Zhust wrote: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?
Zhust wrote: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.
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