2N2222 LED Controller Question

[5280MeV]

So I am using a 2N2222 Transistor to control a bunch of LEDs. Here is the circuit with an Arduino as the controller:

Screen shot 2012-09-09 at 8.07.38 AM.png

This works just fine. I set pin 13 to high and the LEDs turn on. I set pin 13 to low and they turn off. Groovy.

Now I want to use a trick. I want to get a flicker/flame effect, so instead of hooking up an expensive Arduino I use a cheap flicker LED as a controller:

Screen shot 2012-09-09 at 8.09.32 AM.png

This does not work. The main LEDs are always on, even as the flicker LED flickers.

I found that by adding another resistor R I can get it to work:

Screen shot 2012-09-09 at 8.10.46 AM.png

If R is too big (10k Ohm), then the main LEDs just stay on all the time. If R is too small (100 Ohm), then the LEDs all turn off. If R is 1000 Ohms, then the whole setup works, the three LEDs flicker and I can easily make electric candles/lanterns of any color/size/shape that I want. Cool.

The only thing that bugs me is that I cannot figure out exactly why I need R? Any thoughts?

[Zhust]

First note: the 250 ohm resistor is unnecessary if you use a 50K base resistor. Transistors are current amplifiers so 50K at 5 volts is 0.1mA at the base; typical DC current gain is 200, so collector current would be 200 * 0.1mA = 20mA. The collector-emitter voltage will automatically change to ensure exactly 20mA passes through the collector. Neat, huh? I typically connect the LED chain anode directly to the unregulated source to reduce the load on the regulator.

I'm sure the flicker LED has an internal microcontroller so it needs a little juice to run. The 1K resistor you're adding pulls the transistor's base close enough to ground (emitter potential) to shut off (that is, VBE is below about 0.6V), but when the LED is on, the current draw in the flickering LED is high enough to pull that voltage up and current flows from base-to-emitter and turns the transistor on.

In other words, when the flickering LED is off, the flicker LED current might be about 1mA, but with it on, it's about 20mA.

[5280MeV]

First note: the 250 ohm resistor is unnecessary if you use a 50K base resistor. Transistors are current amplifiers so 50K at 5 volts is 0.1mA at the base; typical DC current gain is 200, so collector current would be 200 * 0.1mA = 20mA. The collector-emitter voltage will automatically change to ensure exactly 20mA passes through the collector. Neat, huh?

Very cool.

I'm sure the flicker LED has an internal microcontroller so it needs a little juice to run. The 1K resistor you're adding pulls the transistor's base close enough to ground (emitter potential) to shut off (that is, VBE is below about 0.6V), but when the LED is on, the current draw in the flickering LED is high enough to pull that voltage up and current flows from base-to-emitter and turns the transistor on.

In other words, when the flickering LED is off, the flicker LED current might be about 1mA, but with it on, it's about 20mA.

Well that explains it! I can sleep now. The site where I learned the flicker LED trick used a PNP transistor and now I think I understand why.

Thanks!