and will switch to solid mode.

Start your project by building a 555 timer to blink one LED as a fixed rate. You won't need a transistor to drive one LED, and you won't need any knobs. Then build on the project as you learn.

In my diagram, you see how you can switch from blinking to solid. D1 is any common switching diode, there are usually available at Radio Shack in bundles of 50 for only a few dollars. This is important here to keep the 555's CLK pin from being driven high when you switch to solid mode. R1 is a resistor that will lower the voltage that you are applying to the base of the transistor to less than the differential threshold to switch the transistor. This is important because if the voltage at the base and at the collector are the same, or the base is higher, the transistor will not let current flow from the collector to the emmiter (and you might let the smoke out too )

Hmm now that I look at it I think all the LEDs in my diagram are backwards, anyway just put them in so that their positives are connected to your power supply and the negatives to the transistor (or ground if you are building a circuit without the transistor).

Learn more about the 555 circuit and build a circuit with just a single blinking light. Then try replacing one of the resistors with a potentiometer (which is the "knob", it's a resistor whose resistance value is adjusted depending on how far the knob is turned). This will take some experimentation to get results that suit what you want. The 555 is an easy to find chip that you can get at Radio Shack. They also sell boxes of NPN transistors. (There are two types, NPN will allow current flow when there is voltage at the base, and PNP, which will stop current flow when there is a voltage at the base.)

After you figure out your 555 timer, build a circuit with many LEDs that you can drive with a transistor. If something smells funny, don't use that transistor again . Once you get those two circuits working, just combine them! Allright, well, good luck!

P.S.
I left out the drop resistors in the digram too. For a plain old LED with a 9v supply, you will prolly need a resitor before each LED with a value of about 470 Ohm to 1KOhm. Again, this is dependant on the LED, just play with what gives you the birghtest LED output without getting the resistors real super hot.

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This is the most popular one I found.

There is also this one.

There is one that's close, but I don't get the right side.

This is one from a kit. It's only \$3, is that good?

This is the first one I ever found. It looks big and confusing. I hope this isn't the one I need.

Ya don't need "one". You need to understand the 555 (not internally, but how it works in a circuit) so that you can put in the components you need to make an adjustable astable oscillator. Build one of the designs you have that flashes an LED, then pluck at it, change components, see what happens. It will take time and experimentation, but you will get an idea of what is making what do what. Anyway, start by making an LED blink, the rest will come. Try building the first schematic in your post, and connect a 1 kOhm resistor and in series with an LED to the output. Use any 555 calculator you can find, cause there is a bunch of them, to come up with values for R1, R2, and C2. Using resistors with a specific value for R1 and R2 will make the LED blink at a fixed rate. Eventually you will replace R1 or R2 with a potentiometer so that you can make the rate variable. Start small, you'll get it.

Ohhhhh, I'm getting this. The last schematic in my post was the internal stuff of the 555? I thought I have to build a 555 from nothing. I was having a hard time finding schematics to build one, all I got was schematics for something that uses them. Tomarrow, I will go to Radio Shack and get the stuff. All I need is three resistors, 2 capacators, and an LED to go along with the 555, right? Also, what kind of wire should I use? Would speaker wire work?

Yeah, you buy the 555 and just add to it what you need . Anyway, any kind of cable will work. For breadboards and as general hookup wire I like to buy 24 Gauge computer ribbon cable (the kind without ends, just plain old 40 conductor cable) and then strip it apart. Its cheap cause you are getting 40 times more wire length than the actual ribbon cable length, plus when you tin the ends it works great for jumps on breadboards. Anyway, use whatever you like best, nothign in this project is goung to require cable with any sort of high current carrying capacity.

One quick questions about potentiometers, do they make one that is a push-pull that has a SPDT on the pulled out one?

"They" make everything, if you can think of it, it exists. The key is where ay go to get it. Radio Shack is like shopping for food in a gas station in a 200 person town. There is some stuff there, but it isn't even the beginning of what you can get. Jameco has a decent collection, www.jameco.com, and Digikey is just loaded, www.digikey.com.

I found a pot that is the push-pull kind. The description also says, "It also pulls up to activate its on/on mini switch, which can be wired for a variety of special functions." Would this work or not? The guy said it was a DPDT switch, not SPDT. Does that matter?

This is what the D's, S's, T's and P's mean on the switches:
D and S, Dual and Single Appropriately.
P and T, Pull and Throw.

An SPST (Single Pull Single Throw) will have one pull that is switched to one throw, or to none. A SPDT (Single Pull Dual Throw) will take on input (like power) and switch it between either one of the throws (with an off in the middle on some). DPDT like you mentioned is a switch that takes two inputs and switches them between two throws per input. you can use an DPDT wher you would use an SPDT, only ignore one set of pull and throws.

So once I get the circuit and LEDs working, I can use that pot in place of a transistor and I will have the pot change the speed of blinking, and a switch (which I'm not quite sure how to turn on) that will keep them from blinking. I will have a pull and throw that does nothing, and a pot that can be use for something else? When it says "pulls up to activate its on/on mini switch", does that mean the thing is always on, and therefore never stop blinking?

Hmm, use the pot in place of a transistor? You need to go get a book on basic electronics from your local Barne's & Noble or Radio Shack and read up on the theory and function of the basic electronic components which you are using. I don't in anyway mean this to sound demeaning. It is important to know what each basic electrical component does to understand how a whole circuit works. In short, the transistor is there to drive the LEDs, because the 555 lacks the abbility to put out the neccesary current from its output. If you read up on how a transistor works, you'll get what I will mean, and my earlier diagram will make more sense. Again, best of luck!

I meant resistor, use the pot inplace of a resistor. I probably should get a book or something, but I don't really have any intentions of doing a lot of eletrical stuff. I just thought of an idea and am trying to find a pot that can do it. I don't get the on/on thing. It seems like a stupid switch if there is no off.

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