So if my LED says 3.0-3.4V do I treat it as 3.0V or 3.4V when I do my calculation for a resistor?

 

Wake up your brain (no brain?).
An LED with a low forward voltage draws more current than an LED with a higher forward voltage if the supply voltage is the same and if the current-limiting resistor value is the same. It is simple Ohm's Law.
So calculate the resistor value with an LED that has the lowest forward voltage expected so that the current is not too high, then calculate the current with that resistor if the forward voltage is the highest expected to see if the current is high enough for good brightness.

 

So if I calculate 8.5V with two 3.0V minimum forward voltage LEDs at 24mA current, I get 8.5 - 3 - 3 / .024 = 104 Ohm and round up to a 120 Ohm resistor I'll be getting 20.8mA at 3V fv or 14.1mA at 3.4V fv is my math right?

 

So if I calculate 8.5V with two 3.0V minimum forward voltage LEDs at 24mA current, I get 8.5 - 3 - 3 / .024 = 104 Ohm and round up to a 120 Ohm resistor I'll be getting 20.8mA at 3V fv or 14.1mA at 3.4V fv is my math right?

Yes you are correct.

 

The reason I've been getting confused is because the LEDs list 3.0-3.4V. If the input voltage is constant, and the resistance is constant, why would the forward voltage and current of the LED's change?

 

The forward voltage of one LED might be 3.0V and the forward voltage of another LED might be 3.4V even if they have the same part number because they cannot make them exactly the same. You do not know which LED has a lower voltage or which LED has a higher voltage unless you measure them.
Then with one resistor value for each LED, the LED with the lower forward voltage will have more current.
The forward voltage changes a little when the temperature changes.

 

Ah, I was wondering that. So is there likely to be a significant difference between in brightness between the two? I mean, the 20.8mA to 14.1mA is a 30% difference but I don't know how that relates to actual light levels.

 

It is worse than that, the LED out of spec may just burn out, leaving the other taking all the current. Eventually it will burn out too, causing a cascade type failure.

LEDs, 555s, Flashers, and Light Chasers

LED tutorials on Chapter 1 and 2, circuit ideas on chapter 12.

 

Your vision's sensitivity to brightness is logarithmic like your hearing's sensitivity to loudness. That is why they have a vast range.
Half or double the sound power sounds a little less or a little more loud and half or double the LED power looks a little less bright or a little more bright.

You do not want the current to exceed the maximum allowed current for an LED to prevent it from burning out.

 

Or you could just try one of these....
...
http://www.electronicpeasant.com/projects/ledlamps/ledcolor.html
....
http://makezineblog.files.wordpress.com/2010/10/ledcolororgan-schem_r2.gif
...
..
To get it to work right, you have to essentially build a one-channel colr organ (a non technical term used to indicate an audio light modulator). The color organ will react to the sum of the complex audio wave form and give a pulsing display. A 3-channel color organ will give there pulsing displays, one low range (bass oriented), one mid range, and one high range.
..
Dave
Phoemix, AZ

 

Or you could just try one of these....

The first circuit shows the Japanese PNP transistors backwards.
The second circuit does not need the "virtual ground" opamp since its load is the very high input resistance of the opamps.

 

The weird thing about my audio source is I hooked in an adapter to give me a stereo jack and plugged in a pair of headphones and played a CD. The volume level through them was a very comfortable listening level. Is this typical of line level?

 

Or you could just try one of these....
http://makezineblog.files.wordpress.com/2010/10/ledcolororgan-schem_r2.gif

I really like this schematic, but it's odd that it doesn't mention what voltage/current LEDs to use, so why even bother listing the amount of resistance on the resistors? If I dropped the LEDs I just bough in the blue line of that schematic, a 160 ohm resistor with a 12v source would burn them fast. Anyway, I think I could tweak it to my design. I'll go get to work on that.

By the way, are all those capacitors really necessary? I'm trying to keep this simple.

 

Here's my rough draft. I don't know if I got it quite right. Any input?

 

Opps, I goofed.

A thread belongs to the OP (original poster). Trying to take over someone elses thread is called hijacking, which is not allowed at All About Circuits. I have therefore given you a thread of your very own.

This was split from About sound reactive LED's with TIP31C.

 

Sorry about that, when I originally started posting to this thread, I thought my circuit was pretty much the same as the one the original poster was talking about, and I was hoping the answers would be just as applicable for him as for me, but apparently my designs have turned into a whole new monster and in retrospect I probably should have just started my own thread from the beginning. Won't happen again.

 

And that is why the policy exists.

 

The input of your new circuit is a diode in series with the base-emitter diode of a transistor with no series resistor to limit the current.
If it is connected to the speaker ouput of a power amplifier then it and/or the amplifier will blow up!

 

The only difference between the TIP31, A, B, and C is the increasing amount of BV-CBO/CEO, so in a situation where a TIP31 would work, the TIP31C should work just as well, right?

What does the diode you connected between the emitter and the base do, and why is it necessary?

One thing I still don't understand, the transistor is current driven, but you said that the transistor needs between .6V to 1V to turn on, is this just the TIP31, or all PNP transistors and where on the data sheet does it say this, or how did you calculate these values?

 

I looked at the prices of TIP31 transistors and the TIP31C with a max collector voltage of 100V is the cheapest. This circuit uses a low collector voltage so any TIP31 transistor will work.

The diode between the base and emitter of the transistor conducts when the input is 0.6V or more so it prevents the reverse-biased emitter-base diode from avalanche breakdown (the datasheet says its maximum allowed reverse voltage is only 5V and a power amplifier has much more voltage). Avalanche breakdown of an emitter-base junction slowly destroys a transistor.

ALL silicon transistors need about 0.6V from base to emitter to turn on (it changes a little when the current and temperature change). The TIP31 is an NPN transistor, a TIP32 is a PNP transistor with the same spec's. The datasheets show all the spec's.